Lesson 3 - Methods Used in Overpressure Analysis

7/28/2019 Lesson 3 - Methods Used in Overpressure Analysis

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3. METHODS USED IN

OVERPRESSURE ANALYSIS

03. ABNORMAL PRESSURES

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There are several methods that can be applied to predict, evaluate and

calculate the pressures existing in a well (or below, while drilling is still in

progress). They could be divided into three main categories:

1. (not always reliable) Methods used BEFORE DRILLING A WELL,

and in some cases even instead of drilling a well Seismic Data Analysis and Interpretation

Data of Reference Wells, if available and (again) reliable

2. Methods used WHILE DRILLING A WELL (qualitative vs quantitative)

Drilling Data Processing and Interpretation

MWD, LWD, and also LWF Processing and Interpretation

3. Methods usually used AFTER DRILLING A BOREHOLE SECTION Wireline Log Analysis (Sonic Log, Resistivity Log, etc.)

3.1. INTRODUCTION


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METHOD WHEN WHAT HOW

SURFACE SEISMICS BEFORE DRILLING Interval velocities From reflection times

to transit times

DRILLING

PARAMETERS

WHILE DRILLING ROP, WOB, W, MW,

HS, RPM

Drilling exponent and

Sigmalog

CUTTINGS

ANALYSIS

WHILE DRILLING CUTTINGS Features and size of

drilled cuttings

TEMPERATURE WHILE DRILLING MUD

TEMPERATURE

Measurement of mud

temperature inside and

outside the well

WELL BEHAVIOUR WHILE DRILLING Overpulls,

reaming, drilling

break, gas

detection, torque,

water influx

Anomalies in parameter

values, salinity control

MWD- LWDLOGGING

WHILE DRILLING Resistivity andSonic Log

Resistivity and transittime

WIRE LINE LOGS AFTER DRILLING A

HOLE SECTION

Resistivity and

Sonic Log

Resistivity and transittime

3.1. INTRODUCTION


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3.2. PRESSURES PREDICTION ANDEVALUATION

FROM SEISMIC DATA ANALYSIS

(not always reliable),BEFORE DRILLING A WELL


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A. AVERAGE VELOCITY vs

REFLECTION TIME

B. VELOCITY ANALYSIS

(example)

3.2.1. Seismic Data Presentation Formats ENI example


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Average

Velocity

Two-Way

Time

Cumulative

Depth

Interval

Velocity

Interval

Velocity

Correct

Interval

Depth

Correct

Cumulative

Depth

Correct

Average

Velocity

Correct

Interval

Travel

Time

CORRECTIONS

Vav,i ti Hi Vint, i Vint,c, i Hint,c, i Hi, c Vav,c, i tint,i

m/sec sec m m/sec m/sec m m m/sec sec/ft

(1) (2) (3) (4) (5) (6) (7) (8) (9)

1,600 0.000 0.00

1,815 0.200 181.5 1,815 181.5 181.5 168

1,876 0.300 281.4 1,998 1,992 99.6 281.1 1,874 153

1,936 0.400 387.2 2,116 2,106 105.3 386.4 1,932 145

1,991 0.500 497.7 2,211 2,197 109.9 496.3 1,985 139

2,042 0.600 612.6 2,297 2,280 114.0 610.3 2,034 134

2,095 0.700 733.2 2,413 2,383 119.4 729.7 2,085 128

2,151 0.800 860.4 2,543 2,508 125.4 855.3 2,138 122

2,214 0.900 996.3 2,718 2,665 133.2 988.3 2,198 114

2,291 1.000 1,145.5 2,964 2,893 144.7 1,133.0 2,266 105

3.2.1. Seismic Data Presentation Formats ENI example


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1. The cumulative depth (Column 3) can be easily determined with the following relationship:

Hi = (Vav, i ti)/2

2. The interval velocity (Column 4), that is the velocity with which the sound waves travel within

each single depth interval can be obtained by applying the following relation:

Vint, i = [(Vav, i ti) (Vav,i-1 t i-1)]/(ti t i-1)3. The interval velocity, Vint, c,i , (Column 5) can be corrected by using the following equation:

Vint,c,i = [(V2 av, i ti) (V2 av,i-1 ti-1)]/(ti ti-1)4. Once the interval velocity for each time interval is determined, the thickness of the interval

characterised by given value of Vint,c,i, (Column 6) can be obtained from the relation:

Hint,c, i = [Vint,c,i (ti t i-1)]/25. By summing up the thickness of each single interval, the corrected cumulative depth, Hi,c, is

calculated (Column 7):

H c,i= H int,c, I6. Knowing the interval velocities, corrected according to Dix (Point 3), the corrected average

velocities can be recalculated (Column 8) by using this equation:

Vav,c, i= [Vint,c,i (ti t i-1)]/ti7. At this point the corrected interval velocities are transformed into their reciprocal, that is into

interval travel times, t int,i in sec/ft, (Column 9):t int, I = (0.3048 106)/Vint,c, i

3.2.1. Seismic Data Presentation Formats ENI know how


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Enis Way03. ABNORMAL PRESSURES

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3.2.2. Geopressures Evaluation from Seismics

The seismic data (transformed in terms of depths, interval

velocities and interval travel times), at this point are ready

to be used for pressure gradient calculation, that is for the

determination of:

overburden pressure and gradient;

pore pressure and gradient;

fracture pressure and gradient (when possible).

Accord ing to ENI standards, heresec means seconds ,

to be prop er ly ind icated bys


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As a start point, the following relationship between t and can be written:

t = tmax (1) + tfl

where:

- t = transit time of sound through the formation (sec/ft)- tmax = transit time of sound through the solid frame (sec/ft)- tfl = transit time of sound in the pore fluid (sec/ft)

- = porosity, fraction

The previous equation can be also expressed in terms of porosity:

= (t - tmax)/( tn - tmax)

where (in the opinion of ENI):- tfl = assumed equal to 200 sec/ft (a conservative value for pore pressure

gradient calculations)

- tmax = values which depend on lithology



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BUT: Eni E&P has found from laboratory tests that the previous equationcan better represent the behaviour of non-cemented or undercompacted

formations (i.e. sand, gravel, shales), if written in the following form:

=1.228 [ ( t - tmax) / ( t + 200) ] experimental eq.

By combining this equation with the following equation, which gives the bulk

density of a rock, b:

b= fl + (1) max

the relations expressed in next slide can be derived, which allow thecalculation of bulk densities from travel (transit) times or interval

velocities.



