GM-FP-BOP-01 - Stripping Operations Rev 00

Guidelines and Manuals GM-FP-BOP-01

Stripping operations

Exploration & Production Rev: 00 Date:09/03/2004 Page: 2 of 24

TABLE OF CONTENT

1 INTRODUCTION.........................................................................................................3

2 PROCEDURE TO FOLLOW.......................................................................................3

3 PRACTICAL PROCEDURE........................................................................................4

3.1 Stripping with an annular blowout preventer ..........................................................................5

3.2 Stripping with two combined preventers...............................................................................13

3.2.1 Principle .............................................................................................................................13

3.2.2 Stripping with annular/ ram preventers ..............................................................................13

3.2.3 Stripping with dual ram preventers.....................................................................................14

4 RECOMMENDATIONS FOR STRIPPING ................................................................15

5 STRIPPING AND VOLUMETRIC CONTROL CALCULATION ................................21

This document is the property of Total. It must not be stored, reproduced or disclosed to others without written authorisation from the Company.

stripping operations rev 00.doc




1 INTRODUCTION

A study of blow out frequency in the American waters of the Gulf of Mexico region showed that approximately 40% of all blowouts are a direct result of kicks in the well during tripping. The explanation for this relatively high percentage lies in the difficulty of controlling a well taking a kick when the drill bit is far above the bottomhole.

In this domain, prevention is always better than cure, and therefore everything should be done to detect a kick; drilling teams must be constantly made aware of the risks of swabbing, and of the importance of the correct use of the trip tank during drillstring tripping operations.

These guidelines describe the operations to be carried out and other procedures to follow, to run the drill string back down hole, under pressure below the closed blow out preventers.

2 PROCEDURE TO FOLLOW

Standard procedure formerly used to consist of re-running the bit as far down hole as possible, as long as the flowrate was not excessive. The uncertainty arose when the time came to decide whether or not the flow rate was excessive. In the past, this uncertainty resulted in considerable kicks in the well, making well control more difficult or even impossible.

One factor which is important, but often neglected, is the position of the kick in relation to the drillstring.

If the drill bit and the Drill Collar have, run into a less dense fluid without the driller being aware of it , then there will be a considerable drop in hydrostatic pressure. This may result in overpressure loss in the producing formation, a rapid increase in kick volume and finally, high pressures at the surface.

Consequently, the correct procedure is to shut in the well at the first sign of flow. If this procedure is followed to the letter, then pressures indicated at the surface will be minimal in all cases and therefore ensure more methods available for use in killing the well.

The best way of running the drill bit down to the bottom again as quickly as possible, with pressure in the well, is to choose a combination of the stripping method and the volumetric method.






3 PRACTICAL PROCEDURE

The most frequently heard objections raised against stripping drill pipes back to bottom, through an annular blowout preventer are:

1. Excessive pressure at the surface.

2. Excessive overpressure when the pipe is run in.

3. Difficulty in maintaining a correct bottomhole pressure due to the entry and/or expansion of the gas bubble.

4. Loss of hydrostatic stability when the drillstring is at the same level as the gas bubble.

To avoid excessive pressure at the surface, the correct procedure for closing the well, as described above, should be followed, i.e. shut in the well as soon as the first sign of flow is detected.

When the well has been shut in, if the pressure readings are low, stripping would appear to be the most efficient method.

Bottomhole pressure can be controlled during stripping by combining it with volumetric control.

Figures 1 to 5 illustrate this procedure: the drop in hydrostatic pressure that occurs when the drill collar enters the gas bubble can be compensated by maintaining a constant pressure at the bottom, reducing overpressures to a minimum and adding a safety pressure (S).

The following hypotheses were adopted to simplify the procedure and facilitate its application in tripping operations:

1. The kick is assumed to be a trip gas bubble which forms in the drilling phase.

2. The kick remains in place as a continuous slug, entirely filling the annular section of the well.

3. To calculate the decrease in hydrostatic pressure, the bubble is assumed to migrate to a constant annular space between the wellbore/drill collar (the smallest annulus, although, this said, the wellbore/drill pipe annulus is unknown).

The method is divided into three separate operations, again to make it easier to put into practice on site:

1. The driller runs in the drill pipes at constant speed, avoiding overpressures in the well, after first adjusting the closing pressure of the annular blowout preventer to prevent any leaks occurring when the tool joints pass. The closing pressure can be determined after stripping exercises or from the manufacturers information (see Fig.2).

