Wellbore Stability Analysis Bergermeer Field 2009 351869

Wellbore Stability Analysis

for the Bergermeer Field

Netherlands

Coordinated by: Jelle Wielenga, TAQA

Performed by Zissis Moschovidis, PCM Technical, Inc.

For TAQA Corporation

Report: R2009PCM011 October 28, 2009

Prepared by:

PCM Technical, Inc.4618 E. 55th Street, Tulsa, OK 74135Tel. & fax: 918-494-8986

Report R2009PCM011 October 28, 2009

P CM T echn ica l , Inc . 4618 E . 55 t h S t ree t , T u l sa O K 74135 - Te l . / Fax 918- 494 -8986 1

1. Introduction .................................................................................................................................. 2

Objectives .................................................................................................................................... 3 Note to the reader ........................................................................................................................ 3

2. Conclusions and Recommendations ........................................................................................... 4 3. Methodology ................................................................................................................................ 5

Stress Invariants .......................................................................................................................... 5 Analysis Steps.............................................................................................................................. 6

4. In Situ Stress and strength Profiles ............................................................................................. 8 Calibration of in situ stress profile ................................................................................................ 9 In situ strength profile................................................................................................................. 12 Model Validation......................................................................................................................... 16

BGM8 well stability ................................................................................................................. 17 BGM4 well stability ................................................................................................................. 18 BGM2 well stability ................................................................................................................. 19 Conclusions of model validation............................................................................................. 19

5. WBS Analysis of BGM8............................................................................................................. 22 Conclusions of BGM8 WBS analysis ......................................................................................... 22

6. WBS Analysis of BGM1............................................................................................................. 30 Conclusions of BGM1 WBS analyses........................................................................................ 42

7. WBS Analysis of BGM7 and proposed horizontal well H35....................................................... 42 WBS analysis for BGM7............................................................................................................. 42 WBS analysis for H35 ................................................................................................................ 45 Conclusions of BGM7 & H35 WBS analyses............................................................................. 48

8. WBS Analysis of BGM3 well...................................................................................................... 49 Conclusions of BGM3 WBS analysis ......................................................................................... 52

9. WBS Analyses for BGM2 & V23................................................................................................ 54 WBS analysis of BGM2.............................................................................................................. 54 WBS analysis for V23 ................................................................................................................ 57 Conclusions of BGM2 and V23 WBS analyses ......................................................................... 57

10. Production Stability of the Rotliegendes Reservoir.................................................................. 61 Production stability of Rotliegendes based on lowest UCS of BGM1 well ................................. 61 Production stability of horizontal sections of H35 well................................................................ 64 Conclusions of Production WBS analyses................................................................................. 64

References .................................................................................................................................... 65 Acknowledgements........................................................................................................................ 65



Wellbore Stability Analysis

for the Bergermeer Field The Netherlands

Coordinated by: Jelle Wielenga, TAQA

Performed by Zissis Moschovidis, PCM Technical, Inc.

1. INTRODUCTION

PCM Technical Inc. was contracted by Jan Thijs Keijser of TAQA Energy B.V., the end of July 2009 to review a previous wellbore stability study (ref. 1) for the Bergermeer Field, and provide recommendations for assessing the stability of new wells to be drilled. The new wellbore stability (WBS) study is documented in this report and in Power-Point presentations sent in the course of the study from August to October 2009. The study was coordinated by Jelle Wielenga of TAQA, who also provided the data and contributed valuable discussions and guidance. Figure 1.1: Map of the existing wells of the Berger meer Field

TAQA Energy and its partners are constructing an underground gas storage facility, the Bergermeer gas storage facility, which will accommodate seasonal fluctuations in the supply and demand of gas by storing gas during the summer so that it can then be used in the winter.



The Bergermeer field is located near the cities of Bergen and Alkmaar in the Netherlands. Most of the existing wells of the field (all except BGM4, see Figure 1.1) have produced for a number of years successfully and currently are highly depleted from an initial reservoir pressure of ~ 235 bar down to ~ 10 bar. Presently the field is being converted to a gas storage field. Approximately 14 to 20 new wells will the drilled in the next years to satisfy the injection and production needs of the gas storage operation, which is planned to be operational in 2013. The Bergermeer field is a horse & grabben structure and contains layers of sands, clay-stones and dolomites. The reservoir is the Rotliegendes formation, at a depth of 2000 to 2600 m TVD, which is a relatively weak and permeable sandstone. The top of this formation is more competent and is referred to as the Weissliegendes formation. The Rotliegendes formation is under the Zechstein formation, consisting of claystone, dolomite, salt, and anhydrite layers. The field consists of two blocks separated at the reservoir level by a normal fault (the approximate location is shown in Figure 1) striking in the NW-SE direction. The present analyses are based on the logs of the existing Bergermeer wells (BGM 1, 2, 3, 4, 7 and 8 wells). The in situ stress-profile was estimated using the theory of elasticity from the logs that were provided, and was calibrated to in situ stress data from fracturing tests in the BGM8 and BGM4 wells. The strength profile used in the analysis was estimated from the compressive wave slowness time (Dt) using the Formel-Dt model (ref. 2). The WBS analyses were performed using the software developed by PCM Technical.

Objectives

The review of the FEED study (ref. 1) showed that an in-depth WBS study should be done which will be compared and to events observed while drilling the existing wells. The following plan was adopted for the study:

(1) Analyze available stress tests, leak-off tests (LOTs) and drilling events of wells in the Bergermeer, to establish in situ stresses at different depths inside and outside of the reservoir.

(2) Make a calibrated in situ stress profile using the logs and the in situ stress data and observations collected in step #1.

(3) Perform a WBS analysis to predict a “log of safe mud-window” for drilling operations, for 0, 25, 50, 75 and 90-degree inclination wells. Also repeat the analysis for few depletion scenarios (as required). This objective was amended to include production stability.

The stability of wellbores is affected by both mechanical and chemical effects. Chemical effects are expected to be primarily significant in shales, which are chemically active and require a chemically compatible fluid to avoid hole sloughing. Sands are expected to be chemically more inert than shales (depending if they contain clays). The Bergermeer wells were drilled with water-base mud with additives to make it compatible with the shales, and therefore, we assume that wellbore instability is primarily a mechanical phenomenon. The present study addresses wellbore stability caused by the mechanical failure of the formation, and we assume that any chemical instability has been effectively addressed. The mechanical wellbore stability was calculated using the failure theory by Von Mises, which accounts for the effects of all three principal stresses and uses the entire failure envelope of the rock under different confining conditions (see following section).

Note to the reader

The results of this study were first documented in “PowerPoint” presentations (BGM-A, BGM-B, BGM-C, BGM-D and BGM-E), which were sent as results became available. Subsequently the present report was written to provide more complete documentation of the study. The majority of



the presentation slides appear as figures of this report. However not all the slides of the PowerPoint presentations have been included in this report.

2. CONCLUSIONS AND RECOMMENDATIONS

This report documents the wellbore stability analyses of wells in the Bergermeer Field, based on in situ stress and strength data that were derived from logs. The following wells were analyzed: BGM1, BGM2, BGM3, BGM4, BGM7, and BGM8. The results obtained based on the logs of these wells were consistent and cover the entire field. The trajectories of two proposed wells were also analyzed: the horizontal well H35 based on the logs of BGM7 and the S-shaped V23 well based on the logs of BGM2. The calculations were performed using the wellbore stability software developed by PCM Technical, Inc.

Based on these analyses we reached the following conclusions and recommendations:

Conclusions 1. The analysis reports of the fracture tests of BGM8 and BGM4 wells were critically reviewed,

and found to be very good for use in the WBS analysis. 2. The logs of BGM8 and BGM4 were used to estimate the in situ stress profiles for these wells

and calibrate them to the fracture test data provided and the reported horizontal stress anisotropy, SH_max/Sh_min = 1.1. The calibration gave the tectonic strains (see table 4.2), which characterize the Bergermeer Field, and were used to derive the in situ stress-profiles of all wells.

3. The Formel-Dt model was used to estimate the UCS strength from the compressive wave slowness sonic log. The UCS-profile agrees very well with the one used by the FEED study for the BGM1 well, which was also validated by lab tests at the University of Delft. The UCS then was used to predict the failure envelope of the rock using correlations within the PCM stability model.

4. The wellbore stability assessment of BGM1, BGM2, BG3, BGM4, BGM7 and BGM8 wells, at reservoir depletion when the wells were drilled, predicted drilling instability events that were observed both in the Zechstein (below the Platten dolomite) and in the Rotliegendes when the wells were drilled. This validates the in situ-stress and strength modeling used in the WBS analyses.

5. The Zechstein formation is predicted to have a weak UCS-interval (1200 to 2000 psi) below the Platten dolomite and has a safe mud-weight range only for well inclinations less than ~ 45 degrees and is unstable for higher inclinations (e.g. the H35 trajectory, with inclination 58 to 90 degrees, is predicted to be unstable in the Zechstein).

6. The weakest part of the Rotliegendes (UCS = ~ 1436 psi) is unstable for any reservoir pressure for well inclinations greater than ~ 20 degrees (e.g. the BGM2 trajectory with ~30-degree inclination in the Rotliegendes had stability problems).

7. The horizontal section of the H35 well, in the upper part of the Weissliegendes (the strongest part of the Rotliegendes, UCS = 4830 psi) has a safe mud-weight of 9 to 10 ppg for all reservoir depletions.

8. Wells with near-vertical trajectories within the Zechstein and the Rotliegendes (e.g. the proposed V23 well trajectory) are the most stable for drilling.

9. Well azimuth of horizontal or inclined wells does not affect significantly the safe mud-weight window (~ 0.5 ppg difference between trajectory along SH_max and Sh_min). This is because of the relative magnitudes of the horizontal and the overburden stresses in the Bergermeer Field.

10. Production and injection stability analyses of the weakest layers in the Rotligendes showed that well trajectories of more than ~ 10 degrees are unstable (i.e. shear failure occurs around



the wellbore) and may produce solids. Near vertical wells (of less than 5-degree inclination) are stable at any reservoir pressure for production and injection provided that the injection pressure does not exceed the reservoir pressure by ~ 250 psi.

11. The horizontal section of the H35 well in the Weissliegendes (estimated UCS is 4830 psi at 7216 ft TVD) is predicted to fail during production operations for reservoir pressure less than ~ 800 psi (55 bar).

Recommendations 1. Specific WBS analyses are recommended, accounting for the well trajectory and the depletion

conditions, to select safe wellbore pressure ranges for drilling and production. 2. For new wells, use well trajectories with nearly vertical section in the Rotliegendes (less than 5

degrees) and limit the inclination to less than 45 degrees in the Zechstein (below the Platten dolomite).

3. METHODOLOGY

PCM Technical, Inc. has developed software for wellbore stability based on the elastic solution for the stress field induced by the wellbore (see for example reference 4). The formation stress next to the wellbore is compared to the Failure Envelope (FE) of the formation to determine whether shear failure has occurred (Figure 3.1). In this plot, the formation stress and the FE are expressed in terms of the stress invariants J1 (normal stress) and

2J (shear stress). When a stress state (J1, 2J ) plots below the failure envelope, the formation

has not failed, and conversely when it plots above the failure envelope the formation has failed. The stress next to the wellbore depends on the mud-weight used, the in situ stresses, wellbore

inclination, and azimuth. When varying the mud-weight, the stress state (J1, 2J ) plots on a

trajectory, which (in general) intersects the FE at the shear failure and the fracturing points. These points are the extremes of the “safe mud-window” (wellbore pressure or mud-weight) for stable wellbore while drilling. It is generally assumed that formation failure would cause wellbore problems (loss of returns or hole collapse) and should be avoided for safe drilling or production operations. However, formation failure in many cases does not cause catastrophic wellbore problems. The use of drilling mud aims at preventing loss of returns by forming a “thin and flexible” filter-cake, which stabilizes the hole. Weak ductile formations do not collapse when they fail but gradually deform, a problem that can be successfully addressed during the limited time they may by exposed, until installing a casing, gravel-pack, etc. The FE of the formations drilled in the Bergermeer Field is assumed to be given as a function of the UCS strength (curvilinear failure envelope published by J. Zhang et al., ref. 6).

