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
Equivalent Depth Method
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,
Vsound= 4,600 ft/sec
In low density, high porosity
rocks,
Vsound= 6,000ft/sec
In dense dolomites,
Vsound= 20,000 ft/sec
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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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Equivalent Depth Method
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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