pore pressure

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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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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