2. A choke operator maintains a constant, predetermined pressure in compliance with the instructions given by the toolpusher.

3. One or two operators assigned to the trip tank/stripping tank continuously measure(s) the volume of returns in the trip tank and recover(s) from the stripping tank the volume of mud corresponding to each stand run in the hole.

If necessary, add a measure of foam inhibitor to the mud in the trip tank to help obtain a sound interpretation of the levels in the two tanks.






3.1 Stripping with an annular blowout preventer

1. Once the well is shut in, calculate the volume of the kick and control the pressure at the surface at two-minute intervals. When the pressure has stabilized, it will be recorded every five minutes or at every stripped length.

2. Calculate the specific volume of the mud in the annular space wellbore/drill collar which corresponds to 1 bar of hydrostatic pressure.

Equivalent volume (in l/bar) = 1d

2,10Vas

Vas: Volume of annular space wellbore/drill collar (in l/m) d1: Initial mud density.

3. Select the value of the working pressure level p : between 5 and 10 bars inclusive, it must be readable on the control manometer.

4 Convert this working pressure p into a working volume corresponding to the wellbore/Drill Collar annulus.

v = (1d10.2Vas

) x p

This is the volume of mud that will be used for the levels in the volumetric method (see figure4).

5. Calculate the additional pressure S which will compensate for the loss of hydrostatic pressure when the drill pipe and the drill collar are in the gas bubble (not to be confused with the trip margin).

However, as the exact position of the kick in the well is unknown and it is therefore impossible to say exactly when the additional pressure should be applied, it is recommended that adequate safety measures be taken from the start of the stripping operation. It is essential to know the kick volume observed at the surface in order to calculate S:

S = (bore/DC- wVas

Gain-

bore- wVGain

) x 2.10

dgas - d 1

Gain: in litres

Vas wellbore/DC: in l/m

V w-bore: in l/m

d1 : initial mud density

dgas : gas density






6. Adjust the closing pressure of the annular BOP to an acceptable minimum; no leaks should be tolerated. Check that there is no flow above the annular casing during the adjustment of the closing pressure.

7. Start to lower (strip) the first length of drill pipes, already equipped with the full bore valve open and topped by the Gray type valve and allow the annular pressure to rise to a value Pa:

Pa = Pa1 + S + p Pa1: initial annular pressure

S : additional pressure compensating for hydrostatic loss with bit/D.C. at the same level as the influx.

p : working pressure level NOTE: The bottom of the first stand is stripped with the choke closed until the required annular pressure (Pa) is reached. Only the remainder of the length, which is stripped at a constant pressure, should be taken into consideration in calculating the corresponding exterior volume to be bled off from the stripping tank.

For example, if the annular pressure Pa is reached after stripping two singles in the first stand, only the volume of the 3rd single is recovered in the stripping tank.

The same principle is also applied when a level p is added. 8. Maintain this pressure level Pa constant throughout the stripping process The mud gain corresponding to the external volume (VE) of the lowered drillstring is recovered by purging from the trip tank, then transferred by gravity to the stripping tank once the length has been entirely run in to the well.

This procedure means that any other volume increase in the trip tank is probably due to the gas bubble starting to expand and therefore the onset of hydrostatic pressure loss in the well.

NOTE: If, due to an oversight during the stripping process, the bottomhole pressure dropped below the formation pressure (Pf




10. Once an initial surplus volume of mud v has been recovered, with the choke closed, the annular pressure should be left to increase by a value p, and a new working level of annular pressure is obtained PA = Pa + p and should be kept constant. 11. By reproducing the cycle described above as many times as necessary until the BHA has reached the bottom, the bottomhole pressure Pf remains almost constant, while the migration and expansion of the gas in the annular both develop, i.e.:

PA1 = PA + p PA2 = PA1 + p PA3 = PA2 + p etc, etc

12. Fill the interior of the drillstring after running each stand of drill pipe.

13. Each time a new stand is put on the (manual) chocks, keep a length of flexible cable wound around the handles.

14. The pressures and volumes will be recorded and/or noted on a check sheet throughout the entire stripping operation (see calculation sheet in appendix).

15. Once the drill bit is at the bottom and the drill pipes full, the well is killed using the Drillers Method. To eliminate the possible build-up of gas below the Gray valve, a volume of mud corresponding to the interior volume of the Gray valve /drill bit is pumped out at reduced flowrate. Pt1 can then be measured.

