abnormal pressure

7/29/2019 Abnormal Pressure

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7. Abnormal Pressure 661. Drilling Engineering Slide 1

PETE 661Drilling Engineering

Lesson 7

Abnormal Pressure


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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 Pressure

Pore Pressure, psig

Depth,

ft

10,000??


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Pore Pressure vs. Depth

8 9 10 11 12 13 14 15 16

Pore Pressure Equivalent, lb/gal

0

5,000

10,000

15,000

20,000

{ Density of mud required to control this pore pressure }

Normal

Abormal

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


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Pore Pressure

Gradient

Fracture Gradient


7/657. Abnormal Pressure 661. Drilling Engineering Slide 7

* 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...


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HIGH PRESSURE

NORMAL PRESSURE

Thick, impermeable layers of shale (or salt) restrict the movement

of water. Below such layers abnormal pressure may be found.


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HIGH PRESSURE

NORMAL PRESSURE

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


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When crossing faults it is possible to go from normal

pressure to abnormally high pressure in a short interval.


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Well A found only Normal Pressure ...


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sOB = p + sZ

sob

p sz


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?


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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


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Indications of Abnormal Pore Pressures

Methods, contd:

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


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Prediction and Detection of Abnormal

Pressure Zones

1. Before drilling

Shallow seismic surveys

Deep seismic surveys

Comparison with nearby wells


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Pressure Zones

2. While drilling

Drilling rate, gas in mud, etc. etc.

D- Exponent

DC - Exponent

MWD - LWD

Density of shale (cuttings)


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Pressure Zones

3. After drilling

Resistivity log

Conductivity log

Sonic log

Density log


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.

.SD000085.0

e41.0


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Decreasing ROP

What is d-

exponent?


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D - Exponent

Thedrilling rate

equation:

WhereR = drilling rate, ft/hr

K = drillability constant

N = rotary speed, RPME = rotary speed expon.

W = bit weight, lbs

DB = bit diameter, in

D = bit wt. Exponent

or D - exponent

D

B

E

D

WNKR


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D - Exponent

If we assume that K = 1

and E = 1

Then

D

B

E

D

WNKR

BD

Wlog

N

Rlog

D

D

BDW

NR


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D - Exponent

A modified version of this equation

follows:

B

6 D10W12log

N60

Rlog

d


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Example

Calculate the value of the d - exponent ifthe drilling rate is 35 ft/hr, the rotary RPM is

100, and the weight on the 12 1/4 bit is

60,000 lbs.

B

6 D10W12log

N60

Rlog

d

2308.1

2341.2

25.1210000,60*12log

100*60

35log

6

d = 1.82


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Example

What happens to d if R doubles to 70 ft/hr?

Note that an increase in Rresulted in a decrease in d.

Doubling R decreased d from 1.82 to 1.57

57123081

93311

251210

0006012

10060

70

6

..

.

.

,*log

*logd


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Example

d may be Corrected for mud densityas follows:

)ppg(useinweightmudactual

)ppg(gradientnormalforweightmudddc

37.112

9*82.1

12

9dd.,g.e c


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Procedure for Determining Pore

Pressure From dc - Exponent

Calculate dc over10-30 ft intervals

Plot dc vs depth (use only date from

Cleanshale sections) Determine the normal line for the

dc vs. depth plot.

Establish where dc deviates from thenormal line to determine abnormal

pressure zone


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dc - Exponent

Depth Normal

Abnormal


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If possible, quantify the magnitude of the

abnormal pore pressure using

overlays, orBen Eatons Method

Pore

Pressure

Grad.Overburden

Stress Grad.

Normal Pore

Pressure Grad.

2.1

c

c

nnormald

calculatedd

D

P

D

S

D

S

D

P


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In normally pressured

shales, shale

compaction increaseswith depth


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Shale Density - Mud Cup Method

1. Fill mud cup with shale until the weight is 8.33.2. Fill to top with water, and record the reading Wtot.

Note: Dry sample carefully with towel.

Do not apply heat.

totW66.16

33.8Gravity.Spec


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Alternate Method: Use variable density column.