Stress Invariants

The severity of a given loading on some material point can be quantified by the invariants of the induced stress tensor (stress components). The stress invariants are independent of the coordinate system used for the calculations and allow the comparison of stress states of different stress components. The inference is made here, that stress states of the same stress invariants, although they are different, they are equally severe on the material from the point of view of failure. It is, therefore, possible to compare a given state of stress with known stress states that cause material failure, which can be realized in the laboratory.



Such failure stress states can be determined by performing simple experiments such as the pure tension, the compression, the TWC (thick wall cylinder), and the Brazilian tests. The results of these failure tests may be reported as a curve (failure envelope), which relates the normal and shear stresses that must exist at some point in the material to cause failure. The failure envelope can be determined as the osculating line to the Mohr’s circles of all failure stress states. An alternative way is to report the failure envelope as a functional relation between the first invariant,

J1, and the square root of the second invariant, 2J , of the stress tensor. Both methods are

equivalent, since the Mohr’s circles are also invariant with respect to coordinate transformations.

The invariants J1 and 2J are related to the normal and the shear stresses on the octahedral

planes, i.e., the planes that make the same angle with the principal directions of the stress tensor. The definitions used herein for the first and second invariants of the stress tensor, σij, where i, j = 1, 2, 3 are:

231

223

212

21133

23322

222112

3322111

6/])()()[(

3/)(

σσσσσσσσσσσσ

+++−+−+−=

++=

J

J

Figure 3.1: Failure Plot in Invariant Stress Space

Decreasing Mud Weight

Increasing Mud Weight

Effective Wellbore Stress

(J 2)1/2 Shear Failure

Fracturing

CompressionTension

Failure Envelope

Normal Stress J1

Shear Stress

Decreasing Mud Weight

Increasing Mud Weight

Effective Wellbore Stress

(J 2)1/2 Shear Failure

Fracturing

CompressionTension

Failure Envelope

Normal Stress J1

Shear Stress

Analysis Steps

The objective of the present WBS analysis is to determine the safe mud-weight “window” (range) for drilling (and production). The analysis uses software that calculates the first and second

invariants (J1 and 2J ) of the effective stresses near the wellbore for different mud-weights and

compares it with a curvilinear failure envelope to determine the safe mud-weight range. The methodology used for wellbore stability may be divided into the following steps:

1. Determine the poro-elastic properties, the in situ stresses and formation pressure using formation logs and calibrate with leak-off tests, MDT tests and other direct field measurements.

2. Determine the log-derived formation strength, i.e. the unconfined compressive strength (UCS) of the layers penetrated by the well and compare with laboratory measurements for calibration.



3. Using the UCS from logs as a parameter, define the curvilinear failure envelope based on the non-dimensional failure envelope for weak sands published by J. Zhang et al.

4. Run the Wellbore Stability software (developed by PCM Technical Inc.) to derive a log of the minimum and maximum mud-weight that do not cause failure.

5. Compare the WBS results with the field observations from drilling the existing wells (see ref. 7) and modify the in situ stresses and/or strength, if necessary.

6. Using the calibrated in situ stress and strength profiles, run the WBS software to predict the stability of inclined wells or well drilled later under depleted reservoir conditions.



4. IN SITU STRESS AND STRENGTH PROFILES

The min and max in situ stresses (Sh and SH) in each well are determined from the overburden stress, Sv, the pore pressure, Po, and the elastic properties of the formation (E, ν). The basic parameters common for a block or a field are the tectonic strains εεεεh and εεεεH along the min and max stress (positive when compressive). These are used to match the available stress data from LOTs, minifracs, etc. An additional correction (Tcor) may sometimes be used which was not used in this study. The principal horizontal stresses are given as:

.)(11

21

1 2Tcor

EPSS HhoVh ++

−+

−−×+

−= νεε

νννα

νν

Eq. (4.1)

TcorE

PSS hHoVH ++−

+

−−×+

−= )(

11

21

1 2νεε

νννα

νν

Eq. (4.2)

In these equations, εh and εH are tectonic strains and Tcor is the tectonic stress correction (an ad hoc correction parameter to account for stress differences between the observations and the predictions). The poroelastic parameter α is Biot’s constant given by:

grKE

/63

1

−−=

να Eq. (4.3)

Biot’s constant “α” is determined from the static properties of ν (Poisson’s ratio), E (Young’s modulus), and Kgr (the rock grain bulk modulus, the value of 5.62E06 psi for quartz is usually used). In the above derivations the static values of E and ν are used which are determined from the dynamic values (using a proprietary correlation derived from published data of various lithologies). For the Bergermeer wells, only the compressive wave travel time Dtc was provided. Because no shear wave travel time DTs was available, the Poisson’s ratio could not be derived from the sonic logs, and it necessary to estimate it from a correlation to the gamma-ray log. Then the shear wave travel time DTs was estimated from Dtc and the Poisson’s ratio. The dynamic values E and ν were then determined using well-known relations from Dtc, Dts and density logs. The overburden stress, Sv, and the initial pore pressure, Po, were obtained based on data from previous studies (ref 1 and 2) and were expressed by linear equations in terms of true vertical depth (TVD) by:

( )TVDftpsiSv /00.1= for all wells Eq. (4.4)

( )TVDftpsiPo /47.0= for all wells Eq. (4.5)

Currently the reservoir pressure is depleted in the Rotliegendes reservoir. To account for this a constant depletion term “dpl” was subtracted from Po when TVD is in the Rotliegendes i.e.:

( ) )(/47.0 esRotliegenddplTVDftpsiP −= for all wells Eq. (4.6)



Calibration of in situ stress profile

The equations above and the logs of the BGM8 and BGM4 wells were used to derive the in situ stress profiles given in Figures 4.1 and 4.2 respectively. These profiles were calibrated to the fracture-test data of BGM8 and BGM4 wells (ref. 5) given in Table 4.1. Furthermore they were also constrained to give a value of 1.1 for the ratio of the average max stress (green line) over the min stress (red line). This stress anisotropy ratio was estimated by a previous study (ref. 3). Table 4.1 Fracture-test data for BGM8 and BGM4 well s (ref. 5)

The stress calibration was done by changing the tectonic strains in Eqns 4.1 and 4.2 (assuming Tcor = 0), until approximate predictions of the in situ stresses measured by the fracture tests of BGM8 and BGM4 were achieved. Table 4.2 shows that matching results Table 4.2 Matching parameters for fracture-test dat a for BGM8 and BGM4

Well Overburden (psi) Tectonic strains, Tectonic Correction

Initial Pore Pressure (psi)

All wells in Bergermeer

Sob = 1.00xTVD (TVD in ft)

εh = 1.3E-04 εH = 2.4E-04 Tcor = 0 psi

Po = 0.47 xTVD (TVD in ft)

At the time of the fracture test in BGM8, the Rotliegendes formation was significantly depleted, while the BGM4 was at initial reservoir pressure (BGM4 was drilled in an un-depleted area just outside the reservoir). For this reason the minifrac test for BGM8 does not plot near the minimum in situ stress line (red line, Sh) in Figure 4.1, while the one for BGM4 does (Figure 4.2). Figure 4.3 shows the predicted stress profile at the time of the minifrac test of BGM8, when the reservoir pressure was ~ 10 bar in the Rotliegendes formation. The figure shows that the predicted minimum in situ stress approximates well the minifrac test. The matching of the minifrac tests is also shown in Figures 4.4 and 4.5 for the BGM8 and BGM4 wells respectively in expanded scale. The minifrac test result represents an average of the minimum in situ stress over an interval where the minifrac test fracture has propagated, and this could cover an interval of several tens of feet. The figures show that the stress predictions were reasonably uniform around the minifrac initiation points, and therefore the prediction results for Sh (red line) are in good agreement with the tests. Additional confirmation of the validity of the in situ stress predictions is provided though the wellbore stability analyses of the existing wells, by comparing of the stability predictions with the actual field observations while drilling the wells. This is presented in subsequent sections, where the WBS results from individual wells are discussed. However the WBS predictions also depend on the strength model used. In conclusion, the WBS analyses support the in situ stress modeling presented in this section.



Figure 4.1 In situ stress profile for BGM8 well at initial reservoir pressure

In s itu S tre s s e s for Be rge rme e r 8 We ll

0

1000

2000

3000

4000

5000

6000

7000

8000

1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300

TVDrkb (m)

(psi

)

Res er. Pre s . (ps i) Min. Hor. Stre s s Sh (ps i) Max. Hor. S tre s s SH (ps i) Ove rburden Stre s s Sv (ps i)

Zechs te in Pla tten Dol Weis s liegendes Rotliegendes

BGM8-Sh (ps i) BGM8-Pr (ps i) BGM4-Sh (ps i) BGM4-Pr (ps i)

Figure 4.2 In situ stress profile for BGM4 well at initial reservoir pressure


0

1000

2000

3000

4000

5000

6000

7000

8000

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

TVDrkb (m)

(psi

)

Res er. Pre s . (ps i) Min. Hor. Stre s s Sh (ps i) Max. Hor. S tre s s SH (ps i) Overburden Stre s s Sv (ps i)

Zechs te in Pla tten Dol Weis s liegendes Rotliegendes




Figure 4.3 In situ stress profile for BGM8 well at the time of the fracture test (Rotliegendes Pr = ~10 bar = ~150 psi)

In s itu Stre s s e s for Be rge rme e r 8 We ll at pre s e nt de ple tion of ~3050 ps i

0

1000

2000

3000

4000

5000

6000

7000

8000

1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300

TVDrkb (m)

(psi

)

Res er. Pres . (ps i) Min. Hor. Stres s Sh (ps i) Max. Hor. Stres s SH (ps i) Overburden Stres s Sv (ps i)

Zechs te in Platten Dol Weis s liegendes Rotliegendes


Figure 4.4 In situ stress profile for BGM8 well at the time of the fracture test (Rotliegendes Pr = ~10 bar = ~150 psi)-- Expanded scale

In s itu S tre s s e s fo r B e rg e rme e r 8 We ll at p re s e nt d e ple tio n o f ~ 3050 p s i

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

8 0 0 0

2 0 5 0 2 0 5 5 2 0 6 0 2 0 6 5 2 0 7 0 2 0 7 5 2 0 8 0 2 0 8 5 2 0 9 0 2 0 9 5 2 1 0 0

TVDrkb (m )

(psi

)

Re s e r . Pre s . (ps i) Min . Hor. S tre s s S h (ps i) Ma x . Hor. S tre s s S H (ps i) Ove rburde n S tre s s S v (ps i)

Ze c hs te in Pla tte n Dol We is s lie ge nde s Rotlie ge nde s

BGM8-S h (ps i) BGM8-Pr (ps i) BGM4-S h (ps i) BGM4-Pr (ps i)

Sh-min of the fracture test is close to the in situ stress profile

The pore pressure is depleted and matches the fracture test

In s itu S tre s s e s fo r B e rg e rme e r 8 We ll at p re s e nt d e ple tio n o f ~ 3050 p s i

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

8 0 0 0

2 0 5 0 2 0 5 5 2 0 6 0 2 0 6 5 2 0 7 0 2 0 7 5 2 0 8 0 2 0 8 5 2 0 9 0 2 0 9 5 2 1 0 0

TVDrkb (m )

(psi

)

Re s e r . Pre s . (ps i) Min . Hor. S tre s s S h (ps i) Ma x . Hor. S tre s s S H (ps i) Ove rburde n S tre s s S v (ps i)



Sh-min of the fracture test is close to the in situ stress profile

The pore pressure is depleted and matches the fracture test



Figure 4.5 In situ stress profile for BGM4 well at initial reservoir pressure -- Expanded scale

In s itu S tre s s e s fo r B e rg e rme e r 4 We ll

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

8 0 0 0

2 1 0 0 2 1 0 5 2 1 1 0 2 1 1 5 2 1 2 0 2 1 2 5 2 1 3 0 2 1 3 5 2 1 4 0 2 1 4 5 2 1 5 0

TVDrkb (m )

(psi

)

Re s e r. Pre s . (ps i) Min. Hor. S tre s s S h (ps i) Ma x. Hor. S tre s s S H (ps i) Ove rburde n S tre s s S v (ps i)



The fracture test point falls near the Sh (red line)

The fracture test was done at initial reservoir pressure (well is in an undepleted block of the reservoir)