16. Stop pumping, check the annular pressure and resume circulation while maintaining a pressure PR1 = Pt1 + PC1 + S at the top of the drill pipe until all gas has been completely evacuated.

Pt1 may be zero if the mud density (d1) in the drillpipes balances out the formation pressure at the level of the kick. This may occur when swabbing while tripping.

If there is a risk of sticking due to differential pressure, trip the drillstring through the BOP.






FIG. 1: STANDARD INSTALLATION ON SITE COMBINING STRIPPING AND VOLUMETRIC METHOD

Annular BOP

Choke

DC

Note: the stripping tank must:

Have a net capacity of 1000 to 1200 litres Be equipped with a set of removable graded gauges for 2 7/8, 3, 5 pipes, etc






Fig.2: RECOMMENDED INSTALLATION FOR STRIPPING

WITH A COMPENSATED ANNULAR BOP

1 An accumulator bottle (capacity 10 to 20 gallons, working pressure 3000 psi,) against overpressure is installed on the closure line. The pre-charge of the accumulator should be between 50% and 75% of the required closing pressure

ANNULARPREVENTER

OPEN CLOSE

2 It regulates the closing pressure and prevents pressure surges when the tool joints pass through the annular . 3 Use an accumulator bottle with a capacity that will ensure closure of the annular preventer.






Fig.3: MUD GAIN AT THE SURFACE ARISING EXCLUSIVELY FROM THE EXTERNAL VOLUME OF THE DRILL PIPE

TRIPTANK

GRAYvalve turnvalve

VE= The volume of mud bleed to the stripping tank equal to the external volume of the strip in drill pipe. Annulus pressure to be kept constant. Practical reminder: A stand of three 5 pipes displaces a volume of mud equal to: 13.24 l x 28 m ~ 370 l A stand of three 3 pipes displaces a volume of mud equal to: 6.60 l x 28 m ~185 l






FIG. 4: MUD GAIN AT THE SURFACE DUE TO EXPANSION OF THE GAS EFFLUENT

TRIPTANK

VDC

Annulus pressure to be kept constant.

Gas expansion of V in the annulus hole/DC. The gas expansion is estimated from the volume V received in the trip tank.






Fig. 5: RESULT OF THE PASSAGE OF THE BHA IN THE EFFLUENT

P1

P2

H1: height of the gain at the well closure, the position of the gain is uncertain . H2: height of the gas column with drilling bit and drill collars in front of the gain. P 1: the annulus pressure required when the gain is below the bit. P 2: the annulus pressure required when the gain is in front of the BHA. 1: Anticipate the loss of hydrostatic pressure 2: Calculate the additional annular pressure required






3.2 Stripping with two combined preventers

When the annular pressure is too high or when the tool joints may cause packing to be torn off the annulus, it is sometimes necessary to use a second preventer to pass the tool joints through the BOP. This could be an annulus/ram or ram/ram combination.

As a general rule:

- The upper BOP opens WITHOUT pressure,

- The lower BOP opens UNDER pressure (equilibrium).

3.2.1 Principle

The upper preventer is in fact used like the door of a pressure lubricator, while the lower preventer (Fig.7 et 8) is used exclusively for stripping and is positioned low enough for the tool joint to pass.

The distances or spacing between each preventer and a reference point (e.g. the rotary table) must be known so that the position of the tool-joint can be determined at any time during operations.

A pressurisation line between the two preventers must be provided. It is needed to balance the pressures on either side of the rams of the lower preventer before they are opened during the stripping process. The space between the two preventers can be pressurised with a pressure test pump or with the cementing unit if it is available.

A second line or lateral outflow must be installed between the two preventers allocated to stripping. It is used to carry the mud to the trip tank via the atmospheric degasser (MGS), and enables the pressure in the pressure lock to be totally bled off before opening the upper preventer.

NEVER USE THE PRESSURE IN THE WELL TO BALANCE THE PRESSURES ON EITHER SIDE OF THE RAMS.

REMEMBER THAT THE LOWEST RAM PREVENTER IS CONSIDERED AS THE LAST SAFETY BARRIER AND MUST NEVER BE USED FOR STRIPPING OPERATIONS.

3.2.2 Stripping with annular/ ram preventers

The annular preventer is the main preventer during stripping.

Whenever the annular ram has to be closed, its normal closing pressure (1500 psi) will be applied. This will then be reduced to the required minimum while the pressures on each side of the ram are brought into equilibrium by pumping between the two preventers. (Fig.9).

