Transcript
TAMU - Pemex
Well Control
Lesson 7Pore Pressure
2
Contents
Normal Pore Pressure
Subnormal Pore Pressure
Abnormal Pore Pressure
Origins of Pore Pressure
Origins of Pore Pressure
Origins of Abnormal Pore Pressure
Bulk Density and Porosity vs. Depth
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Abnormal Pressure Gradients
Normal Pressure Gradients West Texas: 0.433 psi/ft Gulf Coast: 0.465 psi/ft
Normal and Abnormal Pore Pressures
Pore Pressure, psig
Dep
th,
ft
10,000 ? ?
Subnormal
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Pore Pressure vs. Depth0
5,000
10,000
15,000
5 10 15 20Pore Pressure, lb/gal equivalent
De
pth
, f
t
Normal Abormal
Density of mud required to control this pore pressure
0.433 psi/ft 8.33 lb/gal
0.465 psi/ft 9.00 lb/gal
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Lost Returns
Kicks
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7
Pore Pressure
= formation pressure
= formation fluid pressure
= pressure in fluid contained in the pore spaces of the rock
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Pore Pressure
Normal pressure gradients correspond to the hydrostatic gradient of a fresh or saline water column
Example 2.1. Determine the pore pressure of a normally pressured formation in the Gulf of Mexico at 9,000’ depth.
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Pore Pressure
pn = gnD = 0.465 psi/ft * 9,000 ft
pn = 4,185 psig
TABLE 2.1 -
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Normal Pressure
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Subnormal Pressures
Formation pressure gradients less than normal gradients for a given area.
Lost circulation problems and differential sticking are common problems in these areas
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Subnormal pressures due to faulting
8,000’9,000’
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Aquifer outcrops below rig
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Production of oil or gas
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Abnormal Pressures
Abnormal Pressures are formation pressures greater than normal pressures
Can cause severe drilling problems
There are many possible causes of abnormal pressure
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Abnormal Pressure
All abnormal pressures require some means of sealing or trapping the pressure within the rock body.
Otherwise hydrostatic equilibrium back to a normal gradient would eventually be restored.
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Abnormal Pressure
Massive shales provide good pressure seals, but shales do have some permeability, so, given sufficient time, normal pressures will eventually be established.
It may take tens of millions of years for a normal pressure gradient to re-occur.
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PressureSeals
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Abnormal pressures
Dense rocks should always be a warning to a driller that the pore pressure may be changing
Many abnormal pore pressure processes are simply the reverse of those which effect subnormal pressures
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Abnormal pressures
For example, the converse to a low piezometric water level is abnormal pressure resulting from an Artesian source.
A thick gas sand that is normally pressured at the bottom of the sand will be abnormally pressured at the top of the sand.
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Pore pressures do not always increase with
depth
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Causes of abnormal pressureTABLE 2.2 -
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Aquifer
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Thick gas sand
2P = 605 - 0.05 * 300
= 605 - 15= 590 psig
1p = 0.465 * 1,300
= 605 psig
3g = 590/1,000 = 0.590 psi/ft
EMW = 0.590/0.05211.3 ppg
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Normal Faulting
10,000 ft9,000 ft
psi650,4
ft000,10*ft
psi465.0
ppg94.9
000,9*052.0
650,4
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DownfaultingTop of
Transition Zone
Pressure may increase
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Salt Diapirs
Salt diapirs plastically “flow” or extrude into the previously deposited sediment layers. The resulting compression can result in overpressure.
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Salt formations
Salt
Pressure at the bottomof the salt is often
extremely overpressured
Normally pressured
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Erosion
Depth*052.0
pEMW
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Caprock Mineral Deposition
Possible precipitation of carbonate and silica minerals
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Man-Made Abnormal PressuresUnderground
blowoutCasing leaks
Faulty cement job
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Compaction Theory of Abnormal Pressure
Best fits most naturally occurring abnormal pressures
In new areas, geologic and geophysical interpretations along with analogy to known areas are always important
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Compaction Theory
During deposition, sediments are compacted by the overburden load and are subjected to greater temperatures with increasing burial depth.
Porosity is reduced as water is forced out.
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Compaction Theory
Hydrostatic equilibrium within the compacted layers is retained as long as the expelled water is free to escape
If water cannot escape, abnormal pressures occur
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Compaction TheoryUndercompacted Shales
Water is expelled from the shales
Pore water expelled because of increasing overburden
If the expelled water is not free to escape, abnormal pressures may result. Sufficient compaction cannot occur so the pore fluids carry more of the overburden
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Compaction TheoryThe overburden load is supported by the vertical stress in the grain framework and by the fluid pore pressure
ob = eV + pp
ob = overburden stress
eV = matrix stress
pp = pore pressure
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Compaction Theory
The average porosity in sediments, generally decreases with increasing depth - due to the increasing overburden
This results in an increasing bulk density with increasing depth, and increasing rock strength
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Compaction Theory
From a porosity log, we can construct a plot of bulk density vs. depth
From this (or directly from a density log, we can calculate overburden stress vs. depth.
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Compaction TheoryTABLE 2.4 -
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Bulk Densities - Santa Barbara Channel
f
e DK
0
De 0001609.037.0
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GOM Bulk
Densities
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Pore Pressure Prediction
Overburden Pressure vs. Depth
Porosity vs. Depth
Pore Pressure Prediction
By Analogy
By Seismic Methods
From Drilling Rate Changes
Factors that Affect Drilling Rates
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Overburden Stress
Dkfmamaob
Dk
D
fmaob
bob
ek
D
egratingand
e
setting
dD
gdD
1052.0
int
1052.0
0
0
0
and integrating
setting
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Example 2.5
Calculate the overburden stress at a depth of 7,200 ft in the Santa Barbara Channel. Compare to Eaton’s prediction.
Assumeo = 0.37
ma = 2.6 gm/cc
k = 0.0001609 ft-1
f = 1.044 gm/cc
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Solution
psig
e
ob
ob
032,7
1*0001609.0
37.0*33.8044.16.2200,7*33.8*6.2052.0 200,7*0001609.0
Eaton’s Fig. 2.21 shows a value of :
gob = 0.995 psi/ft
So,
(ob)eaton = 0.995 * 7,200 = 7,164 psig
{ Difference = 132 psi or 1.9% }
Dk0fmamaob e1
kD052.0
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Overburden stress depends upon porosity, and porosity depends on overburden stress
Shales are more compactible than sandstones.
Young shales are more compactible than older shales.
Limestones and dolomites are only slightly compactible.
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Eaton predicts that an overburden stress gradient of 1 psi/ft be achieved at a depth of 20,000 ft in the GOM
A common assumption for sedimentary deposits is gob = 1.0 psi/ft
This is not a good assumption in young sediments
Rule of Thumb
Eaton predicts that an overburden stress gradient of 1 psi/ft be achieved at a depth of 7,400 ft in the Santa Barbara Channel
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Eaton’s ob stress gradient for GOM
Eaton’s ob stress gradient for Santa Barbara Channel
1 psi/ ftat 20,000’
1 psi/ ftat 7,400’
0.84 psi/ft 0.89 psi/ft
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Shale porosity depends not only on depth
e.g. At 6,000’ depth varies from 3% to 18%
Note the ~ straight line relationship on semilog paper
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Eaton’s porosities from the Santa Barbara Channel.
The straight line is a plot of the equation:
= 0.37e-0.0001609D
At D = 0, = 0.37
At D = 10,000 ft = 0.074
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