See p. 270 in text


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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 ft0.2 0.5 1 2 3

10,000


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From plot, Rn = 1.55 ohms

Robs = 0.80 ohms

From Eaton:

EATON

2.1

n

obs

nR

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 psi0.2 0.5 1 2 3

10,000

Dep

th


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Prediction of

Abnormal Pore Pressure

Resistivity of Shale

Temperature in the Return Mud

Drilling Rate Increase

dc - Exponent

Sonic Travel Time

Conductivity of Shale


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EXAMPLE

Shale Resistivityvs. Depth

1. Establish normal

trend line

2. Look for

deviations

(semi-log)


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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)


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Example

8.2 X

Why?


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DP = (P2 - P1)1,000

Effect of Differential Pressure


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Typical Drilling Rate Profiles - Shale

The drilling rate in a normallypressured, solid shale

section will generally

generate a very steady and

smooth drilling rate curve.

The penetration rate will be

steady and not erratic(normally pressured, clean

shale).

Shale


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Typical Drilling Rate Profiles

Note:

If you are drilling overbalancedin a transition it

will be very difficult to pick up the

transition zone initially. This will allow you to move well into the

transition zone before detecting the problem.

Any deviation from the expected decrease inddilling rate with depth, when you are drilling in a

clean shale, might indicate a transition zone.


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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

21


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Where

trendnormalthefromonentexpdd

plotfromentexpondactuald

psi/ftgradient,stressoverburden

D

S

psi/ft0.465,or0.433e.g.,

areaingradientwaternormalD

P

psi/ftgradient,pressureformationD

P

ccn

cc

n

2.1

cn

c

n d

d

D

P

D

S

D

S

D

P


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Example

Calculate the porepressure 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


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Example

From Ben Eaton:

psi/ft5662.0D

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


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Example

lbm/gal9.1012,000x0.052

6794EMW

psi6794000,12x5662.0P


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E S P b k


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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 actualtravel times.


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Interval Travel Time, msec per ft


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B E t


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Ben Eaton

From Ben Eaton,

psi/ft0.6904

12,000

10,0000.465]-[1.0-1.0

t

t

D

P

D

S

D

S

D

P

3

0.3

n

n

D

D

( Dt a 1/v )

B E t


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Ben Eaton

From Ben Eaton

Note: Exponent is 3.0 this time,NOT 1.2!

r = (0.6904 / 0.052) = 13.28 lb/gal

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

Equations for Pore Pressure Determination


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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

rr

ACTUALNORMAL

B6

C *

D10

W12log

N60

Rlog

d

2.1

o

n

nC

C

D

P

D

S

D

S

D

P

0.3

o

n

nt

t

D

P

D

S

D

S

D

P

D

D


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Pore Pressure Determination


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EXAMPLE 3 - An Application...

Mud Weight = 10 lb/gal. (0.52 psi/ft)Surface csg. Set at 2,500 ft.

Fracture gradient below surf. Csg = 0.73 psi/ft

Drilling at 10,000 ft in pressure transition zone* Mud weight may be less than pore pressure!

DETERMINE Maximum safe underbalance

between mud weight and pore pressure if well

kicks from formation at 10,000 ft.


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Wellbore Pressure, psi

Dep

th,

ft

DP

Casing Seat at 2,500 ft

Kick at 10,000 ft

Before Kick

After Kick and

Stabilization

525

525


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At 2,500 ftInitial mud pressure = 0.52 psi/ft * 2,500 ft = 1,300 psi

Fracture pressure = 0.73 psi/ft * 2,500 ft = 1,825 psi

Maximum allowable increase in pressure = 525 psi

At 10,000 ftMaximum allowable increase in pressure = 525 psi

(since the pressure increases uniformly everywhere).

This corresponds to an increase in mud weight of525 / (0.052 * 10,000) = 1.01 lb/gal

= maximum increase in EMW

= kick tolarance for a small kick size.


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Depth,

ft

DP


Kick at 10,000 ft

1,300 psi

1,825 psi

5,725 psi

5,200 psi


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Dep

th,

ft

DP


Kick at 10,000 ft

Before Kick

After Small Kick

and Stabilization

After Large Kick and Stabilization

abnormal pressure

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