Just as for the annular preventer on its own (see chapter 3.1 above), the pressure regulator on the annular preventer must be adjusted until a minimal pressure is reached, ensuring an effective seal on the drillstring during stripping operations.

The ram BOP is shut with the normal manifold pressure (1500 psi) after purging the space between the two preventers to zero. Pressure on the manifold of the hydraulic accumulator is lowered into a range of 200 to 500 psi (Fig.7). The correct reduced pressure in fact depends on the ambient pressure in the well and is determined such as to increase the life of the packing of the stripping rams.






3.2.3 Stripping with dual ram preventers

The procedure used is identical to the procedure described in paragraph 3.2.1.

The upper ram preventer (closing on pipes) will serve as the main preventer for stripping (the longest stripped length), and the ram preventer located just below will be used only to pass the tool joint into a standby position under the main preventer in the stripping process.

Note: This technique is seldom applied on our drilling sites, given that:

the upper ram preventer is reserved for blind or shear closure (CR FPP 160), the lower ram preventer (closing on pipes) should not be used for stripping (CR FPP 160),

few rigs are equipped with 4 ram preventers, as the space for a tool joint between the two tiers of rams that could theoretically be used may be too small.






4 RECOMMENDATIONS FOR STRIPPING

When a kick is taken during tripping, it is important to implement the requisite procedures as soon as possible.

- If stripping is a viable option, everything must have been thought through so that the operation can be carried out rapidly. This means that the following should be available:

. a means of accurately measuring each volume of mud bled off,

. well adjusted manometers,

. pressure regulators on the BOP should be in working order.

- In certain cases, stripping exercises should be added to the other exercises (pit drill, kick drill) organised to train drilling teams and to evaluate the stripping characteristics of the preventer used (to be done prior to redrilling a casing shoe).

- The drilling rig must be suitably equipped if the procedure described above and the volumetric method associated with it are to be applied.

The annular preventer is generally used for stripping operations since its supple casing allows the tool joints to pass. To reduce wear, the pipes should be greased and the closing pressure exerted on the annulus should be kept to a minimum while ensuring that no leaks occur. Certain manufacturers recommend a slow leak (4 litres/min.) as beneficial in terms of lubrication through the annulus.

It may be advisable to inject a water/bentonite mixture (bentonite plugs previously softened) at the top of the annular casing.

Remove the rubber protectors and file off any rough edges left by the chocks and the tong dies on the body or the tool joints of the drill pipe.

- Install the Gray valve even if there is a float valve on the drillstring.

It is important to measure all fluid leaving the well. - The formation fluid that has entered the well is assumed to be gas that migrates upwards

during stripping operations and may appear when the pressure is bled off at the wellhead.

- If there is no fluid migration in the well, then the volume of mud recovered from the well during stripping of one stand of drill pipes should correspond to the total external volume (VE) of the stand in question and surface pressure should remain the same (Fig.3).

- If the gas migrates into the annular space, the surface pressure would increase, even if the total external volume of mud (VE) leaked off at the surface is exactly equal to the exterior volume of the stripped drillpipes.

- To maintain a constant pressure level, an additional mud volume (V) must be bled off so that the gas can expand, thereby reducing the risk of cracking the formation under the casing shoe (Fig. 4).

- The main principle implemented in purging is to maintain pressure levels constant above the choke during stripping.

When the BHA is in the gas bubble, substantial hydrostatic pressure loss occurs. The possibility of this future pressure loss should be borne in mind and an additional counter-pressure S should be added right at the start of the stripping operations (Fig. 5).






Fig. 6 STRIPPING PROCEDURE ANNULUS / RAMS

wellhead

TRIPTANK

Piperams

1) Reduce the closing pressure of the annular preventer to the minimum pressure to avoid leak. 2)Strip the drill pipe in the well to have the tool joint on top of the annular preventer. 3)Bleed off the mud volume (VE ) of the stripped in drill pipe. 4)The pressure of 140 bars on the gauge is just given as an example.






Fig. 7 PROCEDURE FOR ANNULUS/RAMS STRIPPING

TRIPTANK

Piperams

4)Close pipe rams with a 1500 psi closing pressure. 5)Bleed off to zero the trapped pressure between the two preventers ( annular and pipe

rams). 6)Reduce the closing pressure on the pipe rams down to the lower pressure without leak (200 to 500 psi).






Fig. 8 STRIPPING PROCEDURE ANNULUS/RAMS

TRIPTANK

Piperams

7)Open the annular preventer. 8)Strip down the drill pipe through the pipe rams to have the tool joint completely below the annular preventer.