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3.2.2.1. Overburden Pressures and Gradients Calculation

BULK DENSITIES CALCULATION

Bulk densities can be computed with the following equations, proposed byEni E&P (but NOT shared worldwide):

In terms of interval travel-times:

In terms of interval velocities:

min


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BULK DENSITIES CALCULATION

where:

-b

= bulk density of the formation, g/cm3

-max = matrix density, g/cm3 (an average value of 2.75 g/cm3 is usually assumed)

-tint = interval transit-time obtained from analysis of the seismic data, sec/ft

-tmax = interval transit-time of the rock matrix, sec/ft (assumed between 43.5 47.0

sec/ft)

-tfl = interval transit time of the fluid present in the rock, sec/ft (equal to 200 sec/ft)

-Vint = interval velocity obtained from analysis of the seismic data, m/sec

-Vmax

= velocity of sound in the rock matrix, m/sec (assumed to be 6,485 - 7,000 m/sec)

- Vmin = minimum velocity of sound corresponding to the first superficial layer, m/sec

(generally around 1,500 m/sec).



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OVERBURDEN PRESSURE CALCULATION

The following equation is applied:

which expressed in terms of interval transit-time becomes:



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OVERBURDEN GRADIENT CALCULATION

The Overburden Gradient can be calculated with the following equation:

where:- Gov = overburden gradient, kgf/cm

2 / 10 m

- Pov = overburden pressure, kgf/cm2

- H = depth, m



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OVERBURDEN GRADIENT CALCULATION EXAMPLE

Results of calculations are as here below shown.

Depth Interval Transit Time Bulk Density Overburden

Pressure

Overburden

GradientH, m tint,c,i , sec/ft g/cm

3 kgf/cm2 kgf/cm2/10 m

181.5 168 2.056 37.32 2.056

281.1 153 2.116 58.40 2.078

386.4 145 2.151 81.05 2.098

496.3 139 2.177 104.98 2.116

610.3 134 2.200 130.06 2.131

729.7 128 2.229 156.67 2.147

855.1 122 2.259 185.00 2.163

988.3 114 2.300 215.64 2.182

1133.0 105 2.349 249.63 2.203

1285.1 100 2.377 285.78 2.224

1450.5 92 2.425 325.89 2.247

1629.7 85 2.469 370.13 2.271

1802.7 88 2.450 412.52 2.288

1928.3 85 2.469 456.68 2.305



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BULK DENSITY

(kg/dm3)

OVERBURDEN

GRADIENT

(kgf/cm2/10m)

2.0 2.5zero 0

1000

2000

3000

4000

5000

6000

1.5

Profonditm

BULK DENSITY

INTEGRATED OVERBURDEN

DENSITY



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3.2.2.2. Pore Pressures and Gradients Calculation

Two methods of analysis of the data can be applied for Pore

Pressure calculation (the final goal, of course). They are:

1) (Interval Velocity versus Depth) or, more commonly,Interval Transit Times versus Depth Plots, finally leading

to a quantitative assessment performed either by theEQUIVALENT DEPTH method or by EATON method

2) (empirical method) Interval Velocity / Theoretical VelocityRatio, R = V1/V2, forclastic formations.


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The calculation procedure can be summed up in the following points:

determination of the interval velocity, Vint;

determination of the depth H relative to each interval velocity;

transformation of the interval velocities into interval travel times, tint; construction of the graph tint versus H on semilog paper (see next slide).Generally, for plotting the data a two cycle semilog paper is used, with depth on

the ordinates and tint on the abscissae (logarithmic scale). It is also advisable toplot the depth in such a way as to have 1 cm corresponding to 100 - 200 m;

once interval transit times as a function of depth have been plotted, the

interpretation of the curve results easier if the following points are taken into

consideration:

in normal compaction conditions, tint decreases with depth; Gp isnormal and equal to 1.031 kgf/cm2/10 m. Through these points, thereference trend line can be drawn;

when tint values start to increase, this means abnormal compactionconditions and an abnormal pore pressure gradient, which has to be

calculated.

3.2.2.2.1. Interval Transit Times versus Depth


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0

1000

2000

3000

4000

5000

10 100 1000

Depth(m)

The tint decreases with

depth down to 2900 m,where the data start to

increase.


3 2 2 2 1 I t l T it Ti D th


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0

1000

2000

3000

4000

5000

10 100 1000

Depth(m)

Overpressures

Top

The tint decreases with

depth down to 2900 m,where the overpressure

top is located.

Below 2900 m,

overpressures areencountered.


3 2 2 2 1 I t l T it Ti D th


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The methods used to quantify pore pressure gradients

using the travel time vs depth plots usually are:

A.theEQUIVALENT DEPTHmethod (most used)

B.EATONmethod


3 2 2 2 1 I t l T it Ti D th


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A. EQUIVALENT DEPTH METHOD PRINCIPLE

1. As already pointed out, the overburden pressure acting at a certain depth, H, is the sum of the

effective pressure, Peffor, and the pore pressure, Pp, as given by the equation:

Pov = Peff+ Pp

2. If at the considered depth H1, the rock has been able to dissipate the pressure generated within

its pores as a consequence of its burial at major depth, its compaction is considered normal and

also normal is its pore pressure, assuming a value equal to the hydrostatic trend.

3. If, for instance, the rock laying at the depth H2 was impeded, for any reason, to expel the excess

of water located within its pores during the burial process as a consequence of the increasing

weight of the sediments above it, it will show a porosity higher than normal, and will be therefore

undercompacted and will have a pore pressure above the hydrostatic one.

4. If the two rocks are made both by the same lithology (i.e. both are shales) and have the same

transit time, though being at different depths, this means that they have the same porosity andthat have been subjected to the same compaction pressure.


3 2 2 2 1 Interval Transit Times versus Depth


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5. Therefore, once calculated the overburden pressure at the depth H1 and knowing that here the

pore pressure gradient is normal (equal, for instance to 1.031 kgf/cm2/10 m), by applying the

above mentioned equation the effective compaction pressure can be easily calculated:

Peff,H1 = Pov,H1 Pp,H1 (at depth H1)

6. Moving to the depth H2, the overburden pressure is easily calculated, being the overburden

gradient already available; by subtracting from the overburden pressure the effective

compaction pressure, as obtained at the previous point, the pore pressure is finally determined

by the difference:

Pp,H2 = Pov,H2 Peff,H1=H2(at depth H2)

Operatively you must:




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1.Select the depth at which you want to compute the Pressure Gradient (i.e. point H2 at 3500 m).

2. Intersect vertically the Normal Compaction Trend line (point H1 at 2300 m).

This means that shales at 3500 m are as compacted as those at 2300 m.

