TAMU - Pemex
Well Control
Lesson 7Pore Pressure Prediction
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Contents
Porosity
Shale Compaction
Equivalent Depth Method
Ratio Method
Drilling Rate
dC-Exponent
Moore’s Technique
Comb’s Method
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Pore pressure prediction methods
Most pore pressure prediction techniques rely on measured or inferred porosity.
The shale compaction theory is the basis 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 porosity values 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 “Normal” Trend Line in good
“clean” shale
Transition
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 would tend to be lower (at a particular depth).
Use the trend line closest to the transition.
Lines may or may not be parallel.
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D
De
Equivalent Depth Method
The normally compacted shale at depth De has the same compaction as the abnormally 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 2.6
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’, obe = 1.00 * 9,100 = 9,100 psig
At 10,200’, ob = 1.00*10,200 = 10,200 psig
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Solution pp = pne + (ob - obe) ……………. (2.13)
= 3,940 + (10,200 – 9,100)
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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XnXo
The Ratio Method
uses (Xo/Xn) to predict the magnitude of the abnormal pressure
We can use:
• drilling rate
• resistivities
• conductivities
• sonic speeds
Shale Porosity Indicator
Dep
th
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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 2.6 – Cont’d
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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 with depth. For this reason the interval velocity also increases with depth, so travel time decreases
t = tma(1-) + tf
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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 = 6,000 ft/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 Pennebaker’s empirical correlation
Ignore the data between 9,000’ and 11,000’. Assume Eaton’s Gulf Coast overburden gradient.
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SolutionPlot interval travel time vs. depth on
semilog paper (Fig. 2.31)
Plot normal trend line using the 6,000-9,000 data.
From Fig. 2.20, at 19,000’, gob = 0.995 psi/ft
(ob)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’
obe = 0.875 * 2,000
=1,750 (Fig. 2.20)
But,
pne = 0.465 * 2,000
= 930 psig
pp = 930 + (18,905-1,750)
pp = 18,085 psig
tnto
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Pennebaker’s correlation for Gulf Coast sediments
Higher travel time means more porosity and higher pore pressure gradient
Example 2.7 (Table 2.7)
to = 95 sec/ft @ 19,000’tn = 65 sec/ft @ 19,000’
to/ tn = 95/65 = 1.46
pp = 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 rock faster 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 to estimate the magnitude of 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
Differential pressure is the difference between wellbore pressure and pore fluid pressure
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.
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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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ob is generally the maximum in situ principal stress in undisturbed rock
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Stresses on Subsurface Rocks
ob, H1, H2 and p all tend to increase with depth
ob is in general the maximum in situ principal stress.
Since the confining stresses H1 and H2 increase with depth, rock strength increases.
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Stresses on Subsurface RocksThe pore pressure, p, cannot produce
shear in the rock, and cannot deform the rock.
Mohr-Coulomb behavior is controlled by the the effective stresses (matrix).
When drilling occurs the stresses change.
ob is 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, o
Formation fracture is resisted by the shear stress, o, which is a function of the rock cohesion and the friction between the plates. This friction depends on o.
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Fig. 2.41 - Differential Pressure 0.1 in below the bit.
When ob is 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 psiHorizontal Stress = 7,000 psiPore Pressure = 4,700 psiWellbore Pressure = 4,700 psi
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Drilling Rate as a Pore Pressure Predictor
Penetration 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:
in Diameter,Bit d
lbf ,Bit WeightW
exponentdd
rpmN
ft/hrR
1012
log
60log
b
6
bdWNR
d
d
bd
WNKR
3
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d-exponent
The d-exponent normalizes R for any variations in W, db and N
Under normal compaction, R should decrease with depth. This would cause d to increase with depth.
Any deviation from the trend could be caused by abnormal pressure.
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d-exponent
Mud weight also affects R…..
An adjustment to d may be made:
dc = d (n /c)
where
dc = exponent corrected for mud density
n = normal pore pressure gradient
c = 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*10000,20*12
log
100*6050
logd
6
bdWN
R
d
61012
log
60log
c
nc dd
19.1d
1.10*052.0
465.034.1d
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 McClendon’s correlation
Zamora’s correlation
The equivalent depth method
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TABLE 2.10
d-EXPONENT AND MUD DENSITY DATA FOR A WELL LOCATED OFFSHORE 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. 2.43Slope 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
p = 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
p = 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
ob = 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,
obe = 0.86 * 750 = 645 psi
pne = 0.465 * 750 = 349 psig
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Equivalent Depth Method
From Eq. 2.13, at 6,050 ft pp = pne + (ob - obe)
pp = 349 + (5,536 - 645) = 5,240 psig
p = 19.25 * (5,240 / 6,050) = 16.7 ppg
Perhaps the equivalent depth method is not always suitable for pp prediction 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 for normal 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 when these 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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Moore’s TechniqueFig. 2.45
Moore proposed a practical method for maintaining a pore-pressure overbalance while drilling into a transition.
Drilling parameters must be kept constant for this technique to work.
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Comb’s 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
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Comb’s Method
Nd
a
nb
aa
bd tfpf
dd96
q
200
N
d500,3
WRR
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
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Tooth wear factor
Correction would depend upon bit type, rock hardness, and abrasiveness
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Differential pressure factor
Method is too complicated and too site specific.
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