0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

8 0 0 0

2 1 0 0 2 1 0 5 2 1 1 0 2 1 1 5 2 1 2 0 2 1 2 5 2 1 3 0 2 1 3 5 2 1 4 0 2 1 4 5 2 1 5 0

TVDrkb (m )

(psi

)

Re s e r. Pre s . (ps i) Min. Hor. S tre s s S h (ps i) Ma x. Hor. S tre s s S H (ps i) Ove rburde n S tre s s S v (ps i)



The fracture test point falls near the Sh (red line)

The fracture test was done at initial reservoir pressure (well is in an undepleted block of the reservoir)

In situ strength profile

The WBS model uses the entire failure envelope (FE) of the formation for stability prediction. The

FE is expressed as a 2nd degree polynomial in the (J1 and 2J ) stress invariant space, where the

coefficients of the polynomial are given as functions of the unconfined compressive strength (UCS) of the formation. Therefore prediction of the UCS strength log is a fundamental input for the wellbore stability predictions. Figure 4.6 shows the UCS strength profile for BGM1 well that was used in the FEED study (ref. 1, 2) to predict stability. In the same study, several strength models were compared, among which were the Sarda, the Formel-phi and the Formel –Dt models (ref. 2). The formulation of the Formel-Dt model was provided as follows: UCS = 140 -2.1 Dtc + 0.0083 Dtc2 (4.7) where, the uniaxial strength UCS is given in MPa and the compressive time slowness Dtc in µsec/ft. The model is valid for a sonic transit time of 90 to 140 µsec/ft. Figure 4.7 shows the Formel-Dt model prediction for BGM1 well. As can be seen from the comparison of figures 4.6 and 4.7, the Formel-Dt model predicts similarly with the model used in the FEED study, although the minimum UCS strength of about 1436 psi at 2200 m TVD in the Rotliegendes is slightly higher than ~1200 psi as shown in Figure 4.6. This is probably because the FEED study strength model is a combination of the models used in the study. Our own methodology of predicting UCS based on Dtc and GR, predicted similarly to the Formel-Dt model for the intervals of low strength, but predicted high strength in other areas similarly to the Sarda model. However the higher range of the predictions were thought to be too high and it was decided to use the Formel-Dt model in all WBS analyses in this report, as it provided realistic strength predictions and was in reasonable agreement with strength ranges from lab tests



performed at the University of Delft. Figure 4.6 UCS prediction for BGM1 well used in the FEED study (Ref. 1, 2)



Figure 4.7 UCS prediction for BGM1 well using the F ormel-Dt model

B GM1 W ell U C S streng th

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 2280 2300

TV D rkb (m )

(psi

)

UCS _Form el_Dt (ps i) Zec hs tein P lat ten Dol W eis s liegendes Rot liegendes

Figures 4.8, 4.9, 4.10 and 4.11 show the strength profiles calculated for the GBM1, BGM8, BGM4 and BGM2 wells respectively, for the entire ranges of the Dtc logs that were provided. The figures show very well behaved strength predictions based on the compressive wave travel time. The flat line of figure 4.9 for BGM8 was due to lack of sonic data for that interval. The following observations can be made:

• The minimum strength in the Rotliegendes is least for the GBM1 well (~ 1436 psi) as compared with ~ 2000 psi for the other wells.

• The Weissliegendes (i.e., the upper part of the Rotliegendes) has higher UCS strength than ~ 2000 psi for all wells.

• The Zechstein has a low strength within a 100m-interval below the Platten dolomite where as low UCS as 1000 psi was predicted.

• Weak intervals at shallow depths (1200 to 1600 m TVD) were also predicted for BGM1 and BGM 2 wells

The Formel-Dt model appears to give realistic estimates of the UCS strength and is supported by the WBS results, as will be documented in subsequent sections. In particular, for several wells there were drilling events (stuck pipe) that occurred in the interval below the Platten dolomite, which support the low UCS predictions of the Formel-Dt model there.




B G M 1 W e ll U C S s tre n g th

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1 2 0 0 1 3 0 0 1 4 0 0 1 5 0 0 1 6 0 0 1 7 0 0 1 8 0 0 1 9 0 0 2 0 0 0 2 1 0 0 2 2 0 0 2 3 0 0

T V D rk b (m )

(psi

)

U C S _ F o rm e l_ D t (p s i) Ze c h s te in P la t t e n D o l W e is s lie g e n d e s R o t l ie g e n d e s



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1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300

T V D rkb (m )

(psi

)

U C S _F orm el_D t (ps i) Zec hs te in P la t ten D o l W e is s liegendes R ot liegendes



Figure 4.10 UCS prediction for BGM4 well using the Formel-Dt model

B G M 4 W e ll U C S E s tim a te

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1 5 5 0 1 6 5 0 1 7 5 0 1 8 5 0 1 9 5 0 2 0 5 0 2 1 5 0 2 2 5 0 2 3 5 0

T V D rk b (m )

(psi

)

U C S _ F o rm e l_ D t (p s i) Ze c h s te in P la t t e n D o l W e is s l ie g e n d e s R o t lie g e n d e s

Figure 4.11 UCS prediction for BGM2 well using the Formel-Dt model

B G M 2 W e ll U C S s tr e n g th

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T V D r k b (m )

(psi

)

U C S _ F o rm e l-D t (p s i ) Ze c h s t e in P la t t e n D o l

W e is s l ie g e n d e s R o t l ie g e n d e s

Model Validation

Both the in situ stress and the in situ strength profiles are necessary inputs for the prediction of wellbore stability. To validate the stability inputs, the wellbore stability of BGM8, BGM4 and BGM2



well trajectories was predicted at the reservoir conditions when the wells were drilled and the results were compared with the drilling events reported (ref 5). The in situ stress profiles when the wells were drilled were derived, accounting for the depletion of the Rotliegendes formation and were used with the strength (Figures 4.9, 4.10 and 4.11) profiles described above for the WBS analyses. Furthermore, it was assumed that a mud creating an efficient mud-cake was used, isolating the wellbore fluids from the reservoir (i.e., the reservoir pressure next to the wellbore = the far-field reservoir pressure). All stability calculations were performed for a formation element at the surface of the wellbore.

BGM8 well stability

The BGM8 well was a nearly vertical drilled when the reservoir pressure was ~ 100 bar (= 1450 psi). Figure 4.12 shows the in situ stress profile predicted and Figure 4.13 the safe-mud-weight-window (SMW) for drilling the actual trajectory of this well. The minimum (pink line) and maximum (red line) mud-weight curves define the window of safe operations. By convention in our software, when these curves simultaneously go to zero it means that there is no safe mud-weight window for stability, i.e., that the wellbore will be failed for any mud-weight. Figure 4.13 shows that the Rotliegendes can be drilled with high overbalance using ~ 3.5 to 8.5 ppg. Actually it was drilled with 8.75 to 9 ppg; this is certainly within the uncertainty of the analysis, and therefore this event supports the in situ stresses and UCS strength used. The figure also shows that the Zechstein could be drilled with 8.5 to 9.75 ppg. The Zechstein was actually drilled with about 8.5 – 9.25 ppg and with increased MW to 10.75 ppg at the Platten dolomite. This again is in agreement with the WBS predictions. Figure 4.12: Stresses in BGM8 well when it was init ially drilled at Pr = 100 bar =1450 psi in the Rotliegendes (depletion = 1750 psi)

In s itu S tre s s e s fo r B e rg e rme e r 8 We ll (whe n it was drille d)

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

8 0 0 0

1 6 5 0 1 7 0 0 1 7 5 0 1 8 0 0 1 8 5 0 1 9 0 0 1 9 5 0 2 0 0 0 2 0 5 0 2 1 0 0 2 1 5 0 2 2 0 0 2 2 5 0 2 3 0 0 2 3 5 0

MD rkb (m )

(psi

)

0

4 0

8 0

1 2 0

1 6 0

2 0 0

2 4 0

2 8 0

3 2 0

GR

(G

AP

I)

Ove rb u rd e n S tre s s S v (p s i) Min . Ho r. S tre s s S h (p s i) Ma x. Ho r. S tre s s S H (p s i)

R e s e r. P re s . (p s i) Ze ch s te in P la tte n Do l

We is s lie g e n d e s R o tlie g e n d e s GR (GAP I)



Figure 4.13: WBS of BGM8 well when it was initially drilled at Pr = 100 bar =1450 psi (depletion = 1750 psi) – actual trajectory

BGM8 WBS: Actual Trajectory, Reservoir P ressure when well was drilled @ Depletion 1750 psi (BRM8_wbs_trj-true_dpl1750_formel.dat)

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5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 7200 7400 7600

T VD (ft)

(ppg

)

Sh(ppg) m inMW(ppg) m axMW(ppg) SH m ax(ppg)

Sob(ppg) PP(ppg) Zechs tein Pla tten D o l

R otliegendes Weis s liegendes

NOTE: The minimum horizontal stress (Sh) curve is “hidden” by the maxMW line (red line), since for most cases maxMW = Sh to prevent fracturing. However, in some cases maxMW < Sh, when the formations fails at a lower mud-weight in shear.

BGM4 well stability

The BGM4 is a deviated well drilled in an adjacent block to the SE of the reservoir in an un-depleted area where the reservoir pressure is close to normal (Pr = 242 bar = 3500 psi in the Weissliegendes). Figure 4.2 shows the in situ stress profile predicted and Figure 4.14 the safe-mud-weight-window for drilling the actual trajectory of this well. The BGM4 well trajectory drops in inclination from 44 to 17 deg as it goes from the Zechstein deeper in the Rotliegendes, and this improves stability. The WBS analysis shows that the entire well could be drilled balanced with ~9 ppg at the present (un-depleted) reservoir pressure. The well was drilled with 9 to 9.25 ppg above the Zechstein and with 10.5 graded to 11.25 ppg in the Zechstein and the Rotliegendes. These ranges support the predicted averaged maxMW of ~10.25 in the Zechstein and ~11 ppg in the Weissliegendes (upper part of Rotliegendes).




BGM4 WBS: Actual Trajectory, Normal Reservoir Pressure (no depletion) (BRM4_wbs_trj-true_dpl0_formel.dat)

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5100 5300 5500 5700 5900 6100 6300 6500 6700 6900 7100 7300 7500 7700

TVD (ft)

(ppg

)

Sh(ppg) m inMW(ppg) m axMW(ppg) SHm ax(ppg)

Sob(ppg) PP(ppg) Zechs tein Platten Dol

Rotliegendes Weissliegendes

BGM2 well stability

The BGM2 is a 26 to 30-degree deviated well drilled in the main (SE) block of the reservoir when the reservoir was not depleted (the reservoir pressure in close to normal Pr = 226 bar = 3280 psi in the Rotliegendes). Figure 4.15 shows the in situ stress profile predicted and Figure 4.16 the safe-mud-weight-window for drilling the actual trajectory of this well. The WBS analysis predicts that the Rotliegendes may drilled (with possible instability at ~7250 ft to 7350 ft TVD where the safe-mud-weight-window is very narrow) with ~ 10.25 to 11.25 ppg at virgin reservoir pressure. It was actually drilled with ~ 10.75 ppg. The WBS analysis predicts that the Zechstein could be drilled with ~9.5 to 10 ppg but it will have instability at 6700 to 6900 ft TVD, at the present (un-depleted) reservoir pressure. It was drilled with 10.25 to 10.5 ppg and stuck pipe below the Platten dolomite was reported (ref 5) in about the same interval. The above support the analysis and the validity of the in situ stress profile.

Conclusions of model validation

In conclusion the WBS analyses of the actual trajectories of BGM8, BGM4 and BGM2 wells predicted mud-weight ranges and drilling events in agreement with the mud weights used and the drilling events observed during drilling of these wells. Thus, the in situ stress and in situ strength profiles result in realistic wellbore stability predictions and therefore are used for all the analyses of this report. Additional validation will be provided as more wells are analyzed in subsequent sections. Specifically:



1. That BGM8 analysis predicted that when the well was drilled and the Rotliegendes reservoir pressure was 1450 psi =~100 bar (depletion 1750 psi) the safe mud-weight window was ~ 3.5 to 8.5 ppg. Actually it was drilled with 8.75 to 9 ppg, which is close to the top range and validates the in situ stress profile.

