Pore Pressure - Prediction

8/10/2019 Pore Pressure - Prediction

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TAMU - PemexWell Control

Lesson 7

Pore Pressure Prediction


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Contents

Porosity

Shale Compaction

Equivalent Depth Method

Ratio Method

Drilling Rate

dC-Exponent

Moores Technique

Combs Method


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Pore pressure prediction

methods

Most pore pressure prediction

techniques rely on measured or inferredporosity.

The shale compaction theory is thebasis for these predictions.


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Pore pressure prediction methods

Measure the porosity indicator (e.g.density) in normally pressured, clean

shales to establish a normal trend line.

When the indicator suggests porosityvalues that are higher than the trend, then

abnormal pressures are suspected to be

present.

The magnitude of the deviation from the

normal trend line is used to quantify the

abnormal pressure.


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2. Extrapolate

normal trend

line

1. Establish NormalTrend Line in good

clean shale

ransition

Porosity should

decrease with

depth in normally

pressured shales

3. Determine the

magnitude

of the deviation


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Older shales have had

more time to compact,

so porosities wouldtend to be lower (at a

particular depth).

Use the trend line

closest to the transition.

Lines may or may notbe parallel.


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D

De


The normally compacted

shale at depth Dehas thesame compaction as the

abnormally pressured

shale at D. Thus,

sV= sVe

i.e., sob- pp= sobe- pne

pp= pne+ (sob- sobe)

sob= sV+ pp


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Example 2.6

Estimate the pore pressure at 10,200 if the

equivalent depth is 9,100. The normal pore

pressure gradient is 0.433 psi/ft. The

overburden gradient is 1.0 psi/ft.

At 9,100, pne= 0.433 * 9,100 = 3,940 psig

At 9,100, sobe= 1.00 * 9,100 = 9,100 psig

At 10,200, sob= 1.00*10,200 = 10,200 psig


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Solution

pp= pne+ (sob- sobe) . (2.13)

= 3,940 + (10,2009,100)

pp= 5,040 psig

The pressure gradient,

gp= 5,040/10,200

= 0.494 psi/ft

EMW = 0.494/0.052 = 9.5 ppg


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Xn Xo

The Ratio Method

uses (Xo/Xn) to predict

the magnitude of theabnormal pressure

We can use:

drilling rate

resistivities

conductivities

sonic speeds

Shale Porosit Indicator

Depth


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Pore pressures can be

predicted:

Before drilling (planning)

During drilling.

After drilling


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Before drilling the well

(planning)

Information from nearby wells

Analogy to known characteristics of the

geologic basin

Seismic data


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Table 2.6Contd


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Seismic Surveys, as used in conventional geophysical

prospecting, can yield much information about underground

structures, and depths to those structures. Faults, diapirs, etc.

may indicate possible locations of abnormal pressures


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Typical Seismic Section


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Under normal

compaction, density

increases withdepth. For this

reason the interval

velocity also

increases with

depth, so travel

time decreases

Dt = Dtma(1-f) + Dtf f


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Sound moves faster in

more dense medium

In air at sea level,

Vsound= 1,100 ft/sec

In distilled water,


In low density, high porosity

rocks,

Vsound= 6,000ft/sec

In dense dolomites,



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Example 2.7

Use the data in Table 2.7 to determine

the top of the transition zone, and

estimate the pore pressure at 19,000

using the equivalent depth method

using Pennebakers empirical correlation

Ignore the data between 9,000 and11,000. Assume Eatons Gulf Coast

overburden gradient.


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Solution

Plot interval travel time vs. depth onsemilog paper (Fig. 2.31)

Plot normal trend line using the6,000-9,000 data.

