Well Control

Pore Pressure

2

Contents

Normal Pore Pressure

Subnormal Pore Pressure

Abnormal Pore Pressure

Origins of Pore Pressure

Origins of Abnormal Pore Pressure

3

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

De

pth

, f

t

10,000 ? ?

Subnormal

4

Pore Pressure vs. Depth

0

5,000

10,000

15,000

5 10 15 20

Pore Pressure, lb/gal equivalent

Depth

, ft

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

5

Lost Returns

Kicks

6

7

Pore Pressure

= formation pressure

= formation fluid pressure

= pressure in fluid contained in the pore spaces of the rock

8

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.

9

Pore Pressure

pn = gnD = 0.465 psi/ft * 9,000 ft

pn = 4,185 psig

TABLE 2.1 -

10

Normal Pressure

11

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

12

Subnormal pressures due to faulting

8,000

9,000

13

Aquifer outcrops below rig

14

Production of oil or gas

15

Abnormal Pressures

Abnormal Pressures are formation pressures greater than normal

pressures

Can cause severe drilling problems

There are many possible causes of abnormal pressure

16

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.

17

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.

18

Pressure

Seals

19

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

20

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.

21

Pore pressures

do not always

increase with

depth

22

Causes of abnormal pressure

TABLE 2.2 -

23

Aquifer

24

Thick gas sand

2

P = 605 - 0.05 * 300

= 605 - 15

= 590 psig

1

p = 0.465 * 1,300

= 605 psig

3

g = 590/1,000

= 0.590 psi/ft

EMW = 0.590/0.052

11.3 ppg

25

Normal Faulting

10,000 ft 9,000 ft

psi650,4

ft000,10*ft

psi465.0

ppg94.9

000,9*052.0

650,4

26

Downfaulting

Top of

Transition

Zone

Pressure may

increase

27

Salt Diapirs

Salt diapirs

plastically flow or extrude into the

previously

deposited

sediment layers.

The resulting

compression can

result in

overpressure.

28

Salt formations

Salt

Pressure at the bottom

of the salt is often

extremely overpressured

Normally pressured

29

Erosion

Depth*052.0

pEMW

30

Caprock Mineral Deposition

Possible precipitation of carbonate and silica minerals

31

Man-Made Abnormal Pressures Underground

blowout Casing

leaks

Faulty

cement job