3. From the Integrated Overburden Gradient curve, obtain the Overburden Gradientat 3500 m and 2300 m:

GOV at 3500 m (H2) = 2.42 kgf/cm2/10m GOV at 2300 m (H1) = 2.34 kgf/cm

2/10m

4. Calculate the Compaction Pressure at Depth H1:

(GOVH1 - Gp,n) x H1 (2.34 - 1.03) x 2300Pc = = = 301,3 kgf/cm

210 10

5. Calculate the Overburden Pressure at Depth H2:GOVH2 x H2 2.42 x 3500

POV = = = 847,0 kgf/cm2

10 10

6. Calculate the Pore Pressure at 3500 m: PP = POV - Pc = 847,0 - 301,3 = 545,7 kgf/cm2

7. Calculate the Pore Pressure Gradient at 3500 m:GP = (PP x 10)/H2 = (545,7 x 10)/3500 = 1.56 kgf/cm

2/10 m




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Depth

m

010 100 1000

500

1000

1500

2000

2500

3000

3500

4000

Transit time - sec/ft

H4

H 3

H1

H2

THE EQUIVALENTDEPTH METHOD




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0

500

1000

1500

2000

2500

3000

3500

4000

4500

1.5

Verticaldepthm

2.0 2.5 3.0

Overburden Gradient, kgf/cm2/10 m

THE EQUIVALENT DEPTHMETHOD

H1

H2

Gov,H1 Gov,H2




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B. EATON METHOD

A different and simpler method to calculate the pore pressure gradient is

the method proposed by B. Eaton (1969), which is, however, less carefulwith respect to the equivalent depth method. The Eaton relationship is

based on the ratio, at the examined depth, between the normalt,tNCT,as read on the normal compaction trend line, and the computed value,

tcalc, that is:

n

calc

NCTsedsedp

t

tGGG 03.1

The exponent n depends on the method used to define the normalcompaction trend line and is, usually, taken equal to 3 if the Sonic Log (ora seismic data set) has been performed and evaluated, or to 1.5 if aResistivity Log has been considered.


EATON METHOD


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EATON METHOD

AA

Gov = 2.10 kgf/cm2/10 m

n = 3

tNCT (A) = 50 sec/ft

tcalc (A) = 70 sec/ft

Gp = 1.71 kgf/cm2/10 m

n

calc

NCTsedsedp

t

tGGG 03.1

3 2 2 2 2 Interval Velocity/Theoretical Velocity Ratio R = V1/V2


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It is an empirical method, developed within Eni E&P (but not shared by

other OC) to solve some interpretation problems, sometimes encountered

when using the two previously mentioned techniques in clastic formations.

It is based on the calculation and subsequent graphic representation of theratio, indicated as R, between the interval velocity, Vint, and the soundvelocity, Vs, as follows:

R = Vint/Vswhere:

- Vint= interval velocity as obtained from seismics, sec/ft- Vs = sound velocity, that is the velocity of the sound that should beobserved within normally compacted shales, sec/ft

Depending on the values assumed by the quantity R, the followingconditions will be possible:

if R =1 normally compacted and pressured formations if R < 1 undercompacted and abnormally pressured formations if R > 1 overcompacted formations or carbonates

3.2.2.2.2. Interval Velocity/Theoretical Velocity Ratio, R V1/V2

3.2.2.2.2. Interval Velocity/Theoretical Velocity Ratio, R = V1/V2


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R ratio

0

0,5 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0

500

1,0001.500

2.000

2.500

3.000

3.500

4.000

4.500

5.000

5.500

6.000

6.500

7.000

Very porous oroverpressuredformations (R


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0

0,5 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0

500

1.000

1.500

2.00

02.500

3.000

3.500

4.000

4.500

5.000

5.500

6.000

6.5007.000

Overcompactedformations (R>1)

R ratio

Very porous oroverpressuredformations (R


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Once the overburden and pore pressures gradients have been calculated from

the analysis of seismic data as seen before, the fracture gradients can be

easily derived by applying the following equations:

a) The fracture gradient, showing the rate at which the fracture pressure

changes with depth, is given, for a rock having an elastic behaviour, by the

followng expression:

Gfr= Gp + ( 2 ) (Gov Gp)

1-

b) If the drilling fluid is water or whenever the drilling fluid invades in depth the

formation, we have:

Gfr= Gp+ ( 2 ) (Gov Gp)

c) If the rock has a plastic behaviour, the fracture gradients is given by the

formula:Gfr= Gov

3 3 actu e essu es a d G ad e ts


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3.3. PRESSURE PREDICTION AND

EVALUATION

FROM DRILLING DATA ANALYSIS


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3 3 1 INTRODUCTION


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The processing and interpretation of drilling parameters represents a very

important group of techniques, which have the advantage to be available

more or less in real time and to be referred to any possible well in progress.

These methods can be:

qualitative, which, if analyzed in their completeness, can anyway provide

significant information about the actual status of the well and alert the drilling

engineering staff in case dangerous and abnormal conditions are in progress;

quantitative, which ensure the quantification of the pressures acting in andaround the well and the related risk levels.

3.3.1. INTRODUCTION


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3.3.2. QUALITATIVE INDICATORS


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Among the qualitative techniques, based on drilling and geological parameters

recording, processing and interpretation, we can recall the following ones:

rate of penetration

torque

overpulls

cavings and hole tightening

flow rate and pumping pressure

mud level in the pits

cuttings increase at shale shakers

hole fill up

mud resistivity and chlorides concentration

pH

shale resistivity and shale density

gas shows

mud temperaturemontmorillonite concentration in the mud (MBT test)

Q


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3 3 2 1 QUALITATIVE DRILLING INDICATORS


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DRILLING RATE OR RATE OF PENETRATION ROP

If everything else remains constant, the drilling rate (here also calledpenetration rate orrate of penetration, ROP) gradually declines as depth

increases due to the decreasing porosity caused by the weight of the

overlying sediments and the increase in differential pressure between the

hydrostatic head of drilling mud in hole and the pressure of formation fluids.

But raw penetration rate values are affected by so many influencing factors(characteristics of formations, bit types and wear, WOB, RPM, mud type

and weight, etc), that it is impossible to use them directly within a reliable

and efficient detecting method. Furthermore, penetration rate values

provide only a qualitative indication about formations porosity and do not

allow any quantitative evaluation of existing (or not) abnormal pressures.


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3.3.2.1. QUALITATIVE DRILLING INDICATORS



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TORQUE

The amount of torque required during the drilling operations can supply

reliable information about the presence of abnormally pressured formations.

It must be remembered that the amount of required torque depends on the

resistance met by the bit, which is a function of: the weight on bit, the

coefficient of friction of the formation and the amount of restoring torque, thelatter parameter being significantly dependent on the amount of frictional

force developed against the wellbore walls. Any change in the torque value,

therefore, can be due to a change in weight on bit, to a change in the type

of formation or to bit balling, and not only to hole tightening, which could

be indicative of wellbore instability and of presence of abnormal pressure

values.


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OVERPULLS

When tripping out a string, the amount of hook load is approximatelyproportional to the depth reached. For various causes, not alwaysimputable to the presence of an abnormal pressure, the predicted value

may be exceeded, thus causing an overpullcondition. The main causes of

hook load increase can be represented by the following items:

bit balling, stuck drill string;

unusual swabbing effects;

severe dog-legs.

If these above stated causes can be ruled out, the overpull may be

attributed to underbalance conditions with hole tightening, and then to thepossible presence of overpressures.