2. Also the Zechstein of BGM8 (at initial reservoir pressure) could be drilled with 8.5 to 9.75 ppg. The Zechstein was actually drilled with about 8.5 – 9.25 ppg and increased MW to 10.75 ppg at the Platten dolomite. String became stuck while POH to log.

3. The BGM4 well trajectory drops in inclination from 44 to 17 deg as it goes from the Zechstein deeper in the Rotliegendes and this improves stability. The WBS analysis shows that the Rotliegendes (and the entire well) could be drilled balanced with ~9 ppg at the present (un-depleted) reservoir pressure. The well was drilled with 9 to 9.25 ppg above the Zechstein and with 10.5 graded to 11.25 ppg in the Zechstein and the Rotliegendes. This range is also supported by the maxMW values predicted

4. The BGM2 is a 26-30 degree deviated well. The WBS analysis predicts that the Rotliegendes may drilled with difficulty at ~7250 ft to 7350 ft TVD with ~ 10.25 to 11.25 ppg at virgin reservoir pressure. It was actually drilled with ~ 10.75 ppg. The WBS analysis predicts that the Zechstein could be drilled with ~9.5 to 10 ppg but it will have instability at 6700 to 6900 ft TVD, at the present (un-depleted) reservoir pressure. It was drilled with 10.25 to 10.5 ppg and the Bergermeer summaries (ref 5) indicate stuck pipe below the Platten dolomite in about the same interval. The above support the analysis and the validity of the in situ stress profile.



Figure 4.15: Stresses in BGM2 well when it was init ially drilled at Pr = 226 bar =3280 psi in the Rotliegendes (depletion = 0 psi)

In s itu S tre s s e s fo r Be rg e rme e r 2 We ll

0

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1 75 0 1 80 0 18 5 0 19 0 0 19 5 0 2 0 00 2 05 0 2 10 0 2 15 0 22 0 0 22 5 0 23 0 0 2 3 50 2 4 00 2 45 0 2 50 0 25 5 0 26 0 0 26 5 0 2 7 00 2 7 50

MDrkb (m )

(psi

)

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32 0

GR

(G

AP

I)

Ove rb urd e n S tre s s S v (ps i) Min. Ho r. S tre s s S h (p s i) Ma x. Ho r. S tre s s S H (p s i)

R e s e r. P re s . (p s i) Ze ch s te in P la tte n Dol

We is s lie g e nd e s R o tlie ge n de s GR (GAP I)


BGM2 W BS: Actual Trajectory, Normal Reservoir Pressure (no depletion) (BRM2_wbs_trj-true_dpl0_formel.dat)

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TVD (ft)

(ppg

)


Sob(ppg) PP(ppg) Zechs te in Platten Dol

Rotliegendes Weis s liegendes



5. WBS ANALYSIS OF BGM8

Additional wellbore stability runs were made for various well scenarios: 1. BGM8 at current reservoir pressure Pr =150 psi = 10 bar (depletion 3050 psi) 2. BGM8 at virgin reservoir pressure ~ 3200 psi (depletion 0 psi) 3. Safe-mud-weight vs. reservoir pressure 4. Safe-mud-weight vs. well inclination 5. BGM8 drilled at Pr = 1880 psi = 130 bar (depletion 1320 psi) 6. BGM8 drilled at Pr = 1300 psi = 90 bar (depletion 1900 psi) 7. BGM8 drilled at Pr = 720 psi = 50 bar (depletion 2480

If the BGM8 well trajectory is drilled at present time when the Rotliegendes is severely depleted (at a reservoir pressure of ~ 10 bar), Figures 5.1 and 5.2 show the in situ stress profile and the corresponding safe-mud-weight-window for this case. Figure 5.2 shows that the Zechstein can be drilled with~ 9 to 9.75 ppg fluid and the Rotliegendes can be drilled using ~ 0.5 to 6 ppg with no reservoir failure and therefore no stability problems. Figure 5.3 shows how the safe-mud-weight-window varies with reservoir pressure for the weakest depth of UCS = ~1899 psi at TVD=~7156 TVD in BGM8 vertical well. The plot shoes that the safe-mud-weight-window opens slightly with reservoir depletion in the Rotliegendes which can be drilled both over-balanced and under-balanced even for severe reservoir depletions, provided that the mud-weight is reduced corresponding to the reservoir pressure. Figures 5.4 and 5.5 show the safe-mud-weight-window dependence on wellbore inclination, for two depletion scenarios, when the well was drilled and at present time, respectively. The inclined well is assumed to by drilled in the direction of SH_max, i.e., approximately parallel to the normal fault (Fig. 1.1), which is the direction of future deviated well drilling. The plots show that the safe-mud-weight-window closes completely at ~ 27 deg and 25 deg well inclination for the two cases respectively, and therefore the analysis predicts instability at any mud-weight for wells of higher than about 25 degrees inclination. Figures 5.6, 5.8 and 5.10 show the in situ stress profiles at future reservoir pressure in the Rotliegendes of 130, 90 and 50 bar, respectively, and Figures 5.7, 5.9, and 5.11 the corresponding WBS plots, which show that the Rotliegendes can be drilled with approximately 5 to 9, 3 to 8, and 1.75 to 7.00 ppg respectively for this nearly vertical well.

Conclusions of BGM8 WBS analysis

1. The Zechstein (at initial reservoir pressure) could be drilled with 8.5 to 9.75 ppg. 2. At present reservoir pressure of ~ 150 psi = ~ 10 bar (depletion 3050 psi), the

Rotliegendes can still be drilled with 0.5 to 6 ppg using the actual near vertical well trajectory.

3. At virgin reservoir pressure of ~ 3200 psi = ~ 221 bar (depletion 0 psi), the Rotliegendes can still be drilled with 8.5 to 11.5 ppg. Therefore a nearly vertical well could be drilled both is the Zechstein and the Rotliegendes balanced with ~ 9 ppg.

4. Both the upper and the lower limits of the safe MW decrease nearly linearly with reservoir pressure, and the range opens with depletion. Therefore, near vertical wells like the BGM8 can be drilled in the Rotliegendes even for severe depletions, provided that sufficiently light mud-weight fluids are used.

5. The same is true for small inclination wells of 0 to ~ 25 degrees. However for inclined wells of higher inclination than 25 degrees there is no safe mid-weight window and drilling of such wells is expected to cause formation failure at any mud weight (and possible stability problems). This was based on a min UCS strength of ~1800 psi. The inclination



of the wells should even be lower if UCS is smaller, e.g., for UCS = 1300 psi the max well inclination should be ~ 15 degrees.

6. At future reservoir pressure of 130, 90 and 50 bar, the Rotliegendes can be drilled with approximately 5-9, 3-8, and 1.75-7 ppg respectively for this nearly vertical well.

Figure 5.1: Stresses in BGM8 at present reservoir p ressure, Pr = 155 psi = ~10 bar (depl. = 3050 psi) in Rotliegendes

In s itu S tre s s e s fo r B e rg e rme e r 8 We ll at pre s e nt de ple tio n o f ~ 3050 ps i

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50 0 0

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70 0 0

80 0 0

1 65 0 1 70 0 17 5 0 1 8 00 1 8 50 1 90 0 19 5 0 20 0 0 20 5 0 2 10 0 2 15 0 2 20 0 22 5 0 2 3 00 2 3 50

MD rkb (m )

(psi

)

0

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80

12 0

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28 0

32 0

GR

(G

AP

I)

Ove rb u rd e n S tre s s S v (p s i) Min . Ho r. S tre s s S h (p s i) Ma x. Hor. S tre s s S H (p s i)

R e s e r. P re s . (ps i) Ze chs te in P la tte n Do l

We is s lie g e n d e s R otlie g e nd e s GR (GAP I)



Figure 5.2: WBS in BGM8 at present reservoir pressu re, Pr = 155 psi = ~10 bar (depl. = 3050 psi) in Rotliegendes—actual trajectory

BGM8 WBS: Actual Trajectory, Reservoir Pressure @ present depletion 3050 psi (BRM8_wbs_trj-true_dpl3050_formel.dat)

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TVD (ft)

(ppg

)

Sh(ppg) minMW(ppg) maxMW(ppg) SHmax(ppg)

Sob(ppg) PP(ppg) Zechstein Platten Dol


Figure 5.3: WBS of BGM8 vs. reservoir pressure, wea kest depth in the Rotliegendes – vertical



well

S a fe m ud-w e ight vs. Re se rvoir P re ssure in Rotlige nd e s BGM8TV D=7156.39 ft, UCS = 1799 psi, w e ll de via tion= 0 d e g, w e ll a z im uth=any

(BGM8_trg_W D0_W Aa ny_TVD7156_form e l.da t)

0123456789

10111213141516

0 500 1000 1500 2000 2500 3000 3500

Reservo ir Pressure (psi)

(ppg

)

max_Pw (ppg) m in_P w (ppg) PP(ppg) Sh(ppg)



Figure 5.4: WBS of BGM8 (when it was drilled, deple tion = 1750 psi) vs. well-inclination Weakest UCS in Rotliegendes and well azimuth along SH_max

Safe mud-w e ight vs. w e ll inclina tion in Rotligendes BGM8 TVD=7156 ft, UCS = 1799 psi, w e ll azimuth=135 deg (a long SH_max), depletion

1750 psi (BGM8_dev_W A135_dpl1750_TVD7156_formel.da t )

0123456789

10111213141516

0 10 20 30 40 50 60 70 80 90

Wellbore Inclination (degrees from vertical)

(ppg

)

maxMW (ppg) minMW (ppg) PP(ppg) Sh(ppg)

Figure 5.5: WBS of BGM8 (at present depletion = 305 0 psi) vs. inclination Weakest UCS in Rotliegendes and well azimuth along SH_max

Sa fe m ud-w e ight vs. w e ll inclina tion in Rotligendes BGM8 TVD=7156 ft, UCS = 1799 psi, w e ll azimuth=135 deg (a long SH_m ax), deple tion

3050 psi (BGM8_dev_W A135_dpl3050_TVD7156_forme l.da t )

0123456789

10111213141516

0 10 20 30 40 50 60 70 80 90


(ppg

)




Figure 5.6: Stresses in BGM8 well at Pr = 1880 psi =130 bar (depletion = 1320 psi) in Rotliegendes


0

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20 0 0

30 0 0

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5 2 0 0 54 0 0 5 60 0 5 8 00 60 0 0 6 20 0 6 4 00 6 6 0 0 68 0 0 7 0 00 7 2 0 0 74 0 0 7 60 0

TVDrkb (ft)

(psi

)

Re s er. Pre s . (ps i) Ma x. Hor. S tre s s SH (ps i) Min. Hor. Stre s s S h (ps i) Ove rburde n Stre s s S v (ps i)

Ze c hs te in Pla tte n Dol We is s liege nde s Rotlie ge ndes

BGM8-S h (ps i) BGM8-Pr (ps i) BGM4-Sh (ps i) BGM4-Pr (ps i)

Figure 5.7: WBS of BGM8 at Pr = 1880 psi =130 bar ( depletion = 1320 psi) in Rotliegendes– actual trajectory-


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(ppg

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Rotliegendes Weiss liegendes





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MD rkb (m )

(psi

)

0

4 0

8 0

1 2 0

1 6 0

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2 4 0

2 8 0

3 2 0

GR

(G

AP

I)

Ove rb u rd e n S tre s s S v (p s i) Min . Ho r. S tre s s S h (p s i) Ma x. H o r. S tre s s S H (p s i)



Figure 5.9: WBS of BGM8 well at Pr = 1300 psi = 90 bar (depletion = 1900 psi) in Rotliegendes– actual trajectory

BGM8 W BS: Actual Trajectory, Reservoir Pressure @ present depletion 3050 psi (BRM8_wbs_trj-true_dpl1900_formel.dat)

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68

1012

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1618

2022

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TVD (ft)

(ppg

)








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50 0 0

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1 65 0 1 70 0 17 5 0 1 8 00 1 8 50 1 90 0 19 5 0 20 0 0 20 5 0 2 10 0 2 15 0 2 20 0 22 5 0 2 3 00 2 3 50

MD rkb (m )

(psi

)

