7A. Pore Pressure Prediction

7/21/2019 7A. 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 MethodRatio Method

Drilling Rate

dC-Exponent

Moores Technique

Coms Method


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Pore pressure prediction

methods

Most pore pressure prediction

techniques rely on measured or in!erredporosity"

The shale compaction theory is the

asis !or these predictions"


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Pore pressure prediction methods

Measure the porosity indicator #e"g"density$ in normally pressured% clean

shales to estalish a normal trend line"

&hen the indicator suggests porosityvalues that are higher than the trend% then

anormal pressures are suspected to e

present"

The magnitude o! the deviation !rom the

normal trend line is used to quanti!y the

anormal pressure"


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2. Extrapolate

normal trend

line

1. Establish NormalTrend !ine in "ood

clean shale

Tra siti

Porosit# should

decrease $ith

depth in normall#

pressured shales

%. &etermine the

ma"nitude

o' the de(iation


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'lder shales have had

more time to compact%

so porosities (ould

tend to e lo(er #at a

particular depth$"

)se the trend line

closest to the transition"

*ines may or may note parallel"


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D

De

E)ui(alent &epth Method

The normally compacted

shale at depth Dehas the

same compaction as the

anormally pressured

shale at D" Thus%

V= Ve

i.e., ob- pp= obe- pne

pp

= pne

+ (ob

- obe

)

ob= V+ pp


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Example 2.*

Estimate the pore pressure at +,%,, i! the

equivalent depth is .%+,," The normal pore

pressure gradient is ,"/00 psi1!t" The

overurden gradient is +", psi1!t"

2t .%+,,% pne3 ,"/00 4 .%+,, 3 0%./, psig

2t .%+,,% oe3 +",, 4 .%+,, 3 .%+,, psig

2t +,%,,% o3 +",,4+,%,, 3 +,%,, psig


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+olution

pp3 pne5 #o- oe$ 66666" #"+0$ 3 0%./, 5 #+,%,, 7 .%+,,$

pp, /0/ psi"

The pressure gradient%

gp3 8%,/,1+,%,,3 ,"/./ psi1!t

EM& 3 ,"/./1,",8 , . pp"


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Xn

Xo

The atio Method

uses #9o19n$ to predict

the magnitude o! the

anormal pressure

&e can use:

;drilling rate

;resistivities

;conductivities

;sonic speeds

+hale Porosit 3ndicator

&epth


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Pore pressures can be

predicted4


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5e'ore drillin" the $ell

6plannin"7

=n!ormation !rom neary (ells

2nalogy to >no(n characteristics o! the

geologic asin

Seismic data


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Table 2.* 8 Cont9d


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+eismic +ur(e#s% as used in conventional geophysical

prospecting% can yield much in!ormation aout underground

structures% and depths to those structures" ?aults% diapirs% etc"

may indicate possile locations o! anormal pressures


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T#pical +eismic +ection


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)nder normal

compaction% density

increases (ithdepth" ?or this

reason the interval

velocity also

increases (ith

depth% so travel

time decreases

t = tma(1-) + tf


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+ound mo(es 'aster in

more dense medium

=n air at sea level%

@sound3 +%+,, !t1sec

=n distilled (ater%

@sound

3 /%A,, !t1sec

=n lo( density% high porosity

roc>s%

@sound3 A%,,,!t1sec

=n dense dolomites%

@sound3 ,%,,, !t1sec


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Example 2.:

)se the data in Tale "B to determine

the top o! the transition one% and

estimate the pore pressure at +.%,,,

using the equivalent depth method

using Pennea>ers empirical correlation

=gnore the data et(een .%,,, and++%,,," 2ssume Eatons ul! Coast

overurden gradient"


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+olution

Plot interval travel time vs" depth onsemilog paper #?ig" "0+$

Plot normal trend line using the A%,,,-.%,,, data"

?rom ?ig" ",% at +.%,,,% go3 ,"..8

psi1!t

#o$+.%,,,3 ,"..8 4 +.%,,, 3 +%.,8 psig


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Use

!no"e

E)ui(alent &epth

Method4

?rom the vertical line%De3 %,,,

oe3 ,"B8 4 %,,,

3+%B8, #?ig" ",$


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Penneba


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Comparison

Pore Pressure at a depth o' 1/// 't4

Pennea>er:

