PETE 411 Well Drilling

1

PETE 411

Well Drilling

Lesson 21

Prediction of Abnormal Pore Pressure

2

Prediction of Abnormal Pore Pressure

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

Sonic Travel Time Conductivity of Shale

3

HW #11 Slip Velocity Due 10-28-02

Read:

Applied Drilling Engineering, Ch. 6

4

Shale Resistivityvs. Depth

1. Establish trend line in normally pressured shale

2. Look for deviations from this trend line

(semi-log)

5

EXAMPLE

Shale Resistivity vs. Depth

1. Establish normal trend line

2. Look for deviations

(semi-log)

6

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)

7

Shale Density , g/cc

Dep

th,

ft

8

Mud Temperature in flowline, deg F

Dep

th,

ft

9

Example

8.2 X

Why?

10

Example

8.8 X

Thermal conductivity, heat capacity, pore pressure...

11

Dri

llin

g R

ate,

ft/

min

PHYD - PPORE , psi

12

P = (P2 - P1)1,000

Effect of Differential Pressure

13

Typical Drilling Rate Profiles - Shale

The drilling rate in a normally pressured, 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

14

Typical Drilling Rate Profiles - Sand

The drilling rate in a sand will probably generate an erratic drilling rate curve.

Sands in the Gulf Coast area are generally very unconsolidated. This may cause sloughing, accompanied by erratic torque, and temporarily, erratic drilling rates.

Sand

15

Typical Drilling Rate Profiles - Shaley Sands

This is generally the most troublesome type drilling rate curve to interpret.

Many times this curve will look similar to a solid shale curve that is 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, even though the drilling rate parameter is one of the better parameters.

16

Typical Drilling Rate Profiles

If you are drilling close to balanced, there will probably be a very smooth, (gradual) increase in the drilling rate.

This is due to the difference between the hydrostatic head and the pore pressure becoming smaller. p

Transition Zone Shale

17

Typical Drilling Rate Profiles

Transition Zone Shale

As the pressure becomes very small, the gas in the pores has a tendency to expand which causes the shale particles to pop from the wall. This is called sloughing shale.

The transition zone generally has a higher porosity, making drilling rates higher. In a clean shale the ROP will increase in a smooth manner.

p

18

Typical Drilling Rate Profiles

Note:

If you are drilling overbalanced in 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.

19

Typical Drilling Rate Profiles

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 pending danger that you may not notice the small effect of the drilling rate curve change. This will allow you to move well into that transition zone without realizing it.

20

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

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

22

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

23

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

24

Example

lbm/gal 9.1012,000 x 0.052

6794EMW

psi 6794000,12 x 5662.0P

25

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?)

26

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.

27

Interval Travel Time, sec per ft

Dep

th,

ft

28

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

29

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 )

30

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

31

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

32

Pore Pressure Determination

33

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 well kicks from formation at 10,000 ft.

34Pressure, psi

Dep

th,

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

35

Example 3 - SolutionThe 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

36

Example 3 – Solution – cont’d

This additional pressure, at 10,000 ft, is also 525 psi, and would amount to an additional pressure gradient of:

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

This represents an equivalent mud weight of0.0525 / 0.052 = 1.01 lb/gal

This is the kick tolerance for a small kick!

37

Problem #3 - Alternate Solution

When a well kicks, the well is shut in and the wellbore pressure increases until the new BHP equals the new formation pressure.

At that point influx of formation fluids into the wellbore ceases.

Since the mud gradient in the wellbore has not changed, the pressure increases uniformly everywhere.

38

Wellbore Pressure, psi

Dep

th,

ft

P

Casing Seat at 2,500 ft

Kick at 10,000 ft

Before Kick

After Kick and Stabilization

525

525

39

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 of525 / (0.052 * 10,000) = 1.01 lb/gal

= maximum increase in EMW= kick tolarance for a small kick size.

40

Wellbore Pressure, psi

Dep

th,

ft

P

Casing Seat at 2,500 ft

Kick at 10,000 ft

1,300 psi

1,825 psi

5,725 psi

5,200 psi

41

Wellbore Pressure, psi

Dep

th,

ft

P

Casing Seat at 2,500 ft

Kick at 10,000 ft

Before Kick

After Small Kick and Stabilization

After Large Kick and Stabilization