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PUMPING PRESSURE

If a fluid, having a density which is lower than the mud density in the hole, enters the

well, a decrease in pumping pressure can be observed, because the hydrostatic

pressure in the annulus becomes lower than the pressure in the drill pipes

determining as a consequence a decrease also in pumping pressure for the U tube

effect. In these conditions also the friction losses in the annulus decrease.

FLOW RATE-IN AND FLOW RATE-OUT

If the mud flow rate exiting the hole is higher than the mud flow rate pumped into the

drill string, that clearly indicates underbalance conditions, before any mud pit

volume (say surface level position) change can be appreciated.

HOLE FILL-UP

If, during tripping the drill string out of the hole, less mud than expected is required

to fill the hole, this indicates underbalance conditions and then the necessity to

increase the mud density, unless evident signs of swab are recognized.


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Q



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MUD SALINITY, RESISTIVITY, pH

If an increase in the salinity (mainly

chlorides) of the mud is observed, this

means that water, present in the rockpores, has entered the hole, because of

insufficient pressure exerted by the

mud column. Being the formation water

usually more salty than the water used

for preparing the mud, an increase in

salinity is therefore experienced.

The inflow of salty formation waters into

the hole determines, not only an

increase in salinity, but at the same

time provokes a decrease in mud

resistivity and in its pH. If these

quantities are continuously monitored

and plotted versus depth, theoccurrence of abnormally pressured

formations can be detected.

RESISTIVITY - The resistivity analysis of the salty water that contaminates the drillingmud (from overpressured layers) tends to show slowly decreasing values.

MUD CHLORIDES An increase in Chlorides concentration is an overpressure index.

Depthm

RESISTIVITY

OVERPRESSURETOP

Depthm

CHLORIDES

OVERPRESSURETOP

MUD RESISTIVITY - CHLORIDES


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3.3.2.2. QUALITATIVE GEOLOGICAL INDICATORS


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MUD DENSITY/GAS RELATIONSHIP

The volume of gas released from a drilled formation will be dependent upon: porosity,

permeability, gas saturation and differential pressure of this formation.

The progressive and continuous increase of gas in mud while drilling (background

gas) and while connecting pipes to the drill string (pipe connection gas) could be

regarded as another way to monitor abnormal pressure conditions, provided that other

causes such as swab or drilling through gas bearing rock could be excluded.

The influx of gas into the wellbore determines a decrease in mud density, which can becalculated by using the relation:

with:

- W1 = gas-cut mud density, lb/gal

- W2 = uncut mud density, lb/gal


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CUTTINGS CHARACTERISTICS

When underbalance conditions

are approached, an increase in

penetration rate occurs; in this

case the amount of the cuttingsproduced and lifted to the surface

increases too. The extra-quantity

of produced cuttings depends on:

length of the interval drilled in

underbalance;

differential pressure between themud and the formation;

bit penetration rate.

When drilling in underbalanced

conditions, also cavings are

produced. Cavings are largepieces of formation detached from

the wellbore walls and not due the

bit activity (as the cuttings are).

A. TYPICAL SHALE CAVINGS

PRODUCED BY

UNDERBALANCED DRILLING

B. TYPICAL ARGILLACEOUS

CAVINGS PRODUCED BY

STRESS RELIEF

A B


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SHALE DENSITY

This has been a very popular technique in the past, when sensors and computers were

not so common at rig sites.

The method is based on the measurements of shale density, assuming that

overpressured shales be undercompacted, therefore more porous and with a lower

density than normally compacted shales. By plotting the density, as obtained from

cuttings collected at the shale shakers, versus depth, a graph is built, which shows an

increase in shale density if normal conditions exist and a decrease when overpressures

are entered.

The magnitude of the bulk density change will vary with the type and magnitude of the

geopressure. Bulk density may also decrease, but it may remain constant (due to

lithology) or continue to increase at a lower rate than the previously established trend

due to the geopressure mechanism.


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OVERPRESSURED shales are UNDERCOMPACTED, have a LOWER DENSITYthan what they should have at the depth where they are located.

Verticaldepthm

Verticaldepthm

DENSITY g/cm3

2.2 2.3 2.4 2.5

DENSITY g/cm3

2.2 2.3 2.4 2.5

OVERPRESSURETOP

SHALE DENSITY

NORMAL

COMPACTIONUNDERCOMPACTION


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SHALE DENSITY

Ideal Clay

DensityResponses in

Geopressured

Zones Caused

by Different

Mechanisms


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TEMPERATURE

The geothermal gradient, or the rate at which subsurface temperatures increase with

depth, can be calculated by using the relation:

GT = 100 [(TF2 TF1)/(D2 D1)]

where:

- GT = geothermal gradient (C / 30 m)

- TF1 = temperature (C at depth D1, m)

-TF2 = temperature (C at depth D2, m)

While the average temperature gradient across normally pressured formations may beconstant, pressured formations exhibit abnormally high geothermal gradients, due to

their higher porosity and higher fluid content, which make them very poor heat

conductors. Therefore, overpressured shales will heat the mud much more than other

normally pressured rocks.

Monitoring and recording mud flowline temperature is a practical method to determinetemperature gradient, provided variable factors such as pump rate, lag time,

environment temperature, lithology and temperature changes at the surface (due to

mud mixing and to chemical treatments) can be accounted for.


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TEMPERATURE

A. Geothermal Gradient Change

Through an Insulating Geopressured

Zone

B. Expected Flowline Temperature WhenDrilling Through a Geopressured Interval

A

B

Prior to reaching a

geopressured zone, a

ttz (say a temperature

transition zone) will

be encountered in

which, due to distortion

of the isothermal lines,

there will be areduction in

geothermal gradient,

followed by an

extremely large

increase, occurring as

the geopressured zoneis penetrated.


47Enis Way

TEMPERATURE



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TEMPERATURE

At the rig-site flowline

temperature is

plotted againstdepth. The transition

zone is characterized

by a decrease in

geothermal gradient,

while the entrance into

the overpressuresshows an increase in

geothermal gradient.

The mud temperature

is affected by many

variables and can giveonly qualitative type

responses.


48Enis Way

NORMALIZED PENETRATION RATE

3.3.3. QUANTITATIVE EVALUATION OF GEOPRESSURES


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As seen before, the rate at which a formation can be drilled is determined by a number

of factors, such as:

weight on bit, WOB; rotary speed, RPM;

bit tooth efficiency;

differential pressure;

hydraulics;

rock matrix strength;

formation compaction.

To make penetration rate info more useful, several attempts have been made in the

past to correct them for some of the most important parameters which affect drilling, in

particular WOB, RPM, hole size, differential pressure and mud density and,

consequently, a number ofdrillability ornormalized penetration rate formulationshave been proposed in order to simplify the treatment of the effects of so many drilling

variables.