+%,8, psi or ,".8, psi1!t or +"0 ppg

Equivalent Depth Method:

+%,8 psi or ,".8 psi1!t or +"0 ppg


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?hile &rillin"

dc-exponent

M&D G *&D

Hic>s

'ther drilling rate !actors #Tale "8$


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TABLE 2.5 -


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Penetration rate and abnormal pressure

!aster than through normally pressured

roc> #i! everything else remains the

same$"

&hen drilling in clean shales this !act

can e utilied to detect the presence

o! anormal pressure% and even toestimate the ma"nitudeo! the

overpressure"


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Iote% that many !actors can in!luence the drilling rate%

and some o! these !actors are outside the control o!

the operator"

TABLE 2.8 -


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E''ect o' bit $ei"ht and h#draulics

on penetration rate

=nadequate

hydraulics or

excessive

imedding o!

the it teeth in

the roc>

Drilling rate

increases more

or less linearly

(ith increasing

it (eight"

2 signi!icantdeviation !rom

this trend may

e caused y

poor ottom

hole cleaning

/


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E''ect o' &i''erential Pressure on &rillin" ate

Di!!erentialpressure is the

di!!erence et(een

(ellore pressure

and pore !luidpressure

&ecrease can be due to4

;The chip hold do$n e''ect

;The e''ect o' $ellbore

pressure on roc< stren"th


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&rillin"

underbalanced

can !urther

increase the

drilling rate"

Th hi h ld d '' t


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The chip hold-do$n e''ect

The mud pressure

acting on the

ottom o! the hole

tends to hold the

roc> chips in

place

3mportant hold-do$n parameters4

'veralance Drilling !luid !iltration rate

Permeaility Method o! rea>ing roc> #shear or crushing$


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;Drilling rates are in!luenced y roc> strengths"

;'nly drilling rates in relatively clean shales are use!ul !or

predicting anormal pore pressures"

TABLE 2.9 -


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ois generallythe maximum in

situ principal

stress in

undistured roc>


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+tresses on +ubsur'ace oc


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+tresses on +ubsur'ace oc% and cannot de!orm

the roc>"

Mohr-Coulom ehavior is controlled ythe the e!!ective stresses #matrix$"

&hen drilling occurs the stresses

change"

ois replaced y dynamic drilling !luidpressure"


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The degree o!

overalance no(

controls the

strength o! the

roc> ahead o! the

it"


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oc< 'ailure caused b# roller cone bit.

The di!!erential pressure !rom aove provides

the normal stress% o

'o"mation f"a#t"e is "esiste b* te sea" st"ess, o,i# is a fn#tion of te "o# #oesion an te f"i#tion

beteen te pates. /is f"i#tion epens on o.


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>i". 2.01 - &i''erential Pressure /.1 in belo$ the bit.

&hen ois replaced y phyd#lo(er$ the roc> immediately elo( theit (ill undergo an increase in pore volume% associated (ith a

reduction in pore pressure"

=n sandstone this pressure is increased y !luid loss !rom the mud"

(n#e

iffe"entia

"ess"e in

mpe"meabe

"o#.

' t*)

@ertical +tress , 1//// psi

oriBontal +tress , :/// psi

Pore Pressure , 0:// psi?ellbore Pressure , 0:// psi


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&rillin" ate as a Pore

Pressure PredictorPenetration rate depends on a numer

o! di!!erent parameters"

R 3 H#P+$a+ #P-$a- #P0$a06 #Pn$an

2 modi!ied version o! this equation is:

d

bd

WNKR

= 3


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&rillin" ate as a Pore

Pressure Predictor

'r% in its most

used !orm:

iniamete",5it)

.bf,5it ei!+t

e7ponent))

"pm8

ft$+"9

10

12

.o!

60.o!

b

6

==

=

==

=

bd

W

N

R

d

d

bd

WNKR

= 3


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d-exponent

The d-exponent normalies R !or any

variations in &% dand I

)nder normal compaction% R shoulddecrease (ith depth" This (ould cause

d to increase (ith depth"

2ny deviation !rom the trend could e

caused y anormal pressure"


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d-exponent

Mud (eight also a!!ects R6""

2n adJustment to d may e made:

dc3 d #n1c$(here

dc3 exponent corrected !or mud density

n3 normal pore pressure gradient

c 3 e!!ective mud density in use


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Example

&hile drilling in a ul! Coast shale%

R 3 8, !t1hr

& 3 ,%,,, l!