0

40

80

12 0

16 0

20 0

24 0

28 0

32 0

GR

(G

AP

I)




Figure 5.11: WBS of BGM8 well at Pr = 720 psi =50 b ar (depletion = 2480 psi) in Rotliegendes– actual trajectory


02468

1012141618202224

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TVD (ft)

(ppg

)






6. WBS ANALYSIS OF BGM1

BGM1 well, the first well drilled in the Bergermeer Field is a vertical well, and therefore its stability analysis should give similar results as the nearly vertical BGM8 well, which was analyzed in a previous section. However, some of the WBS analysis results of this well are presented herein for comparison with similar cases of the FEED study (Ref. 2). The following runs were done for GBM1 well:

1. BGM1 actual trajectory, when Pr = ~3200 psi = 221 bar (depletion 0 psi) and when Pr =~150 psi = 10 bar (depletion 3050 psi)

2. Horizontal BGM1 well scenarios parallel to SH-max and SH-min, when Pr =~150 psi = 10 bar (depletion 3050 psi)

3. 45-degree inclined BGM1 well scenarios parallel to SH-max and SH-min, when Pr =~150 psi = 10 bar (depletion 3050 psi)

4. Safe-mud-weight vs. well inclination for several weak TVDs in the Zechstein (5013, 6332, 6878 ft TVD) at initial reservoir pressure and well drilled parallel to SH_max

5. Safe-mud-weight vs. well inclination for the weakest TVD in the Rotliegendes (7289 ft TVD) at initial (221 bar), present (10 bar) and future (50, 90 and 130 bar) and well drilled parallel to SH_max

6. Safe-mud-weight vs. horizontal well azimuth for the weakest TVD in the Rotliegendes (7289 ft TVD) at 130 bar

7. Safe-mud-weight vs. reservoir pressure for 7289 ft TVD When the BGM1 well was drilled, the Rotliegendes was at initial reservoir pressure (at a reservoir pressure of ~ 221 bar). Figures 6.1 and 6.2 show the in situ stress profile and the corresponding safe-mud-weight-window for this case. Figure 6.2 shows that the entire BGM1 well (including the Rotliegendes and the Zechstein) could be drilled balanced with about 9.5 ppg. When the well was drilled at virgin (un-depleted) reservoir pressure, no drilling problems were reported. This agrees well with the WBS predictions and further validates the in situ stress modeling and the Formel-Dt strength profile (see Fig. 4.8) used in this analysis. Figures 6.3 and 6.4 show that at the present depleted state, the Rotliegendes formation can be drilled with 0.5 to 6 ppg for this vertical well (similar results as for BGM8). However, inclined well scenarios are likely to cause formation shear failure, resulting in drilling instability problems. The WBS analyses considered also the azimuthal variation of the typical trajectories since the horizontal stresses are unequal (SH

> Sh). In general the azimuthal direction along the maximum horizontal stress is expected to be the most unstable for drilling stability in a normal stress regime (SV > SH > Sh). However, the WBS analyses for Bergermeer showed that the stability is not affected significantly by wellbore azimuth. This result, specific to Bergermeer, is due to the specific magnitudes of the in situ stresses. Figures 6.5, 6.6, 6.7 and 6.8 show the safe-mud-weight-window plots for 90-degree (horizontal) and 45-degree inclination wells at all depths, in the azimuthal direction of SH_max and Sh_min principal in horizontal stresses. The plots show that there are extensive intervals of instability both in the Zechstein and the Rotliegendes, where the safe-mud-weight-window is completely closed (where the maxMW = minMW = 0). Furthermore there is no safe MW-window at ~5000 ft TVD either for horizontal well trajectories. This is because instability increases with well inclination, with the horizontal wells being the most unstable in a normal stress regime (SV > SH

> Sh).





0

10 0 0

20 0 0

30 0 0

40 0 0

50 0 0

60 0 0

70 0 0

80 0 0

1 20 0 1 25 0 13 0 0 1 3 50 1 40 0 1 45 0 15 0 0 15 5 0 1 6 00 1 65 0 1 70 0 17 5 0 18 0 0 1 8 50 1 90 0 1 95 0 20 0 0 20 5 0 2 1 00 2 15 0 2 20 0 22 5 0 2 3 00 2 3 50

MD rkb (m )

(psi

)

0

40

80

12 0

16 0

20 0

24 0

28 0

32 0

GR

(G

AP

I)




Figure 6.2: WBS of BGM1 well at Pr = 3200 psi =221 bar (depletion = 0 psi) in Rotliegendes– actual trajectory

BGM1 W BS: Actual Trajectory, Normal Reservoir Pressure (no depletion) (BRM1_wbs_trj-true_dpl0_formel.dat)

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Figure 6.3: Stresses in BGM1 well at Pr = 150 psi = 10 bar (depletion = 3050 psi) in Rotliegendes


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Figure 6.4: WBS of BGM1 well at Pr = 150 psi =10 ba r (depletion = 3050 psi) in Rotliegendes– actual trajectory

BGM1 WBS: Actual Trajectory, Rotliegendes Pr = 150 psi ( depl = 3050 psi ) (BRM1_wbs_trj-true_dpl3050_formel.dat)

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Sob(ppg) PP(ppg) Zechstein Platten Dol




Figure 6.5: WBS of BGM1 well at Pr = 150 psi =10 ba r (depletion = 3050 psi) in Rotliegendes– Horizontal well at all depths with az imuth along SH_max

BGM1 WBS: Horizontal Well in the direction of Shmax at all depths (WD = 90 deg, WA = 135 deg), Rotliegendes Pr = 150 psi

(BRM1_wbs_WD90_WA135_dpl3050_formel.dat)

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Figure 6.6: WBS of BGM1 well at Pr = 150 psi =10 ba r (depletion = 3050 psi) in Rotliegendes– Horizontal well at all depths with az imuth along Sh_min

BGM1 WBS: Horizontal well in the direction of Sh_min at all depths (WD = 90 deg, WA = 45 deg), Rotliegendes Pr = 150 psi


02468

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Figure 6.7: WBS of BGM1 well at Pr = 150 psi =10 ba r (depletion = 3050 psi) in Rotliegendes– 45-degree inclination at all depths w ith azimuth along SH_max

BGM1 WBS: 45-deg inclined well in the direction of Shmax at all depths (WD = 45 deg, WA = 135 deg), Rotliegendes Pr = 150 psi


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Figure 6.8: WBS of BGM1 well at Pr = 150 psi =10 ba r (depletion = 3050 psi) in Rotliegendes– 45-degree inclination at all depths w ith azimuth along Sh_min

BGM1 WBS: 45-deg inclined well in the direction of Sh_min at all depths (WD = 45 deg, WA = 45 deg), Rotliegendes Pr = 150 psi


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To investigate further the limitations imposed by wellbore inclination, some representative unstable depths were selected from Figures 6.5 through 6.8, and the safe-mud-weight-window was plotted as a function of well inclination, assuming a well azimuth along the SH-max direction. The following depths were selected:

• BGM1 at TVD 5013 ft, UCS = 2601 psi, shallow point in the logged interval (see figure 6.9).

• BGM1 at TVD 6332 ft, UCS = 1047, the weakest depth of the entire well, at the middle part of the Zechstein (see figure 6.10).

• BGM1 at TVD 6878 ft, UCS = 7804, an unstable point in the Z1 carbonate of the Zechstein just above the Rotliegendes (see Figure 6.11).

• WBS of BGM1 at TVD 7289 ft, UCS – 1436 psi, the weakest depth of the Rotliegendes. For this depth, reservoir pressures 10, 50, 90, 130 and 221 bar were considered, (see figures 6.12, 6.13, 6.14, 6.15 and 6.16).

Figure 6.9 shows the plot for TVD 5013 ft. The reservoir pressure at this depth is normal (~9 ppg) and is not affected by Rotliegendes reservoir depletion. At this relatively weak interval of UCS ~ 2600 psi there is a safe mud-weight window 7.5 – 12 ppg for well inclinations 0 to 30 deg. This safe-MW window becomes smaller for higher inclinations 30 to 70 degrees and closes completely at ~ 79 deg. The analysis shows that there is little chance of instability at shallow depths (above the Zechstein) even for higher inclination wells (up to 70 degrees), if a mud of ~ 10 ppg is used. Figure 6.9: WBS of BGM1 at TVD 5013 ft (no depletio n) vs. well inclination Well azimuth along SH-max

Safe mud-weight vs. well inclination, shallow in BG M1 TVD=5013.76 ft, UCS = 2601 psi, well azimuth=135 d eg, depletion 0 psi

(BGM1_dev_WA135_dpl0_TVD5013_formel.dat)

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(ppg

)

maxMW(ppg) minMW(ppg) PP(ppg) Sh(ppg)



Figure 6.10 shows the plot for TVD 6332 ft. The reservoir pressure at this depth is normal (~9 ppg) and is not affected by Rotliegendes reservoir depletion. At this very weak interval of UCS ~ 1047 psi (weakest in the entire well) there is a safe mud-weight window 8.75 – 12 ppg for well inclinations 0 to 15 deg. This window becomes smaller for higher inclinations 15 to 50 degrees. The safe–mud-weight-window closes completely at ~ 51 deg well inclination. The analysis shows that there is difficulty of drilling the middle of the Zechstein with well inclination above ~ 50 degrees Figure 6.10: WBS of BGM1 at TVD 6332 ft (no depleti on) vs. well inclination weakest depth of the well (at the middle of the Zec hstein). Well azimuth along SH-max

Safe mud-weight vs. w ell inclination, a t middle of Zechste in in BGM1 TVD=6331.89 ft, UCS = 1047 psi, well azimuth=135 d eg, depletion 0 psi


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(ppg

)




Figure 6.11 shows the plot for TVD 6878 ft. The reservoir pressure at this depth is normal (~9 ppg) and is not affected by Rotliegendes reservoir depletion. At this strong interval of UCS ~ 7800 psi there is a safe-mud-weight-window 5.75 – 10 ppg for well inclinations 0 to 40 deg. This window becomes smaller for higher inclination 40 to 80 degrees. The safe MW-window closes completely at ~ 83 deg well inclination. The analysis shows that there is difficulty of drilling the Z1 carbonate with well inclination above ~ 50 degrees using balanced drilling (~ 9 ppg). Figure 6.11: WBS of BGM1 at TVD 6878 ft (no depleti on) vs. well inclination Z1 carbonate of the Zechstein above the Rotliegende s. Well azimuth along SH-max

Safe mud-weight vs. well inclination, at the Z1 car bonate of Zechstein in BGM1 TVD=6378.39 ft, UCS = 7804 psi, well azimuth=135 d eg, depletion 0 psi


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Figures 6.12, 6.13, 6.14, 6.15 and 6.16) show the WBS plot for TVD 7289 ft, UCS = 1436 psi, the weakest depth of the Rotliegendes, for reservoir pressures 10, 50, 90, 130 and 221 bar. The corresponding reservoir pressure gradients for at this depth are: ~1.0, 2.5, 4.0, 5.5, and 9 ppg, respectively. There is a safe-mud-weight-window for well inclinations 0 to about 7 to 9 degrees, which becomes smaller for higher inclinations and closes completely at ~ 19 to 23 deg depending on reservoir pressure. The analysis shows that there is difficulty of drilling wells of deviation higher than ~ 19 to 23 degrees in the Rotliegendes for any reservoir pressure. Figures 6.17 shows an example of the dependence of the safe-mud-weight-window on wellbore azimuth, for TVD 7289 ft. The plot shows that well azimuth has a small effect on the safe mud-weight window. Drilling parallel to the main fault (azimuth of 90 degrees with respect to Sh_min) has a small advantage allowing a lower mud weight by ~ 0.5 ppg.