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Among the many drillability formulations, proposed by several authors to correctpenetration rates for the variables affecting them, one particularly used in the Industry,

at least as a starting point, was that developed by G.F. Combs (1968):

where:

- R0= penetration rate with a new bit and P=0, ft/h- W = weight on bit per hole size unit, lbs/in- Dh = hole diameter, in

- N = rotary table revolutions per minute, rpm

- Q = flow rate, gal/min

- Dn = bit nozzle diameter, 1/32ths of an inch

- aw = exponent of weight on bit

- an = exponent of rotary table speed- aq = exponent of hydraulics

- P = differential pressure, psi- T = bit teeth wear index


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From this basic drillability equation proposed by Combs, two are the most commonsimplified methods derived to obtain a quantitative evaluation of pressures and relevant

gradients while drilling a well, that is:

the drilling exponent, d-exp, with its more advanced version, that is the corrected

drilling exponent, dc-exp, method;

the Sigmalog method, developed also by Eni E&P (but different from other versions).

1. Both methods are semi-empirical and are based on a relationship between

penetration rates and the selected drilling parameters.

2. If all other conditions remain the same, these drillability indices are proportional to

the depth. In other words, in normal compaction conditions, they increase as the depth

of the well increases, because the rock becomes harder and the differential pressure

between the mud in hole and the formation pressure increases too, while in presence of

overpressures the drillability index decreases, because the porosity of the rockincreases and the differential pressure decreases..


51Enis Way

3.3.3.1. THE DRILLING EXPONENT AND

CORRECTED DRILLING EXPONENT METHOD


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M.G. Bingham (1965) proposed the following relationship between the drilling rate,

weight on bit, rotary speed and bit diameter.

where:

- R = drilling (penetration) rate, ft/h- N = rotary speed, rpm

- D = bit diameter, ft

- W = Weight on bit, lbs force

- a = matrix constant, dimensionless

- d = drillability exponent, dimensionless


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This mathematical relationship was revised and adapted to field requirements by Jorden

and Shirley (1966), solving ford and by introducing constants to take into account the

units of measurement commonly used in the petroleum industry in order to obtain valuesthat vary within an acceptable range. It assumed the following form which is known as

drilling exponent or simply d-exp:

where:

- d-exp = drilling exponent. dimensionless

- R = drilling or penetration rate. ft/h

- N = rotary speed, rpm

- W = weight on bit, Ibs force

- D = bit diameter, in


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Using metric units and changing from natural to common logarithms, the previous

equation becomes:

where:

- d-exp = drilling exponent. dimensionless

- R = drilling or penetration rate, m/h

- W = weight on bit, ton force- D = bit diameter, in

- N = rotary speed, rpm


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Since differential pressure depends upon the mud density and formation pore pressure,

whenever there is any change in the mud density this will promote an unwanted change

in the d-exp; for this reason, the corrected drilling exponentordc-exp was proposed(Rehm and McClendon, 1971), whose expression, in metric units, is the following:

- Gpn = normal pore pressure (equal to 1.031), kgff/cm2/10 m

- ECD = Equivalent Circulating Density (mud density plus friction losses), kg/litre

or more simply:

dc-exp = d-exp/MW

with:- d-exp = uncorrected drilling exponent

- MW = mud weight (referring to the density of the mud in use, given in kg / litre)

log


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The calculation sequence generally adopted in the field is the following:

the drilling parameters (ROP, WOB, RPM, HS) are recorded every 1 meter drilled

and processed by using the equations previously seen;

the resulting d-exp and dc-exp values are plotted versus depth on a semilogarithmic

paper;

in normal compaction conditions, the d-exp, and also the dc-exp, will increase with the

depth and all points, taken in correspondence of shales, will lay on what is called the

normal compaction trend line;

as soon as the d-exp and dc-exp values, always taken in correspondence of shalelevels, start to decrease, this means that abnormal formations are entered and the

more these values depart from the reference trend line the higher is their overpressure.

Of course, the true overpressure values have to be calculated on the dc-exp curve,

because it has been corrected for the mud weight present in the well. The following slide

shows that the d-exp curve, with respect to the dc-exp curve, is masked as the mudweight seems to result increased, thus indicating (apparently) a lower pressure regime.


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d Exponentd Exponent dc

Exponentdc Exponent

Depth

Normally

Compacted Zone

Mud Masking

Effect

Overpressured

Zone

Overpressure

Top

d-exp and dc-exp

trends versus depth


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The quantitative calculation of pore pressures and gradients can be performed with

one of the three following methods, that is:

A.the equivalent depth principle, as seen when dealing with the interval travel timesin seismics;

B. (slightly modified) the so-called Eaton method;

C. the r ratio method (abacus construction) which is based on the expression:

Po = PN [(dc-exp)N/(dc-exp)p]

where:

- Po = the actual pore pressure at the depth, H, of interest, kgf/cm2

- PN = normal pore pressure, obtained from (H x 1,031)/10, kgf/cm2

- (dc-exp)N = actual dc-exp value at the depth of interest;

- (dc-exp)p = value at the depth of interest as read on the normal compaction trend line


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ddcc--exponentexponent

Vertical

Depth

Vertical

Depth

A2

A1H1

H2

dc-EXPONENT: A) EQUIVALENT DEPTH METHOD

Pc,H1 = Pov,H1 Pp,H1

Pov,H1 = (G ov,H1 H1)/10

Pp,H1 = (1.03 H1)/10

Pc,H1 = Pc,H2

Pov,H2 = (G ov,H2 H2)/10

Pp,H2 = Pov,H2 Pc,H1=H2

Gp,H2 = (Pp,H2 10)/H2

Gov,H1 = 2.13 kg/cm2/10 m

Gov,H2 = 2.28 kg/cm2/10 m

H1 =2300 m

H2 = 3400 m

Pov,H1 = 490 kg/cm2

Pp,H1 = 237 kg/cm2

Pc,H1 = Pc,H2 = 253 kg/cm2

Pov,H2 = 775 kg/cm2Pp,H2 = 522 kg/cm

2

Gp,H2 = 1.54 kg/cm2/10 m


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2.1

03.1

norm

compovbdovbdp

dcdcGGG

dc-EXPONENT: B) EATON METHOD

A A

Gov = 2.10 kg/cm2/10 m

dccomp (A) = 0.4

dcnorm (A) = 0.6

Gp = 1.44 kg/cm2/10 m


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dc-exp

DEPTH

dc-EXPONENT: C) r RATIO METHOD (ABACUS)


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211

131

131

131

111

Bitrundepthm

Sand

Line

Gp Lines

1.7 1.5 1.3 1.1

SHALES

Line

bit

Wear

Bit Wear

Top of Overpressure

Porosity effect

The interpretation of the

dc-exp curve is facilitatedby taking into

consideration:

lithology;

bit runs,

casing setting points;

hole difficulties;

knowledge of the area.


