1.10 Abnormal Pressure

Abnormal Pressure 10.1- 1

1.10-1

Abnormal Pressure


Abnormal Pressure - Definition, Causes -

Normal Pore Pressures

Abnormal Pore Pressure Gradients

Fracture Gradients

Mud Weights

Casing Seat Depths

What Causes Abnormal Pressure?


Abnormal Pressure Gradients

Normal Pressure Gradients West Texas: 0.433 psi/ft Gulf Coast: 0.465 psi/ft

Normal and Abnormal Pore Pressure

Pore Pressure, psig

Dep

th,

ft

10,000’ ? ?


Pore Pressure vs. Depth

8 9 10 11 12 13 14 15 16Pore Pressure Equivalent, lb/gal

0

5,000

10,000

15,000

20,000

{ Density of mud required to control this pore pressure }

Dep

th,

ft

Normal

Abormal

0.433 psi/ft 8.33 lb/gal0.465 psi/ft 9.0 lb/gal


Pore Pressure Gradient

Fracture Gradient


* Pore Pressure

Gradients

* Fracture Gradients

* Casing Setting Depths


Some Causes of Abnormal Pressure

1. Incomplete compaction of sediments Fluids in sediments have not

escaped and are still helping to support the overburden.

2. Tectonic movements

Uplift

Faulting


Some Causes of Abnormal Pressure

3. Aquifers in Mountainous Regions Aquifer recharge is at higher

elevation than drilling rig location.

4. Charged shallow reservoirs due to nearby underground blowout.

5. Large structures...


HIGH PRESSURE

NORMAL PRESSURE

Thick, impermeable layers of shale (or salt) restrict the movement of water. Below such layers abnormal pressure may be found.


HIGH PRESSURE

NORMAL PRESSURE

Hydrostatic pressure gradient is lower in gas or oil than in water.


When crossing faults it is possible to go from normal pressure to abnormally high pressure in a short interval.


Well “A” found only Normal Pressure ...


OB = p + Z

ob

pz


?


Indications of Abnormal Pore Pressures

Methods:

1. Seismic data

2. Drilling rate

3. Sloughing shale

4. Gas units in mud

5. Shale density

6. Chloride content


Indications of Abnormal Pore Pressures

Methods, cont’d:

7. Change in Mud properties

8. Temperature of Mud Returns

9. Bentonite content in shale

10. Paleo information

11. Wire-line logs

12. MWD-LWD


Prediction and Detection of Abnormal Pressure Zones

1. Before drilling

Shallow seismic surveys

Deep seismic surveys

Comparison with nearby wells



2. While drilling

Drilling rate, gas in mud, etc. etc.

D - Exponent

DC - Exponent

MWD - LWD

Density of shale (cuttings)



3. After drilling

Resistivity log

Conductivity log

Sonic log

Density log


. – .SD000085.0e41.0


Decreasing ROP

What is d-exponent?


D - Exponent

The

drilling rate

equation:

Where

R = drilling rate, ft/hr

K = drillability constant

N = rotary speed, RPM

E = rotary speed expon.

W = bit weight, lbs

DB = bit diameter, in

D = bit wt. Exponent

or D - exponent

D

B

E

D

WNKR


D - Exponent

If we assume that K = 1

and E = 1

Then

D

B

E

D

WNKR

BDW

log

NR

logD

D

BD

W

N

R


D - Exponent

A modified version of this equation follows:

B6 D10W12

log

N60R

log

d


Example

d may be Corrected for mud density as follows:

)ppg(useinweightmudactual

)ppg(gradientnormalforweightmudddc

37.112

9*82.1

12

9dd.,g.e c


Procedure for Determining Pore Pressure From dc - Exponent

Calculate dc over 10-30 ft intervals

Plot dc vs depth (use only date from Clean shale sections)

Determine the normal line for the dc vs. depth plot.

Establish where dc deviates from the normal line to determine abnormal pressure zone



dc - Exponent

Dep

th Normal

AbnormalN

ormal Trend



If possible, quantify the magnitude of the abnormal pore pressure using overlays, or Ben Eaton’s Method

Pore Pressure Grad.

Overburden Stress Grad.

Normal Pore Pressure Grad.