Figure 6.12: WBS of BGM1 at TVD 7289 ft, Pr = ~10 b ar (depl = 3050 psi) vs. well inclination- Weakest depth of the Rotliegendes. Well azimuth alo ng SH-max

Safe mud-w eight vs. w e ll inclina tion, w eakest depth in Rotliegendes of BGM1 TVD=7288.97 ft, UCS = 1436 psi, w e ll azimuth=135 d eg, Pr = 10 ba r

(BGM1_dev_W A135_dpl3050c_TVD7289_forme l.dat)

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Safe mud-w eight vs. w ell inclina tion, w eakest depth in Rotliegendes of BGM1 TVD=7288.97 ft, UCS = 1436 psi, w ell azimuth=135 d eg, Pr = 50 bar

(BGM1_dev_W A135_dpl2480_TVD7289_formel.da t)

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Sa fe m ud-w e ight vs. w e ll inclina tion, w e a ke st de pth in Rotlie ge ndes of BGM1 TVD=7288.97 ft, UCS = 1436 psi, w e ll a z im uth=135 d e g, Pr = 90 ba r

(BGM1_de v_W A135_dpl1900_TVD7289_form e l.da t)

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Figure 6.15: WBS of BGM1 at TVD 7289 ft, Pr = ~130 bar (depl = 1320 psi) vs. well inclination- Weakest depth of the Rotliegendes. Wel l azimuth along SH-max



Safe mud-weight vs. well inclination, weakest depth in Rotliegendes of BGM1 TVD=7288.97 ft, UCS = 1436 psi, well azimuth=135 d eg, Pr = 130 bar


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Figure 6.16: WBS of BGM1 at TVD 7289 ft, Pr = ~221 bar (depl = 0 psi) vs. well inclination- Weakest depth of the Rotliegendes. Well azimuth alo ng SH-max

Safe mud-w eight vs. w e ll inclina tion, w eakest depth in Rotliegendes of BGM1 TVD=7288.97 ft, UCS = 1436 psi, w e ll az im uth=135 d eg, Pr = 221 ba r

(BGM1_dev_W A135_dpl0_TVD7289_form e l.da t)

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Figure 6.17: WBS of BGM1 at TVD 7289 ft, Pr = ~130 bar (depl = 1320 psi) vs. well-azimuth- Weakest depth of the Rotliegendes. Well deviation 8 degrees

Safe m ud-w e ight vs. w e ll az imuth, w eakest depth in Rotliegendes of BGM1 TVD=7288.97 ft, UCS = 1436 psi, w e ll devia tion =8 deg, Pr = 221 ba r

(BGM1_Az_W D8_dpl1320_TVD7289_form e l.da t)

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Wellbore az imuth (deg. from Sh_min)




Conclusions of BGM1 WBS analyses

1. The WBS analysis predicts that the entire BGM1 well (including the Rotliegendes and the Zechstein) could be drilled balanced with about 9.5 ppg at initial reservoir pressure. When the well was drilled at virgin (un-depleted) reservoir pressure, no drilling problems were reported. This agrees well with the WBS predictions.

2. The depleted Rotliegendes at the current reservoir pressure of ~150 psi = ~10 bar could be drilled with ~0.5 to 6 ppg for the near vertical BGM1 trajectory.

3. WBS analysis of horizontal wells shows that there are several instability interval is the Zechstein (at Initial reservoir pressure) and in the Rotliegendes (at the current pressure of 10 bar).

4. Stable balanced drilling (~ 9 ppg) is predicted above the Zechstein even for high well inclinations. The weakest depth at 5013 ft TVD would be stable until 50-degree inclination with 9-ppg mud and up to 79-degree inclination with heavier mud 10 to 12 ppg.

5. Stable drilling is expected in the Zechstein with over-balanced drilling (9 to 10 ppg) for well inclinations less than ~ 40 deg and assuming nearly normal pore pressure. For higher inclinations there may be stability problems as the inclination increases and specific WBS analyses should be performed to select the mud-weight.

6. Stable drilling is possible in the Rotliegendes for well inclinations of 0 to ~ 20 degrees with overbalanced drilling (mud-weight 9-10.75 ppg for Pr = 221 bar, 1 – 5 ppg for Pr=10 bar, 3 to 6 ppg for Pr = 50 bar and 4 to 7.5 ppg for Pr = 130 bar). Wells of higher inclination are expected to have instabilities if they intersect the lower UCS layers of the Rotliegendes.

7. Well azimuth has a small effect on the safe mud-weight window. Drilling parallel to the main fault has a small advantage allowing a lower mud weight by ~ 0.5 ppg.

7. WBS ANALYSIS OF BGM7 AND PROPOSED HORIZONTAL WEL L H35

The stability of the proposed horizontal well H35 was based on the logs of BGM7, which is the closest existing well on the map for which logs were provided (see Figure 7.1). The azimuth of trajectory of the H35 well is approximately parallel to the normal fault and the SH-max in situ stress. The TVD was used as the marker depth to identify the values from the BGM7 logs. Then the TVD was used to interpolate the MD, inclination, and azimuth from the H35 trajectory and use those in the WBS analyses. In the following sections the stability of BGM7 is shown first, and then the stability of the H35 trajectory is presented. The following WBS runs were made (however the ones marked by * are not presented since the BGM7 trajectory has small inclination of ~ 13 degrees in the Rotliegendes and no instability is predicted):

1. BGM7 when Pr = 234*, 114, 10*, 131*, 90* and 50* bar in the Rotliegendes. 2. H35 when Pr is normal 3. Safe-mud-weight vs. well inclination for 6600.34 ft TVD (unstable depth in the Zechstein for H35

trajectory) 4. Safe mud-weight vs. depletion for the horizontal section of the H35 trajectory at 7216 ft TVD

WBS analysis for BGM7

Figure 7.2 shows the in situ strength profile (predicted using the Formel-Dt model) and Figure 7.3 the in situ stress profile at the time the BGM7 well was drilled next. Figure 7.4 shows the safe-mud-weight-window for this case. The plot shows that the well could be drilled balanced with ~ 9 ppg in the (un-depleted) Zechstein. However, drilling difficulties are predicted in the Zechstein (about 6100 and 6500 ft TVD) where the BGM7 trajectory is inclined by ~ 45 degrees. In the Rotliegendes (where the well has inclination of ~ 13 degrees) the analysis shows that there would be no instability when drilled with



~4.0 to 8.5 ppg mud. The BGM7 well was actually drilled with ~9.5 ppg with no problem, which is close to the upper range of the WBS results and validates the analysis. For all other states of depletion in the Rotliegendes formation there is a safe-mud-weight-window for stable drilling. Figure 7.1: Spider Plot of Proposed Wells (in blue) and existing wells (in brown)

H35

BGM7

V23BGM2

H35

BGM7

V23BGM2



Figure 7.2: UCS prediction for BGM7 well

B G M 7 W e ll U C S s tr e n g th

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U C S _ F o rm e l_ D t (p s i) Z e c h s t e in P la t t e n D o l W e is s l ie g e n d e s R o t l ie g e n d e s

Figure 7.3: Stresses in BGM7 when the well was dril led, Pr = 1650 psi = ~114 bar (depl. = 1750 psi) in Rotliegendes


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Re s e r. P re s . (ps i) Ze chs te in Pla tte n Dol

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Figure 7.4: WBS of BGM7 when the well was drilled, Pr = 1650 psi =114 bar (depletion = 1750 psi) in Rotliegendes

BGM7 W BS: Actual Trajectory, Pr = 1650 psi (1750 psi depletion) depletion) (BRM7_wbs_trj-true_dpl1750_formel.dat)

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WBS analysis for H35

The stability of the proposed horizontal well H35 was based on the logs of BGM7, by assigning to them the proposed H35 well trajectory, based on TVD. Figure 7.2 shows the in situ strength profile (predicted using the Formel-Dt model) and Figure 7.5 the in situ stress profile of the H35 (and BGM7) well, when the Rotliegendes was at initial reservoir pressure. Figure 7.6 shows the safe-mud-weight-window for this case, including the well inclination. The plot shows drilling instability in the Zechstein between 6050 –7200 ft TVD (except in the Platten dolomite). Note that a 90-degree inclination was assumed for TVD >7216 ft, where the horizontal section of the H35 trajectory terminates. The plot shows that for TVD > 7230 ft, the safe-mud-weight-window is closed. Figure 7.7 shows an expanded scale of the plot bracketing the TVD of the horizontal section. The horizontal section of H35 at 7216 ft TVD could be drilled with about 8.0 to 12.0 ppg. Furthermore, if the horizontal section is drilled between 7210 and 7230 TVD, there is a safe mud-weight of ~9 to 10 ppg. Figure 7.8 shows the dependence of the safe-mud-weight-window vs. reservoir pressure of the horizontal section of the H35 well. Due to the relatively high estimated UCS of 4380 psi the horizontal section could be drilled with ~ 9.0 to 10.0 ppg fluid regardless of depletion.



Figure 7.5: Stresses in H35 & BGM7 at initial reser voir pressure Pr = 234 bar 3400 psi (depl = 0 psi) in the Rotliegendes

In s itu S tre s s e s fo r B G M7 & H35 We ll

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Figure 7.6: WBS of H35 well trajectory using BGM7 s tress-profile, Pr = 3400 psi =234 bar (depl= 0 psi)

H35 W BS: P roposed T rajec tory, Norm al R eservo ir P ressure (no dep le tion) (h35_wbs_ trj_dp l0_ fo rm e l.dat)

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NOTE: the horizontal section of H35 is at 7216 ft TVD (dashed line) at top of the Rotliegendes



Figure 7.7: WBS of H35 well trajectory using BGM7 s tress-profile, Pr = 3400 psi =234 bar (depl= 0 psi)—Expanded scale

H 3 5 W B S : P ro p o s e d T raje c to ry, N o rm al R e s e rvo ir P re s s ure (no d e p le tio n) (h3 5 _ w b s _ trj_ d p l0 _ fo rm e l.d at)

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NOTE: the horizontal section of H35 is at 7216 ft TVD (dashed line) at top of the Rotliegendes Figure 7.8: Safe-mud-weight-window vs. reservoir pr essure of horizontal section of H35 at 7216.34 ft TVD

S a fe m u d -w e ig h t vs. R e se rv o i r P re ssu re fo r H 35 h o r i z o n ta l se c tio nT V D = 721 6 .34 ft, U C S = 48 30 p si , w e l l d e v ia tio n = 90 d e g , w e l l a z im u th = 3 08 .87

(H 3 5_W D 90 _W A 309 _T V D 7216 _ fo rm e l .d a t)

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There are several intervals of instability in the Zechstein, that are caused by the high inclination of the H35 trajectory. For example it has an inclination of 58.1 degrees at 6600.34 ft TVD where UCS = 5060 psi. Figure 7.9 shows that the safe-mud-weight-window closes completely at ~ 48 deg well inclination, and therefore the analysis predicts instability. It is recommended to decrease inclination of the H35 trajectory in the Zechstein formation



Figure 7.9: Safe-mud-weight-window vs. well inclina tion (depl. = 0 psi) at 6600 ft TVD, Unstable depth in the Zechstein below the Platten D olomite

S a fe m u d -w e ig h t vs. w e l l in c l in a tio n fo r H 3 5 w e l l T V D = 6600 .34 ft, U C S = 5060 p si , w e l l a z im u th = 315 d e g (a c tu a l ), d e p le tio n 0 p si

(h 35_ d e v_ W A 31 5_d p l0 _T V D 66 00_ fo rm e l .d a t)

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m ax M W (ppg ) m inM W (pp g) P P (ppg ) S h (ppg)

Conclusions of BGM7 & H35 WBS analyses

1. The WBS analysis predicted, that when the BGM7 well was drilled and the Rotliegendes reservoir pressure was 1650 psi =~114 bar (depletion 1750 psi), the safe mud-weight window was ~ 4 to 8.5 ppg. Actually it was drilled with ~9.5 ppg, which is close to the top range and validates the in situ stress profile.

2. Also for the BGM7 well the Zechstein (at initial reservoir pressure) could be drilled balanced with ~9 ppg. Some drilling difficulties are expected in the Zechstein (about 6100 and 6500 ft TVD) where the well is inclined by ~ 45 degrees and the safe mud-window closes.

3. At present reservoir pressure of ~ 150 psi = ~ 10 bar (depletion 3250 psi), the Rotliegendes can still be drilled with 1 to 6 ppg for the 13-deg inclination trajectory of BGM7.

4. At virgin reservoir pressure of ~ 3400 psi = ~ 234 bar (depletion 0 psi), the Rotliegendes can still be drilled with 8.5 to 11.5 ppg for the trajectory of BGM7.