62Enis Way

3.3.3.2. THE SIGMALOG METHOD


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The Sigmalog method has been developed during the 1970s also (but not

only) by Eni E&P to overcome the limits of the dc-exp technique experienced

while drilling deep wells in the Po Valley Basin, in particular its deficiency in

identifying overpressures within carbonatic reservoirs.

The Sigmalog takes into account the mud density effect on penetration rates

and is based on the drillability concept, as all other methods which process

drilling data. Again the parameters involved are ROP (m/h), RPM (rpm), WOB

(ton force) and bit or hole size HS (in).

The Sigmalog does not allow only the calculation of the pore pressure

gradients, but also of the overburden and fracture gradients through steps

very long and tedious indeed; for this reason, the overburden gradients can be

more easily determined from seismic interpretation or from Sonic Log analysis.


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The basic equation of Sigmalog is as follows:

t= WOB0,5 x RPM0,25

Dh x ROP0,25

where:

- WOB : weight on bit, ton force

- RPM: revolutions per minute of the rotary table

- Dh: hole size (also indicated as HS), in

- ROP: rate of penetration, m/h

To compensate for values excursion at shallow depth, a correction factor,

depending from depth, has been introduced:

t = t + 0,028 (7 H/1000)

where:- H: depth, m


64Enis Way

In order to take into account the effect of the differential pressure, P, on the



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65

penetration rate, the following relationship is introduced:

P = (MWGp) x H/10

where:

- MW: mud weight, kg / litre

- Gp: pore pressure gradient at the depth H, kgf/cm2/10m (taken equal to 1.031)

Because ROP does not change linearly with P, a correction factor takes into

account this occurrence:

F* = 1 + 1 - 1 + n2P2

n Pwhere:

- n = 3,2/(640 t) if t 1

- n = 1 x (4 0,75 ) ift > 1640 t


65Enis Way

The value of Sigmalog which is plotted versus depth is given by the following



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The value of Sigmalog, which is plotted versus depth, is given by the following

equation:

o = F t

The interpretation criterion is the same as per the dc-exponent method., that is:

- if the o values increase with depth in homogeneous formations (shales), it meansthat normal conditions exist and the pore pressure is normal;

- if the o values tend to decrease with depth, always in homogeneous formations, it

means that abnormal conditions are encountered and that overpressures arepresent.

A reference trend line r, indicating normal compaction, can be drawn throughthe o values in the section of the hole where they constantly increase with depth.

The departure of the ovalues from the r trend line is proportional to theamount of overpressure.


66Enis Way

Th l ti t d li h t t i li ti



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The normal compaction trend line has a constant inclination:(angular coefficient a = 0,088)

Trend line equation:

where: H = depth, mb = intersection between the trend line and the x-axis

sr = aH

1000+ b

Pore Pressure Gradient Calculation

3. Calculate the PORE PRESSURE GRADIENT

2. Recalculate the mud/poredifferential pressure

F*t

r=

'

s

sP = 2 (1-F)

1 (1-F)

1

n2

sO s't1. Once youknow

GP =d - p 10H

Mud


67Enis Way

TRUE SIGMALOG



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NORMAL COMPACTION

TREND LINE

ABNORMALLY

COMPACTED AND

OVERPRESSURED ZONE

TRUE SIGMALOG

DEPTH

OVERPRESSURE TOP

True Sigmalog

Versus Depth

Plot Example


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Sigmalog:

Determination of

the ReferenceTrend Line

DETERMINATION OF THE

NORMAL COMPACTION TREND

LINE:

Vertic

aldepthm

Normal

compaction

trend

sO

sr

THE SIGMALOG METHOD: INTERPRETATION

b

SLOPE: 0.088


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Correction of the

Reference Trend

Line by Shiftsin the Normally

Compacted

Zones

sO

Vertic

alDepthm

Vertic

alDepthm

SHIFTS OF THESIGMALOG CURVE

ALLOW THE

CORRECTION OF

THE REFERENCE

TREND LINE

b1 b2b3b4

SLOPE: 0.088


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Correction of the

Reference Trend

Line by Shiftsin the

Abnormally

Compacted

Zones

VerticalD

epth

m

0,2 0,4 0,6b1

2

0s

0s

1

0s

b2


CORRECTION OF THEREFERENCE TREND LINE BYSHIFTS IN THEOVERPRESSUREDINTERVALS

OVERPRESSURE

TOP


71Enis Way

2. THE SIGMALOG METHOD

CALCULATION OF PORE PRESSURES AND GRADIENTS



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72

CALCULATION OF PORE PRESSURES AND GRADIENTS

Once the term b is defined for each shift recognizable on the Sigmalogcurve, the pore pressure gradient at any particular depth is calculated usingthe following procedure:

calculate the values assumed by the reference trend line as seen above;

calculate the term F from the equation:

define the differential pressure P as:P = [2 (1 F)]/[1 (1 F)2] (1/n)

calculate the pore pressure gradient as:

Gp = df- [(P 10)/H]

where:

- df= mud density, kg / litre


72Enis Way

THE SIGMALOG METHOD: PRESENTATION EXAMPLE Example of Pore



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73

1.1 1.3

1.50.

8

1.71.

2

1.81.

6

2.22.

00.1 0.3 0.5 0.7 0.9

124

134

128

136 231

126

124

527

134

124

124

131

131

131

131

111

111

111

500

1000

1500

2000

2500

3000

3500

A

C

B

Kick

A B C

A: Sigmalog before

interpretation with

representation of the

reference trend lines

B: Sigmalog after

interpretation with

representation of

only one reference

trend line

C: Pore pressuregradient trend with

representation of the

mud densities used

while drilling the well

At the left and right

of the curves, bit

runs, hole sizes andlithology are shown

Pressure

Gradient

Development

from

an Interpreted

Sigmalog Curve


73Enis Way


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3.4. PRESSURES PREDICTION

AND EVALUATION

FROM WIRELINE LOGS


74Enis Way

3.4.1. INTRODUCTION

The analysis of wireline logs allows the calculation of:


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- Overburden Gradients

- Pore Pressure Gradients

- Fracture Gradients

The wireline logs generally used for pressure prediction and evaluation are:

Sonic Log (SL)

Induction Log (IES)

Formation Compensated Density Log (FDC)


75Enis Way

The Sonic Log is interpreted in the same way as the seismic data; in fact

3.4.2. SONIC LOG


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both are based on the use of travel times, t, plotted versus depth.

