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## PETE 625 Well Control

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### 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. ... – PowerPoint PPT presentation

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Title: PETE 625 Well Control

1
PETE 625Well Control
• Lesson 5
• Pore 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

3
Assignments
• Homework 3
• Ch 2, Problems 1 - 10
• due Wednesday, Sept 22, 2004
• Read Chapter 2 to p. 60

4
Normal and Abnormal Pore Pressures
Normal Pressure Gradients West Texas 0.433
psi/ft Gulf Coast 0.465 psi/ft
Depth, ft
Subnormal
10,000
? ?
Pore Pressure, psig
5
0.433 psi/ft 8.33 lb/gal
0.465 psi/ft 9.00 lb/gal
Normal
Abormal
Density of mud required to control this pore
pressure
6
Lost Returns
Kicks
7
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8
Pore Pressure
• formation pressure
• formation fluid pressure
• pressure in fluid contained in the pore
spaces of the rock

9
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.

10
Pore Pressure
TABLE 2.1 -
pn gnD 0.465 psi/ft 9,000 ft pn 4,185
psig
11
Normal Pressure
12
Subnormal Pressures
• Formation pressure gradients less than normal
• Lost circulation problems and differential
sticking are common problems in these areas

13
Subnormal pressures due to faulting
8,000
9,000
14
Aquifer outcrops below rig
15
Production of oil or gas
16
Abnormal Pressures
• Abnormal Pressures are formation pressures
greater than normal pressures
• Can cause severe drilling problems
• There are many possible causes of abnormal
pressure

17
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.

18
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

19
PressureSeals
20
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

21
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.

22
Pore pressures do not always increase with depth
23
Causes of abnormal pressure
TABLE 2.2 -
24
Aquifer
25
Thick gas sand
3 g 590/1,000 0.590 psi/ft EMW
0.590/0.052 11.3 ppg
2 P 605 - 0.05 300 605 - 15 590 psig
1 p 0.465 1,300 605 psig
26
Normal Faulting
9,000 ft
10,000 ft
27
Downfaulting
Top of Transition Zone
Pressure may increase
28
Salt Diapirs
Salt diapirs plastically flow or extrude into
the previously deposited sediment layers. The
resulting compression can result in overpressure.
29
Salt formations
Normally pressured
Salt
Pressure at the bottom of the salt is often
extremely overpressured
30
Erosion
31
Caprock Mineral Deposition
Possible precipitation of carbonate and silica
minerals
32
Underground blowout
Faulty cement job
Casing leaks
33
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

34
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.

35
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

36
Compaction Theory
Undercompacted 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
37
Compaction Theory
The overburden load is supported by the vertical
stress in the grain framework and by the fluid
pore pressure
sob seV pp sob overburden stress seV
matrix stress pp pore pressure
38
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

39
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.

40
Compaction Theory
TABLE 2.4 -
41
Bulk Densities - Santa Barbara Channel
42
GOM Bulk Densities
43
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

44
Overburden Stress
setting
and integrating
45
Example 2.5
• Calculate the overburden stress at a depth of
7,200 ft in the Santa Barbara Channel. Compare
to Eatons prediction.
• Assume
• fo 0.37
• rma 2.6 gm/cc
• kf 0.0001609 ft-1
• rf 1.044 gm/cc

46
Solution
Eatons Fig. 2.21 shows a value of gob 0.995
psi/ft So, (sob)eaton 0.995 7,200 7,164
psig Difference 132 psi or 1.9
47
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.
48
Rule of Thumb
A common assumption for sedimentary deposits is
gob 1.0 psi/ft This is not a good assumption in
young sediments
Eaton predicts that an overburden stress gradient
of 1 psi/ft be achieved at a depth of 20,000 ft
in the GOM
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
49
0.84 psi/ft
0.89 psi/ft
Eatons ob stress gradient for Santa Barbara
Channel
Eatons ob stress gradient for GOM
1 psi/ ft at 7,400
1 psi/ ft at 20,000
50
Shale porosity depends not only on depth e.g. At
6,000 depth f varies from 3 to 18
Note the straight line relationship
on semilog paper
51
Eatons porosities from the Santa Barbara
Channel. The straight line is a plot of the
equation f 0.37e-0.0001609D At D 0, f
0.37 At D 10,000 ft f 0.074
52
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