5. At future reservoir pressure of 130, 90 and 50 bar, the Rotliegendes can be drilled with approximately 5.0 to 8.5, 3.0 to 8.0, and 1.75 to 7.0 ppg, respectively, for BGM7 well trajectory.

6. The H35 well trajectory is predicted to be unstable in the Zechstein due to the large inclinations of 58 to 90 degrees.

7. A trajectory of less than 40-degrees inclination in the Zechstein would be required for safe-mud-weight-window to exist based on the WBS analysis.

8. The horizontal section of the H35 well trajectory at 7216 ft TVD is predicted to have a safe-mud-weight-window of 8 to 12 ppg. Furthermore, if the horizontal section is drilled between 7210 and 7230 TVD there is a safe mud-weight of ~9 to 10 ppg.

9. For the horizontal section of the H35 well trajectory at 7216 ft TVD, both the upper and the lower limits of the safe-mud-weight-window decrease nearly linearly with reservoir pressure, and the range opens with depletion. Therefore, reservoir depletion increases the stability of the horizontal section, which could be drilled with ~ 9.0 to 10.0 ppg fluid, regardless of depletion.



8. WBS ANALYSIS OF BGM3 WELL

The BGM3 is an inclined well in the NW part of the field (see Figure 1.1) drilled across the normal fault from BGM7 well and completed in the Weissliegendes formation (i.e., the upper part of the Rotliegendes). This formation is stronger than the deeper layers of the Rotliegendes formation, encountered for example by BGM1 well. The BGM3 well was drilled early in the development of the field, when the Rotliegendes was at initial reservoir pressure. The following WBS runs were made:

1. when Pr = ~3400 psi = 234 bar (depletion 0 psi) in Weissliegendes (upper Rotliegendes) 2. when Pr = 150 = ~10 bar (depletion 3250 psi) in Weissliegendes (upper Rotliegendes) 3. Safe-mud-weight vs. reservoir pressure in Weissliegendes (upper Rotliegendes) 4. Safe-mud-weight vs. inclination in Weissliegendes (upper Rotliegendes)

Figure 8.1 shows the in situ strength profile (predicted using the Formel-Dt model). Figure 8.2 shows the in situ stress profile at the time when the BGM3 well was drilled and Figure 8.3 shows the safe-mud-weight-window for this case. Similarly figures 8.5 and 8.6 show the in situ stresses and the safe-mud-weight-window at present depletion in the Weissliegendes. Figure 8.3 shows a shallow unstable interval at ~ 4850 ft TVD. The Rotliegendes and the Zechstein have several depth ranges where the safe-mud-weight-window is very small or nearly closed. The optimal mud-weight predicted for the well is ~10 ppg, but instability is expected in some depths of the Zechstein and the Weissliegendes. When the well was drilled with 9.75 to 10.5 ppg, there were drilling difficulties in the Zechstein and the Rotliegendes. This is in general agreement with the predictions of the WBS analysis. The un-depleted Weissliegendes (top part of the Rotliegendes) has an n almost closed safe-mud-weight-window. The optimal mud-weight for drilling would be ~10 to 11 ppg. There are depths like 7325 ft TVD, where the safe-mud-window is almost closed and instability is expected Figure 8.1: UCS prediction for BGM3 well based on s onic log using the Formel-Dt model


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T V D r k b (m )

(psi

)

U C S _ F o rm e l_ D t (p s i) Z e c h s t e in P la t t e n D o l W e is s l ie g e n d e s R o t l ie g e n d e s



Figure 8.2: Stresses in BGM3 at virgin (normal) Pr = 3400 psi =234 bar (depl.= 0 psi) in Weissliegendes (upper Rotliegendes)

In s itu Stre s s e s for Be rge rme e r 3 We ll

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Re s e r. P re s . (ps i) Ze chs te in Pla tte n Dol


Figure 8.3: WBS of BGM3 at virgin (normal) Pr = 340 0 psi =234 bar (depletion = 0 psi) in Weissliegendes (upper Rotliegendes)- actual well tr ajectory

BGM3 W BS: Actual Trajectory, Norm al Reservo ir Pressure (no dep le tion) (BGM3_wbs_trj_true_dpl0_form el.dat)

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Figure 8.4: Stresses in BGM3 at Pr = 150 psi =~10 b ar (depletion = 3250 psi) in Weissliegendes (upper Rotliegendes)

In s itu S tre s s e s for Be rg e rme e r 3 We ll

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Re s e r. P re s . (ps i) Ze chs te in P la tte n Do l

We is s lie ge nde s R otlie ge nde s GR (GAP I)

Figure 8.5: WBS of BGM3 at Pr = 150 psi =~10 bar (d epletion = 3250 psi) in Weissliegendes- actual well trajectory

BGM3 W BS: Ac tual T rajec to ry, Norm al Reservo ir P ressure (no deple tion) (BGM3_wbs_trj_ true_dpl3250_ fo rm e l.dat)

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The depleted Weissliegendes (upper Rotliegendes) has safe-mud-window of 5 to 6 ppg (see Figure 8.5). The safe-mud-weight-window at 7325 ft TVD slightly opens with depletion (see Figure 8.6). However drilling the BGM3 trajectory in the Weissliegendes (36 degree inclination) could present instability problems because there is a small mud-weight range for stable drilling. The safe MW-window closes completely at ~ 39 deg well inclination (see Figure 8.7). It is recommended to decrease inclination of the trajectory in the Weissliegendes to less than ~35 degrees for stable drilling.

Conclusions of BGM3 WBS analysis

1. The WBS analysis for the BGM3 well when it was drilled at initial reservoir pressure shows that the Weissliegendes (upper Rotliegendes) and the Zechstein formations have several depth ranges where the safe-mud-weight-window is very small or nearly closed. The optimal mud-weight predicted for the well is ~10 ppg, but instability is expected in some depths of the Zechstein and the Weissliegendes. When the well was drilled with 9.75 to 10.5 ppg, there were drilling problems in the Zechstein and the Rotliegendes. This is in general agreement with the predictions of the WBS analysis.

2. For the BGM3 trajectory, the un-depleted Weissliegendes could probably be drilled with 10 to 11 ppg. However, there are depths like 7325 ft TVD, where the safe-mud-window is almost closed, and instability could occur.

3. At present reservoir pressure of ~ 150 psi = ~ 10 bar (depletion 3250 psi) the Weissliegendes could be drilled with 5 to 6 ppg for the 36-deg inclination trajectory of BGM3. The best mud-weight is ~ 5.5 ppg. However there are some depths like at (~7325 ft TVD) where drilling is likely to cause stability problems.

4. The safe-mud-weight-window at 7325 ft TVD vs. reservoir pressure shows a limited safe-mud-weight-window for stability. The BGM3 trajectory in the Weissliegendes (36 degree inclination) will present instability problems for all reservoir pressures because of the large inclination.

5. It is recommended to decrease inclination of the trajectory of BGM3 in the Weissliegendes to less than ~35 degrees for stable drilling.



Figure 8.6: Safe-mud-weight-window vs. reservoir pr essure for a weak depth at 7325.41 ft TVD in Weissliegendes (upper Rotliegendes)

Safe m ud-w eight vs. Reservoir Pressure in Rotligend es BGM3TVD=7325.41 ft, UCS = 3074 psi, w e ll devia tion= 36 deg, w e ll azimuth=332

(BGM3_trg_W D36_W A332_TVD7325_forme l.da t)

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10111213141516

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max_Pw (ppg) min_PW (ppg) PP(ppg) Sh(ppg)

Figure 8.7: Safe-mud-weight-window vs. inclination, for a weak depth at 7325 ft TVD of BGM3 in Weissliegendes (upper Rotliegendes)--deplet ion = 0 psi- actual azimuth 332 deg

Safe mud-w eight vs. w e ll inclina tion in Rotligendes BGM3 TVD=7325.41 ft, UCS = 3074 psi, w e ll azim uth=253 d eg (actua l), deple tion 0 psi

(BGM3_dev_W A332_dpl0_TVD7325_forme l.da t)

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(ppg

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9. WBS ANALYSES FOR BGM2 & V23

The stability of the proposed vertical well V23 was based on the logs of BGM2, which is the closest existing well on the map for which logs were provided (see Figure 7.1). The trajectory of V23 has an inclined shallower section and intersects vertically the Zechstein and the Rotliegendes formations. The TVD was used as the marker depth to identify the values from the BGM2 logs. Then the TVD was used to interpolate the MD, inclination, and azimuth from the H35 trajectory and use those in the WBS analyses.

WBS analysis of BGM2 In this sections some additional WBS stability runs of BGM2 are presented for various depletions of the Rotliegendes formation, which may be realized in the future gas-storage operations of the field. The reader is referred to Chapter 4 for the WBS analysis when the well was drilled (see Fig 4.11 for the strength profile, Figure 4.15 for the in situ stress profile and figure 4.16 the safe-mud-weight-window for drilling the actual trajectory of this well). This analysis predicted events observed during drilling the well, and validated the in situ strength and stress profiles used in the WBS analyses. Figures 9.1 and 9.2 show the in situ stress profile at present reservoir pressure and the corresponding safe-mud-weight-window for stability. The plot shows that the Rotliegendes at Pr = 150 psi = 10 bar (~ 1 ppg) has a very narrow and/or closed safe-mud-weight-window in the interval 7100 to 73 75 ft TVD and when drilled with the mud-weight of ~5-6 ppg predicted by WBS analysis will probably have some instabilities. Similarly, Figures 9.3 and 9.4 show that Rotliegendes at Pr = 1880 psi = 130 bar (~ 5.75 ppg) will have instability problems. The same is true for all intermediate reservoir pressures. This is due to the ~30-degree inclination of the well in the Rotliegendes, which is higher than the 20-degree threshold identified by previous analysis of the BGM1 well.



Figure 9.1: Stresses in BGM2 at present Pr = 150 ps i = 10 bar (depl. = 3130 psi) in Rotliegendes


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(G

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R e s e r. P re s . (p s i) Ze ch s te in P la tte n D o l

We is s lie g e n d e s R o tlie g e n d e s GR (G AP I)

Figure 9.2: WBS of BGM2 at Pr = 150 psi = 10 bar (d epl. 3130 psi) in Rotliegendes – actual trajectory

BGM2 W BS: Actual Trajectory, Pr = 150 psi = 10 bar (depl. = 3130 psi) (BGM2_wbs_trj_dpl3130_formel.dat)

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Figure 9.3: Stresses in BGM2 at Pr = 1880 psi =~130 bar (depletion = 1400 psi) in Rotliegendes


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R e s e r. P re s . (p s i) Ze ch s te in P la tte n Dol

We is s lie g e nd e s R o tlie ge n de s GR (GAP I)

Figure 9.4: WBS of BGM2 at Pr = 1880 psi =130 bar ( depl. = 1400 psi) in Rotliegendes– actual trajectory

B G M 2 WB S : Actual T raje ctory, P r = 1880 psi = 130 ba r R e se rv oir P re ssure (de p l. 1400 psi) (B G M 2_wbs_tr j_dp l1400_forme l.dat)

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WBS analysis for V23

The stability of the proposed vertical well V23 was based on the logs of BGM2, by assigning to them the proposed V23 well trajectory, based on TVD. Figure 4.11 shows the in situ strength profile (predicted using the Formel-Dt model) and Figure 9.5 the in situ stress profile of the V23 well, when the Rotliegendes was at initial reservoir pressure. Figure 9.6 shows the safe-mud-weight-window for this case, including the well inclination. The V23 well trajectory is one of the more stable trajectories for the Bergermeer Field, because of smaller inclinations both in the Zechstein and the Rotliegendes. A clearly open safe-mud-window is predicted for the well (including the Zechstein and the Rotliegendes). The Zechstein could be drilled ~8 to 9.25 ppg. The un-depleted Rotliegendes with 8.5 to 11 ppg because of the 0-degree inclination of the trajectory. Figures 9.7 and 9.8 show the present in situ stresses and the associated safe-mud-weight-window. The plot shows that the depleted Rotliegendes at present Pr = 150 psi = 10 bar (= ~ 1 ppg) could be drilled with 0.25 to 6 ppg with no stability problems. Similar results are valid for intermediate reservoir pressures in the Rotliegendes. A small interval at ~ 5004 to 5006 ft TVD, where the V23 trajectory has a 40-degree inclination is predicted to have very small safe-mud-weight-window 11.25 to 11.37 ppg (see Figure 9.9). At this depth it was predicted that UCS = 1578 psi. The safe-mud-weight-window closes completely at ~ 43 deg well inclination

Conclusions of BGM2 and V23 WBS analyses

1. The BGM2 is a 26-30-degree deviated well. The WBS analysis predicts that the Rotliegendes may be drilled with difficulty at ~7275 ft to 7350 ft TVD with ~ 10.25 to 11.25 ppg at virgin reservoir pressure. It was actually drilled with ~ 10.75 ppg.