The Sonic Log allows the calculation of:

overburden pressures and gradients

pore pressures and gradients

fracture pressures and gradients


76Enis Way

As already seen, the following relationship between t and can be written:

3.4.2.1. SONIC LOG: OVERBURDEN GRADIENT CALCULATION


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t = tmax (1) + tfl

where:

- t = transit time of sound through the formation (sec/ft)- tmax = transit time of sound through the solid frame (sec/ft)-tfl = transit time of sound in the pore fluid (sec/ft)

The previous equation can be also expressed in terms of porosity:

= (t - tmax)/( tn - - tmax)

where:

- tfl = assumed equal to 200 sec/ft (a conservative value for pore pressure

gradient calculations)- tmax = values depend on lithology


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SOLID Density tMATTER g/cm3 microseconds/ft

Dolomite 2.87 43.5

Limestone 2.71 43.5 - 47.5

Anhydrite 2.96 50

Shale 2.70 47

Transit Times and Density for Some Rock Matrices


78Enis Way

Laboratory tests proved the following relations correct.



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y p g

Relat ion between transi t t ime and poro si ty

for conso l idated and cemented rocks

for uncon sol idated sands

for shales

Relat ion between densi ty and trans i t t ime

for cemented and compacted formations

for non-cemented formations

f = (1)t- t153

ma

ft +

(2)= 1.228 t- tma

200

f (3)= 1.568 t - tt +

ma 200

d = 3.28 - t89

(4)

d =2.75-2.11 tt+

- tma

200

(5)


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Comparisons with density values detected by the FDClog (formation

density compensated log) attested and verified that the following relation

is valid for all formations:

b +2 75 2 11

47

200. .

t

t

where:

47 = t max (transit time through the rocky matrix assumed to be equal to 47 sec/ft

200 = t in water, sec/ft


80Enis Way

To determine the exact transit time in an interval ofH thickness, the following procedurecan be applied:

th b f illi d l d f th d t th f ti i t l



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81

the number of milliseconds elapsed for the sound wave to cross the formation interval

having thickness H is read (this is done by counting the ITT pips on the SL to the left)

on the log; this value is multiplied by 1,000 to change milliseconds into microseconds, and is

divided by the value ofH; in order to obtain the values oft, measured in sec/ft, hereH must be changed from meters into ft by dividing by 3.28; the final expression represents the average transit time in the H interval and takes thesimple form:

t = (K 1000)/(3.28 H)

where:

- K = milliseconds required by the sound wave to pass through a section of height H:- H = depth interval, m

The t values can be also more rapidly read on the log by taking into account theintervals characterized by more or less the same values.


81Enis Way

SONIC LOG INTERPRETATION



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1 82

SONIC LOG INTERPRETATION

ITT = INTEGRATED TRANSIT TIME

H = 15.5 m

K = 8.2

t = 161 sec/ft


82Enis Way

Once determined the bulk density versus depth with the equations seen above,

the procedure for the calculation of the overburden pressure and relevant



83/104


83

the procedure for the calculation of the overburden pressure and relevant

overburden gradient using the transit times read on a Sonic Log is the same as

discussed when dealing with seismic data interpretation, to which the reader

has to refer. Therefore, the basic equations to consider are, as already known,

the following:

Enis Way

Overburden Gradient,kgf/cm2/10 m



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84

BULK DENSITY

0

1000

2000

3000

4000

5000

1,6 1.8 2.0kg/dm3

2.2 2.4

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1.5

V

erticaldepthm

2.0 2.5 3.0kgf/cm2/10 m

Enis Way

20 60 100 200The plot of Transit Times t, asobtained from Sonic Log readings,

t (s/ft)

3.4.2.2. SONIC LOG: PORE PRESSURE GRADIENT CALCULATION


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85

500

1000

1500

2000

2500

obtained from Sonic Log readings,

versus depth is the most common

method for overpressure detectionand calculation, being a quick and

reliable tool. As in the case of

seismics, the transit times in shales

decrease regularly with depth in

normally compacted and normally

pressured formations. This is due tothe fact that the density of the rocks

increases with depth in normal

conditions and increases also the

velocity with which the sound waves

propagate through them. The transit

times will lay on a straight line, whichis the normal compaction trend line.

Enis Way

10080 200

Considering always valid the assumption

that density, porosity and relevantt (s/ft)



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86

Ve

rticalDepthm

0

500

1000

1500

2000

2500

60 10080 200y, p y

pressures (effective and pore pressure)

are correlated, it derives very clearly that

the t will decrease regularly with thedepth when normal conditions exist and

that an increase in its values, on the

contrary, will determine its departure from

the reference trend line and will be

indicative of abnormal conditions

(undercompaction and overpressures). OVERPRESSURES TOP

OVERPRESSURED shales are

UNDERCOMPACTED and have aHIGHER PORE WATER CONTENT.

Therefore they have a HIGHER t,compared to the depth at which theylie.

01008060 200t (s/ft)

In the t vs depth plots, it is possibleti t b th hift d



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87

Verticalde

pthm

500

1000

1500

2000

sometimes to see both shifts and

slope changes in the reference trend

line. Very often these anomalies,clearly indicated by an abrupt

dislocation of the points rightwards or

leftwards, are not related to

overpressures but to particular

geological conditions (overcompacted

formations, for instance in the upper

part of the well) and, therefore, have to

be corrected during interpretation.

TRUE OVERPRESSURES TOP

ERRONEOUS OVERPRESSURES TOP

OVERCOMPACTED

SECTION

Once the curve has been interpreted and adequately corrected, the pore



88/104


88

pressure gradients are calculated by the usual equivalent depth method orby the Eaton method, as already seen.

microseconds/ft

0

01.5 2.0 2.5 3.0

kgf/cm2/10 m

2 36 2 50



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89

Verticaldepth(m)

10 20 30 50 100 200 300 500 1.000

500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

5.000

500

1000

1500

2000

2500

3000

3500

4000

4500

Verticaldepthm

POINT A:- Pov = 1125 kgf/cm

2

- Pc = 332 kgf/cm2

- Pp = 1125 - 332 =

793 kgf/cm2

- Gp = 1.76 kgf/cm2/10m

POINT B:- Pov = 590 kgf/cm2

- Pp = 258 kgf/cm2

- Pc = 590 - 258 =

332 kgf/cm2

POINT A

POINT B

2.36 2.50

2500 m

4500 m

4500 m

2500 m

EQUIVALENT DEPTH PRINCIPLE

Once the overburden and pore pressures gradients have been calculated, the

fracture gradients can be easily derived by applying the following usual

ti

3.4.2.3. SONIC LOG: FRACTURE GRADIENT CALCULATION


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90

equations:

a) The fracture gradient, showing the rate at which the fracture pressurechanges with depth, is given, for a rock having an elastic behaviour, by the

followng expression:

Gfr= Gp + ( 2 ) (Gov Gp)1-

b) If the drilling fluid is water or invades in depth the formation, the relationships

becomes:

Gfr= Gp + ( 2 ) (Gov Gp)

c) If the rock has a plastic behaviour, the fracture gradients is given by the

formula:

Gfr= Gov

The resistivity of a rock depends on its porosity and on the amount of fluids

present in the pores; therefore low porosity rocks are usually high resistivity

3.4.3. SHALE RESISTIVITY: PORE PRESSURE GRADIENTS CALCULATION


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present in the pores; therefore low porosity rocks are usually high resistivity

formations (for instance, compacted limestone, volcanic rocks, etc).

