abnormal pore pressure prediction

7/30/2019 Abnormal Pore Pressure Prediction

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PETE 411

Well Drilling

Lesson 21

Prediction ofAbnormal Pore Pressure


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Prediction ofAbnormal 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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HW #11

Slip Velocity

Due 10-28-02

Read:

Applied Drilling Engineering, Ch. 6


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Shale Resistivityvs. Depth

1. Establish trend

line in normally

pressured shale

2. Look fordeviations from

this trend line

(semi-log)


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EXAMPLE

Shale Resistivity

vs. Depth

1. Establish normal

trend line

2. Look for

deviations

(semi-log)


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Shale Resistivityvs. Depth

1. Establish normaltrend line

2. Look fordeviations

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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Shale Density , g/cc

Depth,

ft


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Mud Temperature in flowline, deg F

Depth,

ft


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Example

8.2 X

Why?


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Example

8.8 X

Thermal conductivity, heat capacity, pore pressure...


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PHYD - PPORE , psi


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P = (P2 - P1)1,000

Effect of Differential Pressure


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Typical Drill ing Rate Profiles - Shale

The drilling rate in a normally

pressured, solid shalesection will generallygenerate 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 Drill ing Rate Profiles - Sand

The drilling rate in a sand willprobably generate an erraticdrill ing rate curve.

Sands in the Gulf Coast area

are generally veryunconsolidated. This maycause sloughing, accompanied

by erratic torque, andtemporarily, erratic drill ingrates.

Sand


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Typical Drill ing Rate Profiles - Shaley Sands

This is generally the mosttroublesome type drilling rate curve

to interpret.

Many times this curve will looksimilar to a solid shale curve thatis moving into a transition zone.

Shaley Sands

Note:This is a prime example why you should not base

your decision on only one drilling parameter, eventhough the drilling rate parameter is one of the betterparameters.


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Typical Drill ing Rate Profiles

If you are drilling close to

balanced, there will probablybe a very smooth, (gradual)

increase in the drilling rate.

This is due to the differencebetween the hydrostatichead and the pore pressurebecoming smaller. p

Transition Zone

Shale


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Typical Drilling Rate Profiles

Transition Zone

Shale

As the pressure becomesvery small, the gas in the pores

has a tendency to expand whichcauses the shale particles to popfrom the wall. This is calledsloughing shale.

The transition zone generallyhas a higher porosity, makingdrilling rates higher. In a clean

shale the ROP will increase in asmooth manner.

p


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Note:

If you are drilling overbalanced in a transition itwill 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.


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This could cause you to move into a permeable

zone which would probably result in a kick.

The conditions you create with overbalanced

hydrostatic head will so disguise the pendingdanger that you may not notice the smalleffect of the drilling rate curve change. This

will allow you to move well into that transitionzone without realizing it.


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Determination of Abnormal Pore

Pressure Using the dc - exponent

From Ben Eaton:

2.1

cn

c

n dd

DP

DS

DS

DP

=


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Where

trendnormalthefromonentexpdd

plotfromentexpondactuald

psi/ftgradient,stressoverburdenD

S

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

areaingradientwaternormalD

P

psi/ftgradient,pressureformation

D

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 withnormal 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. Pennebaker

Used seismic field data for thedetection ofabnormal pressures.

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

(why?)


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E.S. Pennebaker

Any departure from this trend is an

indication of possible abnormalpressures.

Pennebaker used overlays to estimateabnormal pore pressures from the

difference between normal and actualtravel times.


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Interval Travel Time, sec per ft


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Ben Eaton

also found a way to determine pore pressurefrom interval travel times.

Example:In a Gulf Coast well, the speed of sound is 10,000ft/sec at a depth of13,500 ft. The normal speed

of sound at this depth, based on extrapolatedtrends, would be 12,000 ft/sec. What is the porepressure at this depth?

Assume: S/D = 1.0 psi/ft


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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

=

=

=

( t 1/v )


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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


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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

=

=ACTUAL

NORMAL

B6

C *

D10

W12log

N60

Rlog

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

=


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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/ftDrilling 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 wellkicks from formation at 10,000 ft.


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D

epth,

ft

Casing Seat

10,000

Mud Wt. Grad

= 0.52 psi/ft

FractureGradient= 0.73 psi/ft

0.73 0.52 = 0.21 (psi/ft)

5,200

2,500


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Example 3 - Solution

The danger here is fracturing the formation near

the casing seat at 2,500 ft.

The fracture gradient at this depth is 0.73 psi/ft,

and the mud weight gradient is 0.52 psi/ft.

So, the additional permissible pressure gradient

is 0.73 0.52 = 0.21 psi/ft, at the casing seat.

This corresponds to an additional pressure of

P = 0.21 psi/ft * 2,500 ft = 525 psi


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Example 3 Solution contd

This additional pressure, at 10,000 ft, is also

525 psi, and would amount to an additionalpressure gradient of:

525 psi / 10,000 ft = 0.0525 psi/ft

This represents an equivalent mud weight of

0.0525 / 0.052 = 1.01 lb/gal

This is the kick tolerance for a small kick!


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Problem #3 - Alternate SolutionWhen a well kicks, the well is shut in

and the wellbore pressure increasesuntil the new BHP equals the newformation pressure.

At that point influx of formation fluidsinto the wellbore ceases.

Since the mud gradient in the wellborehas not changed, the pressureincreases uniformly everywhere.


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Wellbore Pressure, psi

D

epth,

ft

P

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

Maximum allowable increase in pressure = 525 psi

At 10,000 ft

Maximum allowable increase in pressure = 525 psi(since the pressure increases uniformly everywhere).

This corresponds to an increase in mud weight of

525 / (0.052 * 10,000) = 1.01 lb/gal= maximum increase in EMW= kick tolarance for a small kick size.


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D

epth,

ft

P


Kick at 10,000 ft

1,300 psi

1,825 psi

5,725 psi

5,200 psi


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D

epth,

ft

P


Kick at 10,000 ft

Before Kick

After Small Kick

and Stabilization

After Large Kick and Stabilization

abnormal pore pressure prediction

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