2. The WBS analysis predicts that the Zechstein could be drilled with ~9.5 to 10 ppg but it will have instability at 6700 to 6900 ft TVD, at the present (un-depleted) reservoir pressure. It was drilled with 10.25 to 10.5 ppg and the Bergermeer summaries indicate stuck pipe below the Platten dolomite in about the same interval. The above support the analysis and the validity of the in situ stress profile

3. The depleted Rotliegendes at any reservoir pressure of 10 to 130 bar has a very narrow and/or closed safe-mud-window in the interval 7100 to 73 75 ft TVD and when drilled with optimal mud-weight predicted by WBS analysis will probably have some instabilities. The optimal mud-weight ranges are: 5 –6 ppg for Pr = 10 bar; 8 –9 ppg for Pr = 130 bar; 7 –8 ppg for Pr = 90 bar; and 6 –7 ppg for Pr = 50 bar.

4. The V23 well trajectory is one of the more stable trajectories for the Bergermeer Field, because of smaller inclinations both in the Zechstein and the Rotliegendes.

5. A clearly open safe-mud-window is predicted for the well (including the Zechstein and the Rotliegendes). The Zechstein could be drilled ~8 to 9.25 ppg. The un-depleted Rotliegendes with 8.5 to 11 ppg because of the 0-degree inclination of the trajectory

6. The depleted Rotliegendes could be drilled with no stability problems using the following mud-weights: 0.25 – 6 ppg for Pr = 10 bar; 5 – 8.5 ppg for Pr = 130 bar; 3 – 7.75 ppg for Pr = 90 bar; and 2 – 7 ppg for Pr = 50 bar.

7. A small interval at ~ 5004 to 5006 ft TVD, where the V23 trajectory has a 40-degree inclination is predicted to have very small -mud-window 11.25 to 11.37 ppg.



Figure 9.5: Stresses in V23 at initial reservoir pr essure Pr = 226 bar 3280 psi (depl = 0 psi) in the Rotliegendes

In s itu Stre s s e s for Be rge rme e r V23 We ll

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Figure 9.6: WBS of V23 at initial reservoir pressur e Pr = 226 bar 3280 psi (depl = 0 psi) in the Rotliegendes

B G M-V23 W B S : A c tual T rajec tory, Norm al R eservo ir P ressure (no dep le tion) (BG M-V 23_wbs_trj_dp l0_fo rm el.dat)

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Figure 9.7: Stresses in V23 at present Pr = 150 psi = 10 bar (depl. = 3130 psi) in Rotliegendes

In s itu S tre s s e s fo r B e rg e rme e r V23 We ll

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Figure 9.8: WBS of V23 at Pr = 150 psi = 10 bar (de pl. 3130 psi) in Rotliegendes-actual trajectory

BGM-V23 WBS: Actual Trajectory, Pr = 150 psi = 10 bar (depl. = 3130 psi) (BGM-V23_wbs_trj_dpl3130_formel.dat)

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Figure 9.9: WBS of V23 (depletion = 0 psi) vs. well -inclination--Unstable depth in shallow part of the well at 5004.41 ft TVD

Safe mud-weight vs. well inclination for V23 well TVD=5004.41 ft, UCS = 1578 psi, well azimuth=173.3 8 deg (actual), depl. 0 psi

(BGM-V23_dev_WA173_TVD5004_dpl0_formel.dat)

8.008.258.508.759.009.259.509.75

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10. PRODUCTION STABILITY OF THE ROTLIEGENDES RESERV OIR

The main objective of the present study was to assess the drilling stability of Bergermeer wells. However, in this section we address briefly some aspects of wellbore stability during production operations. Although the methodology is the same as for drilling stability, the pressure transmission conditions in the wellbore are entirely different. Drilling stability is enhanced by the presence of the mud-cake, which prevents to a great degree the transmission of pressure from the wellbore to the formation. During production, the wellbore pressure is transmitted in the reservoir because there is no filter-cake. The Rotliegendes reservoir pressure will range for 10 to 130 bar during the operation as a storage field. Furthermore, the stability of an individual well will depend on the wellbore pressure, which will be higher than the (far-field) reservoir pressure during injection or lower than the reservoir pressure during production. Wellbore instability could occur during either stage and is characterized again by a safe-mud-weight-window, where the mud-weight represents the bottom-hole-wellbore pressure. To assess production stability of the Rotliegendes, we consider the stability of the lowest UCS strength depth encountered in the BGM1 well at 7239 ft TVD. The following production stability runs were made (i.e., assuming no -filter-cake): vertical well, 5-degree inclined well. 15-degree inclined well, and horizontal well. Furthermore the safe-mud-weight-window vs. reservoir pressure was also determined for the horizontal section of the H35 well in the Weissliegendes (upper Rotliegendes) at 7216 ft TVD.

Production stability of Rotliegendes based on lowes t UCS of BGM1 well

Figures 10.1 to 10.4 show the production stability plots for various well inclinations (0, 5, 15 and 90 degrees), at the lowest-UCS-depth (1436 psi) estimated in the Rotliegendes from the logs of BGM1. Figure 10.1 shows that for a vertical well there is a safe-well-pressure-window (from min_Pw to max_Pw) for any reservoir pressure. Since the reservoir pressure lies within this range, no production or injection stability problems are expected if the well is operated within this range. Note that the injection pressure should not exceed the far-field reservoir pressure by more than ~ 250 psi (difference between green and blue lines) to maintain wellbore stability. Figure 10.2 shows similar results for a 5-degree inclination well. However, there is a more limited injection pressure range that diminishes from ~ 200 psi to 0 psi as the reservoir pressure goes to zero. As well inclination increases, the production stability deteriorates. Figure 10.3 shows similar results for a 15-degree inclination well. The plot shows that there is a safe-well-pressure range only for reservoir pressure higher than ~1800 psi. However this is lower than the reservoir pressure and therefore the well could not be practically operated in this range. Furthermore, for lower reservoir pressure than ~ 1800 psi, the well will be at failure for any wellbore pressure. Figure 10.4 shows that a horizontal well, there will be failure for all reservoir pressures. Consequently, only nearly vertical wells (like the BGM1 or the V23) are predicted to be stable during production operations if they encounter weak rock layers of the Rotliegendes (e.g. UCS = 1436 psi).



Figure 10.1: Safe-mud-weight-window vs. reservoir p ressure for 7289 ft TVD in Rotliegendes, for vertical BGM1 well--Production St ability (no filter-cake)

S a fe m u d -w e ig h t vs. Re se rvo ir P re ssu re in Rotl ig e n d e s BG M 1T V D= 7288.97 ft, UCS = 1436 p si , w e l l d e via tion = 2 d e g , w e l l a z im uth = 192 d e g

(BG M 1_trg _W D1_W A192_T V D7289_p ro d .da t)

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Figure 10.2: Safe-mud-weight-window vs. reservoir p ressure for 7289 ft TVD in Rotliegendes, for 5-degree inclined BGM1 well--Prod uction Stability (no filter-cake)

Safe mud-weight vs. Reservoir Pressure in Rotligend es 5-deg incl. BGM1wellTVD=7288.97 ft, UCS = 1436 psi, well deviation= 5 d eg, well azimuth=192 deg

(BGM1_trg_WD5_WA192_TVD7289_prod.dat)

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Max_Pw (psi) min_PW (psi) Res. Pres.(psi) Sh(psi)



Figure 10.3: Safe-mud-weight-window vs. reservoir p ressure for 7289 ft TVD in Rotliegendes, for 15-degree inclined BGM1 well--Pro duction Stability (no filter-cake)

Safe mud-w eight vs. Reservoir Pressure in Rotligend es 15-deg incl. BGM1w ellTVD=7288.97 ft, UCS = 1436 psi, w e ll devia tion= 15 deg, w ell azimuth=192 deg

(BGM1_trg_W D15_W A192_TVD7289_prod.dat)

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Figure 10.4: No safe-mud-weight-window exists for 72 89 ft TVD in Rotliegendes, for horizontal BGM1 well--Production Stability (no filte r-cake)

Sa fe m ud-w e ight vs. Re servoir Pre ssure in Rotlige nd e s Horizonta l BGM1w e llTVD=7288.97 ft, UCS = 1436 psi, w e ll de via tion= 90 de g, w e ll a z im uth=192 de g

(BGM1_trg_W D90_W A192_TVD7289_prod.da t)

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Max_Pw (psi) min_PW (psi) Res . Pres.(psi) Sh(psi)



Production stability of horizontal sections of H35 well

The horizontal section of the H35 well is located in a stronger rock layer (UCS = 4830 psi) at the top of the Weissliegendes. Figure 10.5 shows that for the horizontal section of the H25 well there is a safe-well-pressure-window (from min_Pw to max_Pw) for any reservoir pressure. However, failure is predicted to occur (and possibly wellbore instability) in the Weissliegendes for reservoir pressure less than ~ 800 psi. For reservoir pressure higher than ~800 psi it is possible to inject and produce the well without inducing failure in the rock. Figure 10.5: Safe-mud-weight-window vs. reservoir p ressure for horizontal section of the H35 well in the Weissliegendes (upper Rotliegendes) at 7216 ft TVD.--Production Stability (no filter-cake)

Safe mud-weight vs. Reservoir Pressure for H35 hor izontal sectionTVD=7216.34 ft, UCS = 4830 psi, well deviation= 90 deg, well azimuth=308.87

(H35_WD90_WA309_TVD7216_prod.dat)

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Conclusions of Production WBS analyses

1. No production or injection instabilities are predicted for nearly vertical wells (0 to 5 degrees inclination) in the Rotliegendes (for the lowest UCS = 1436 psi at 7289 ft TVD in BGM1 well). The injection pressure could be at most ~250 psi higher than the far-field reservoir pressure for stability.

2. For higher inclination than 5 degrees, production and injection stability problems are predicted by the analysis.

3. There is a safe-well-pressure-window down to the complete depletion of the reservoir for the horizontal section of the H35 well in the Weissliegendes (estimated UCS is 4830 psi at 7216 ft TVD). However, failure is predicted to occur (and possibly wellbore instability) in



the Weissliegendes for reservoir pressure less than ~ 800 psi. For reservoir pressure higher than ~800 psi it is possible to inject and produce the well without inducing failure in the wellbore.

REFERENCES

1. Part of “Drilling FEED Study: Description of the Geomechanical Work” file “6.DESCRIPTION OF GEOMECHANICAL WORK.pdf” provided in July 2009 for initial review.

2. Technical Note prepared by Philip McCurdy of Senergy “High Level Geomechanics Review of Bergermeer Gas Storage Project”, March 2008.

3. TAQA Report, “Overview of Zechstein Lithology, Bergermeer Field”, file “Bergermeer Field Zechstein Summary-rev1.pdf”.

4. Jaeger, J. C., and Cook, N. G. W., “Fundamentals of Rock Mechanics,” Third Edition, June 1979, Chapman and Hall Publishers

5. BGM8 4 and BGM4 Minifrac Analysis, PowerPoint presentation of 13 October 2008, file “BGM_4 and 8 Frac analysis Final rev26-11-08.ppt”.

6. Zhang, J. J., Rai, C. S., and Sondergeld, C. H. “Mechanical Strength of Reservoir Materials: Key Information for Sand Prediction”, SPE Reservoir Eval. & Eng. 3 (2), April 2000, SPE paper 62499.

7. Drilling summaries (file “bergermeer summaries.xls”).

ACKNOWLEDGEMENTS

The author thanks Jan Thijs Keijser for defining the scope of this study and authorizing this study, and to Jelle Wielenga for coordinating and providing valuable discussions and guidance during the study.

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Wellbore Stability Analysis Bergermeer Field 2009 351869

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