If all other conditions are the same, the resistivity of a rock depends on:

saline concentration of the fluids within the pores;

rock composition;

formation temperature.

As depth increases, shales are more and more compacted and less porous,

therefore their resistivity tends to progressively increase.


91Enis Way

There are two methods of overpressure analysis based on

3.4.3. SHALE RESISTIVITY: PORE PRESSURE GRADIENTS CALCULATION


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Shale Resistivity:

Method 1 - Find shale resistivity on the recorded electric log,

and plot it directly on a semi-logarithmic scale,

without any elaboration.

Method 2 - Analyze the F shale factor (shale formation

factor).


92Enis Way

3.4.3.1. SHALE RESISTIVITY PLOT


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93

RESISTIVITY LOG


93Enis Way

METHOD 1The resistivity of the cleanest shales is plotted Vs depth, on a semi-logarithmic scale. The

inversely proportional relationship existing between resistivity and porosity (meant as fluid content)



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y p p p g y p y ( )

will produce a diagram, whose values will increase with the depth in case of normal compaction

conditions.

Depth

Shale resistivityTherefore, in case of normally

compacted and normally pressured

formations, the resistivity values

will increase with the depth and will

lie on the normal compaction trend

line, as shown in Figure aside.

Formation with a NORMAL

pressure gradient.

As depth increases, so does

compaction, while porosity

decreases


94Enis Way

METHOD 1In case of overpressured formations, the resistivity values, as obtained from the log

readings, will tend to depart from the normal compaction trend line, assuming lower values



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than expected for that depth of burial.

Overpressure Top

Shale resistivity

Depth

This depends on the fact that more

fluid with more salt is present in the

pores of the rock and this makes the

resistivity to decrease. The more the

points depart from the reference

trend line, the higher the pore

pressure gradient will be.

OVERPRESSURED formations

Undercompacted shales with

high porosity, compared to

the depth where they lie.


95Enis Way

METHOD 2In this case, it is not made reference only to the resistivity values as read on the log,

but it is

necessary to determine the Shale Formation Factor, or simply the Fshale factor, which is the

3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH


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F shale- Normal-Gradient Formations

y p y

ratio between the shale resistivity and the formation water resistivity, as shown below:

Vertical

depth

m

00.1000.0800.060 0.200

500

1000

1500

2000

2500

wshalew

shaleshale

RCR

RF

1

where:

- Rshale = shale resistivity

- Cshale = shale conductivity

- Rw = formation water resistivity


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0 1000 0800 060 0 200

METHOD 2Also in this case, if overpressured

formations are encountered, a decrease in



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,

00.1000.0800.060 0.200

500

1000

1500

2000

2500

F shale-

OverpressuredFormations

Verticaldepth

m

OVERPRESSURE

In presence of undercompacted

shales, hencein OVERPRESSURE,

Fshale values decrease whencompared to the normalcompaction trend.

,

Fsh values can be observed; again, higher

is the departure of the points from thereference trend line, higher will be the pore

pressure gradient values.

OVERPRESSURES

TOP


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CALCULATION SEQUENCE

The operational sequence required to compute pore pressure gradients by means of the



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Fshale technique is as follows:

1. Calculate RW, that is the formation water RESISTIVITY along the whole well profile.

2. PlotRWvalues on a semi-logarithmic scale.

3. Read the Condu ct iv i tyvalue in the login correspondence of clean shales, along the whole

well profile.

4. Calculate the F shalevalue.

5. Plot the F shale on semi-logarithmic paper.

6. Draw the F shalenormal compaction trend.

7. Interpret the F shale variation and observe the possible presence of overpressures.


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1. FORMATION WATER RESISTIVITY (RW) calculation along the entire well profile

CALCULATION SEQUENCE



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1. Create a chart, copying at different depths the well temperatures recorded on the electric log.

2. Read the Spontaneous Potential (SP) variation value in correspondence of the cleanestsands.

3. Find the Rmf(resistivity of mud filtrate in well) value with the corresponding temperature.on the log heading

4. Enter the mud and temperature Rmfvalues read on the log heading in theSCHLUMBERGER Gen 9 A-6 diagram , and read the Rmfvalues at depth andtemperature of SP values.

5. Calculate the (Rmf)evalue: if Rmf at 75 F is > than 0.1 ohm-m(Rmf)e=Rmfx 0.85. If Rmf at 75 F is < than 0.1 ohm-m, (Rmf)e is to be foundin the Schlumberger SP-2 A12 diagram.

Please note: This is true for water-based muds with the exclusion of lime and

gypsum-based muds.

6. Enter the pair ofSP and temperaturevalues in the SCHLUMBERGER SP-1 A-10diagram, and read the (Rmf)e/ (RW)e ratio values.


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CALCULATION SEQUENCE (continued)



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8. Obtain RW for the entire well. Entering the (RW)e values and correspondingtemperatures in the Schlumberger SP-2 A12 diagram, obtain RW values along thewhole well profile.

( )RW e

( )Rm f e

( )Rmf e

( )RW e

=

7. Once the (Rmf)e terms and the (Rmf)e/ (RW)e ratio are known,

calculate (RW)e using this equation:


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010 100 1000

The Rw values, as

obtained with the

d d t il d



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1000

2000

3000

4000

5000

Verticaldepth

(m)

MEAN RW VALUES(RAVENNA SEA ZONE) OHMm

procedure detailed

above, are then plotted

versus depth, obtaining a

curve, similar to that

shown in Figure.

Each well or area is

characterized by a

specific Rw curve.


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CALCULATION SEQUENCE (continued)



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9. In the Resistivity/Conductivity log read the Conductivity value in correspondence ofclean shales .

10. Compute the F shale value using the following relation:

11. Plot the F shale values on semi-logarithmic paper.

12. Draw the shale normal compaction trend and interpret F shale variation to detect thepossible presence of overpressures.

F C Rshale shale W

=1

x Cshale= Shale condu ct iv i ty


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" Fshale

0,01 0,10 1,00

The Fshale values

plotted versus

d th i di t



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Verticaldepth

m

0

1000

2000

3000

4000

5000

depth indicate

the presence of

overpressures

when they

depart

(decreasing)

from the normal

compaction

trend line.

Overpressures Top

Normally Pressured

Zone

Overpressured Zone


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The main limits of the methods based on Resistivity Measurements

can be summarized as here follows:



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They can never be applied reliably to CARBONATES.

They can only be applied in presence of frequent interbeddings

of shales and sands.

The SP (Spontaneous Potential) value between shales andsands must be clearly detectable.

Shalesmust be clean.

Fluids contained in shales (gas or oil) modify the Conductivity value.

Borehole must not be caved in (say its geometry must be regular).


104Enis Way

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Lesson 3 - Methods Used in Overpressure Analysis

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