2.1

c

c

n normald

calculatedd

D

P

D

S

D

S

D

P


In normally pressured shales, shale

compaction increases with depth


Pore Pressure from Resistivity

Shale resistivity plots may be developed from (i) logs or

(ii) cuttings

What is the pore pressure at the point indicated on the plot?

[Assume Gulf Coast]. Depth=10,000 ft

0.2 0.5 1 2 3

10,000’

Dep

th


From plot, Rn = 1.55 ohms

Robs = 0.80 ohms

From Eaton:

EATON

2.1

n

obs

n R

R

D

P

D

S

D

S

D

P

2.1

55.1

80.0465.095.095.0

D

P

= 0.7307 psi/ft = 14.05 lb/gal

P = 0.7307 * 10,000 = 7,307 psi 0.2 0.5 1 2 3

10,000’

Dep

th


Prediction of Abnormal Pore Pressure

Resistivity of Shale Temperature in the Return Mud Drilling Rate Increase dc - Exponent

Sonic Travel Time Conductivity of Shale


EXAMPLE

Shale Resistivity vs. Depth

1. Establish normal trend line

2. Look for deviations

(semi-log)


Shale Resistivity vs. Depth

1. Establish normal trend line

2. Look for deviations

3. Use OVERLAYto quantify pore pressure

(use with caution)

Pore Pressure(lb/gal equivalent)

16 14 12 10

9 ppg (normal)


Example

8.2 X

Why?


Determination of Abnormal Pore Pressure Using the dc - exponent

From Ben Eaton:

2.1

cn

c

n d

d

D

P

D

S

D

S

D

P


Where

trendnormal thefrom onentexpd d

plot from entexpond actual d

psi/ft gradient, stress overburden D

S

psi/ft 0.465,or 0.433 e.g.,

areain gradient water normalD

P

psi/ft gradient, pressureformation D

P

ccn

cc

n

2.1

cn

c

n d

d

D

P

D

S

D

S

D

P


Example

Calculate the pore pressure at depth X using the data in this graph.

Assume:

West Texas location with normal overburden of

1.0 psi/ft.

X = 12,000 ft.

X

1.2 1.5

dc


Example

From Ben Eaton:

psi/ft 5662.0D

P

5.1

2.1]433.00.1[0.1

d

d

D

P

D

S

D

S

D

P

2.1

2.1

cn

c

n


Example

lbm/gal 9.1012,000 x 0.052

6794EMW

psi 6794000,12 x 5662.0P


E.S. Pennebaker

Used seismic field data for the detection of abnormal pressures.

Under normally pressured conditions the sonic velocity increases with depth. (i.e. Travel time decreases with depth)

(why?)


E.S. Pennebaker

Any departure from this trend is an indication of possible abnormal pressures.

Pennebaker used overlays to estimate abnormal pore pressures from the difference between normal and actual travel times.


Interval Travel Time, sec per ft

Dep

th,

ft


Ben Eaton

also found a way to determine pore pressure from interval travel times.

Example:In a Gulf Coast well, the speed of sound is 10,000 ft/sec at a depth of 13,500 ft. The normal speed of sound at this depth, based on extrapolated trends, would be 12,000 ft/sec. What is the pore pressure at this depth?

Assume: S/D = 1.0 psi/ft


Ben Eaton

From Ben Eaton,

psi/ft 0.6904

12,000

10,0000.465]-[1.0-1.0

t

t

D

P

D

S

D

S

D

P

3

0.3

n

n

( t 1/v )


Ben Eaton

From Ben Eaton

Note: Exponent is 3.0 this time,

NOT 1.2!

= (0.6904 / 0.052) = 13.28 lb/gal

p = 0.6904 * 13,500 = 9,320 psig


Equations for Pore Pressure Determination

2.1

c

c

n normald

calculatedd

D

P

D

S

D

S

D

P

2.1

n

obs

n R

R

D

P

D

S

D

S

D

P

ACTUAL

NORMAL

B6

C *

D10W12

log

N60R

log

d

2.1

o

n

n C

C

D

P

D

S

D

S

D

P

0.3

o

n

n t

t

D

P

D

S

D

S

D

P


Pore Pressure Determination

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1.10 Abnormal Pressure