From Fig. 2.20, at 19,000,

gob= 0.995 psi/ft

(sob)19,000= 0.995 * 19,000 = 18,905 psig


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Use

Ignore

Equivalent Depth

Method:

From the vertical line,De= 2,000

sobe= 0.875 * 2,000

=1,750 (Fig. 2.20)

But,

pne= 0.465 * 2,000

= 930 psigpp = 930 +

(18,905-1,750)

pp= 18,085 psig

Dtn Dto


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Pennebakers

correlation for Gulf

Coast sediments

Higher travel time meansmore porosity and higher

pore pressure gradient

Example 2.7 (Table 2.7)

Dto= 95 msec/ft @ 19,000

Dtn= 65 msec/ft @ 19,000

Dto/ Dtn= 95/65 = 1.46pp= 0.95 * 19,000

= 18,050 psig

0.95

Fig. 2.30


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Comparison

Pore Pressure at a depth of 19,000 ft:

Pennebaker:

18,050 psi or 0.950 psi/ft or 18.3 ppg

Equivalent Depth Method:

18,085 psi or 0.952 psi/ft or 18.3 ppg


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While Drilling

dc-exponent

MWD & LWD

Kicks

Other drilling rate factors (Table 2.5)


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TABLE 2.5 -


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Penetration rate and abnormal pressure

Bits drill through overpressured rockfaster than through normally pressured

rock (if everything else remains the

same).

When drilling in clean shales this fact

can be utilized to detect the presence

of abnormal pressure, and even toestimate the magnitudeof the

overpressure.


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Note, that many factors can influence the drilling rate,

and some of these factors are outside the control of

the operator.

TABLE 2.8 -


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Effect of bit weight and hydraulics

on penetration rate

Inadequate

hydraulics or

excessive

imbedding of

the bit teeth in

the rock

Drilling rate

increases more

or less linearly

with increasing

bit weight.

A significant

deviation from

this trend may

be caused by

poor bottom

hole cleaning

0


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Effect of Differential Pressure on Drilling Rate

Differentialpressure is the

difference between

wellbore pressure

and pore fluidpressure

Decrease can be due to:

The chip hold down effect

The effect of wellbore

pressure on rock strength


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Drilling

underbalanced

can further

increase the

drilling rate.

Th hi h ld d ff t


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The chip hold-down effect

The mud pressure

acting on the

bottom of the hole

tends to hold the

rock chips in

place

Important hold-down parameters:

Overbalance Drilling fluid filtration rate

Permeability Method of breaking rock (shear or crushing)


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Drilling rates are influenced by rock strengths.

Only drilling rates in relatively clean shales are useful for

predicting abnormal pore pressures.

TABLE 2.9 -


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sobis generally

the maximum insitu principal

stress in

undisturbed rock


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Stresses on Subsurface Rocks

sob, sH1, sH2and p all tend to increase

with depth

sobis in general the maximum in situ

principal stress.

Since the confining stresses sH1andsH2increase with depth, rock strength

increases.


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Stresses on Subsurface Rocks

The pore pressure, p, cannot produceshear in the rock, and cannot deform

the rock.

Mohr-Coulomb behavior is controlled bythe the effective stresses (matrix).

When drilling occurs the stresses

change.

sobis replaced by dynamic drilling fluid

pressure.


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The degree of

overbalance now

controls the

strength of the

rock ahead of the

bit.


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Rock failure caused by roller cone bit.

The differential pressure from above provides

the normal stress, so

Formation fracture is resisted by the shear stress, to,

which is a function of the rock cohesion and the friction

between the plates. This friction depends on so.


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Fig. 2.41 - Differential Pressure 0.1 in below the bit.

When sobis replaced by phyd(lower) the rock immediately below the

bit will undergo an increase in pore volume, associated with a

reduction in pore pressure.

In sandstone this pressure is increased by fluid loss from the mud.

(Induced

Differential

Pressure in

Impermeable

rock.

FEM Study)

Vertical Stress = 10,000 psi

Horizontal Stress = 7,000 psi

Pore Pressure = 4,700 psiWellbore Pressure = 4,700 psi


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Drilling Rate as a Pore

Pressure PredictorPenetration rate depends on a number

of different parameters.

R = K(P1)a1 (P2)

a2 (P3)a3 (Pn)

an

A modified version of this equation is:

d

bd

WNKR

3


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Drilling Rate as a Pore

Pressure Predictor

Or, in its most

used form:

inDiameter,Bitd

lbf,Bit WeightW

exponentdd

rpmN

ft/hrR

10

12

log

60log

b

6

bd

W

N

R

d

d

bd

WNKR

3


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d-exponent

The d-exponent normalizes R for any

variations in W, dband N

Under normal compaction, R shoulddecrease with depth. This would cause

d to increase with depth.