I 3 +,, RPM

ECD 3 +,"+ ppg #Equivalent Circulating Density$

d3 "8 in

Calculate d and dc


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+olution

0/"+d

88/"+

,B."

8"4+,

,,,%,4+log

+,,4A,

8,log

d

A

=

=

=

bd

W

N

R

d

*1/

12lo"

*/lo"

c

n

c dd

+."+d

+"+,4,8",/A8",0/"+d

c

c

=

=


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Example 2.

Predict pore pressure at A%,8, !t #ppg$:

!rom data in Tale "+, using:

Rhem and McClendons correlation

Kamoras correlation

The equivalent depth method


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TA5!E 2.1/

d-E9P'IEIT2ID M)D

DEIS=TL D2T2

?'R 2 &E**

*'C2TED'??SF'RE

*')=S=2I2


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+tep 1 is to plot the

data on Cartesian

paper 6>i". 2.0%7.

Transition at /%B,, !t

6or is it a !ault

Seismic data and

geological indicators

suggest a possile

transition at 8%B,, !t"


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>i". 2.0%

ope of 0.000038 ft-1


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ehm and McClendon

gp3 ,"0. log #dcn-dco$ 5 ,"A

3 ,"0. log #+"+ - ,".8$ 5 ,"A"p, /.*/* psi't

p3 ,"A,A 1 ,",8 , 11.: pp"


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Damora

>rom >i". 2.00

gp3 gn#dcn1dco$

3 ,"/A8 4 #+"+1".8$

gp3 ,"8B psi1!t

p3 ,"8B1,",8

p, 11.1 pp"

1.180.95


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E)ui(alent

&epth Method

?rom ?ig" ",% at

A%,8, !t%

go3 ,".+8 psi1!t

o3 ,".+8 4 A%,8,3 8%80A psi


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E)ui(alent

&epth Method

?rom ?ig" "/0%

Equivalent Depth

3 B8, !t

2t B8, !t%

oe3 ,"A 4 B8,

3 A/8 psipne3 ,"/A8 4 B8,

3 0/. psig


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E)ui(alent &epth Method

?rom Eq" "+0% at A%,8, !t

pp3 pne5 #o- oe$

pp3 0/. 5 #8%80A - A/8$ 3 8%/, psigp3 +."8 4 #8%/, 1 A%,8,$ 3 +A"B ppg

Perhaps the equivalent depth method isnot al(ays suitale !or ppprediction

using dc NN


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'verlays such as this can e

handy% ut

e care!ul that the scale is

correct !or the graph paper

eing usedO

the slope is correct !ornormal trendsO

the correct overlay !or the

!ormation is utilied"


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To impro(e pore pressure predictions

usin" (ariations in drillin" rate4

Try to >eep it (eight and rpm relatively

constant (hen ma>ing measurements

)se do(nhole #M&D$ it (eights (hen

these are availale" #?rictional drag in

directional (ells can cause large errors$

2dd geological interpretation (hen

possile" M&D can help here also"


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3mpro(ed pore pressure

predictions

Heep in mind that tooth (ear can

greatly in!luence penetration rates"

)se common sense and engineering

Judgment"

)se several techniques and compare

results"


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Moores Technique

>i". 2.0

Moore proposed a practical

method !or maintaining a

pore-pressure overalance

(hile drilling into atransition"

Drilling parameters must e>ept constant !or this

technique to (or>"


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Comb9s Method

Coms attempted to improve on the

use o! drilling rate !or pore pressure y

correcting !or:

hydraulics

di!!erential pressure

it (ear

in addition to &% d% and I

C b9 M th d


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Comb9s Method

( ) ( )Id

a

n

aa

d t!p!dd.A

q-,,I

d8,,%0&RR

qI&

=

: = #i"#atin! "ate

n= iamete" of one bit no;;e

f(p) = fn#tion "eate to te iffe"entia p"ess"e

f(t) = fn#tion "eate to bit ea"

a= bit ei!t eponent = 1.0 fo" offso"e


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Tooth $ear 'actor

Correction(ould depend

upon it type%

roc> hardness%

and

arasiveness

&i''erential press re 'actor


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&i''erential pressure 'actor

7A. Pore Pressure Prediction

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