Fig 9. ANNULUS/RAMS STRIPPING PROCEDURE

Piperams

Pump toequalisepressures

TRIPTANK

9)Close the annular preventer with the maximal operating pressure of 1500 psi. 10)Pump through the kill line to equalise the pressure below and above the pipe rams. 11)Reduce the closing pressure on the annular preventer at the lower pressure without leak.






Fig. 10 ANNULUS/RAMS STRIPPING PROCEDURE

TRIPTANK

Piperams

12)Open the annular preventer. 13)Continue stripping operations.






5 STRIPPING AND VOLUMETRIC CONTROL CALCULATION

1- WELL DATA OPEN HOLE

open hole measured length = m. open hole vertical length = m. open hole diameter = inch.

SG OF THE MUD IN HOLE d1 =

CASING DATA last casing diameter = inch. measured depth of the casing shoe = m. vertical depth of the casing shoe = m. p max at shoe = bar.

WELL DEPTH drilled depth = L m. vertical depth = Z m.

2- PUMP DATA PUMP CAPACITIES

pump n 1 : cap1 = litre/stroke. pump n 2 : cap2 = litre/stroke.

CONTROL FLOW RATE Qr = l/min and pump speed = stroke per minute.

CIRCULATING PRESSURE AT CONTROL FLOW RATE pump n 1 = bar. pump n 2 = bar.

3- KICK DATA Drill pipe closing pressure : Pd1 = bar. Annulus pressure : Pa1 = bar. Gain = litre






4- CALCULATION OF THE VOLUMES AND PUMPING TIME VOLUMES

Drill string internal volume : Vint = litre. Annulus volume : Van = litre. Active volume on surface : Vsurf = litre. Total circulating volume : Vtot = Vint+Van+Vsurf = litre.

PUMPING TIMES Pumping time from surface to drilling bit = Vint/Qr = min. Pumping time from drilling bit to surface = Van/Qr = min. Pumping time for total active volume = Vtot/Qr = min.

NUMBER OF PUMP STROKES Number of pump strokes from surface to drilling bit = Vint/cap= strokes Number of pump strokes from drilling bit to surface = Van/cap= strokes Number of pump strokes for total active volume = Vtot/cap= strokes

5- CALCULATION OF AVERAGE VE FOR ONE DRILL PIPE VE : external volume of one drill pipe = average drill pipe length x drill pipe

capacity

6-CALCULATION OF VOLUMETRIC BLEED OFF Height for 1 bar of hydrostatic pressure: h = 10.2/d1= m. Calculation of equivalent volume (Veq) for 1 bar of hydrostatic pressure:

Veq = hxVas ( w-bore/DC)

Selected value of the working pressure level p = bar. calculation of the bleed off volume for p : v = Veqxp = litre.

7- CALCULATION OF THE ADDITIONAL PRESSURE S The additional pressure S which will compensate for the loss of hydrostatic pressure when the drill pipe and the drill collar are in the gas bubble (not to be confused with the trip margin).

- S = (bore/DC- wVas

Gain - bore- wV

Gain ) x 2.10

dgas - d 1

8- STRIPPING OPERATION 1- Start stripping operation with Pa = Pa1+S+p 2- Keep Pa constant up to an increase of trip tank volume of v. 3- Choke close, let Pa increases of p, then keep Pa constant as in 2- up to a

second increase of trip tank volume of v. Then continue.






9-FOLLOW UP OF THE ANNULUS PRESSURE IN FUNCTION OF THE BLEED OFF VOLUMES

Annulus Pressure (bar)








v = VE = Vol.int. = Vol.an = Vol.tot Well bore =

Vol.tot In circ. =

MUD LEAK OFF VOLUME

TIME LENGTH

No. TRIP TANK STRIPPING

TANK

ANNULAR PRESSURE

RETURN DENSITY

TRIP TANK

OBSERVATION

Back to GM SummaryHomeTABLE OF CONTENT1 INTRODUCTION2 PROCEDURE TO FOLLOW3 PRACTICAL PROCEDURE3.1 Stripping with an annular blowout preventer3.2 Stripping with two combined preventers3.2.1 Principle3.2.2 Stripping with annular/ ram preventers3.2.3 Stripping with dual ram preventers

4 RECOMMENDATIONS FOR STRIPPING5 STRIPPING AND VOLUMETRIC CONTROL CALCULATION

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GM-FP-BOP-01 - Stripping Operations Rev 00

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