Any deviation from the trend could becaused by abnormal pressure.


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d-exponent

Mud weight also affects R..

An adjustment to d may be made:

dc= d (rn/rc)

where

dc

= exponent corrected for mud density

rn= normal pore pressure gradient

rc = effective mud density in use


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Example

While drilling in a Gulf Coast shale,

R = 50 ft/hr

W = 20,000 lbf

N = 100 RPM

ECD = 10.1 ppg (Equivalent Circulating Density)

db= 8.5 in

Calculate d and dc


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Solution

34.1d

554.1

079.2

5.8*10

000,20*12log

100*60

50log

d

6

bd

W

N

R

d

610

12log

60log

c

n

c dd

19.1d

1.10*052.0465.034.1d

c

c


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Example 2.9

Predict pore pressure at 6,050 ft (ppg):

from data in Table 2.10 using:

Rhem and McClendons correlation

Zamoras correlation

The equivalent depth method


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TABLE 2.10

d-EXPONENTAND MUD

DENSITY DATA

FOR A WELL

LOCATEDOFFSHORE

LOUISIANA


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Step 1 is to plot the

data on Cartesian

paper (Fig. 2.43).

Transition at 4,700 ft?

or is it a fault?

Seismic data and

geological indicators

suggest a possible

transition at 5,700 ft.


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Fig. 2.43

Slope of 0.000038 ft-1


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Rehm and McClendon

gp= 0.398 log (dcn-dco) + 0.86

= 0.398 log (1.18 - 0.95) + 0.86

gp= 0.606 psi/ft

rp= 0.606 / 0.052 = 11.7 ppg


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Zamora

From Fig. 2.44

gp= gn(dcn/dco)

= 0.465 * (1.18/.95)gp= 0.578 psi/ft

rp= 0.578/0.052

p= 11.1 ppg

1.180.95


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Equivalent

Depth Method

From Fig. 2.20, at

6,050 ft,

gob= 0.915 psi/ft

sob= 0.915 * 6,050

= 5,536 psi


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Equivalent

Depth Method

From Fig. 2.43,

Equivalent Depth

= 750 ft

At 750 ft,

sobe= 0.86 * 750

= 645 psipne= 0.465 * 750

= 349 psig


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From Eq. 2.13, at 6,050 ft

pp= pne+ (sob- sobe)

pp= 349 + (5,536 - 645) = 5,240 psigrp= 19.25 * (5,240 / 6,050) = 16.7 ppg

Perhaps the equivalent depth method isnot always suitable for ppprediction

using dc !!


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Overlays such as this can be

handy, but

be careful that the scale is

correct for the graph paper

being used;

the slope is correct fornormal trends;

the correct overlay for the

formation is utilized.


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To improve pore pressure predictions

using variations in drilling rate:

Try to keep bit weight and rpm relatively

constant when making measurements

Use downhole (MWD) bit weights whenthese are available. (Frictional drag in

directional wells can cause large errors)

Add geological interpretation when

possible. MWD can help here also.


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Improved pore pressure

predictions

Keep in mind that tooth wear can

greatly influence penetration rates.

Use common sense and engineering

judgment.

Use several techniques and compare

results.


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Moores Technique

Fig. 2.45

Moore proposed a practical

method for maintaining a

pore-pressure overbalance

while drilling into atransition.

Drilling parameters must bekept constant for this

technique to work.


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Combs Method

Combs attempted to improve on the

use of drilling rate for pore pressure by

correcting for:

hydraulics

differential pressure

bit wear

in addition to W, db, and N

Combs Method


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Combs Method

Nd

a

nb

aa

b

d tfpfdd96

q200N

d500,3WRR

qNW

q = circulating rate

dn= diameter of one bit nozzle

f(pd) = function related to the differential pressure

f(tN) = function related to bit wear

aW= bit weight exponent = 1.0 for offshore Louisiana

aN= rotating speed exponent = 0.6 for offshore Louisiana

aq= flow rate exponent = 0.3 for offshore Louisiana

Tooth wear factor


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Tooth wear factor

Correctionwould depend

upon bit type,

rock hardness,

and

abrasiveness

Differential pressure factor


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Differential pressure factor

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Pore Pressure - Prediction