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PETE 411Well Drilling

Lesson 20Abnormal Pressure

Abnormal Pressure

- Normal Pore Pressures
- Abnormal Pore Pressure Gradients
- Fracture Gradients
- Mud Weights
- Casing Seat Depths
- What Causes Abnormal Pressure?
- Detection of Abnormal Pressure
- Quantification of Abnormal Pressure

HW 11 Slip VelocityDue 10-28-02

ReadApplied Drilling Engineering, Ch. 6

Normal and Abnormal Pore Pressure

Normal Pressure Gradients West Texas 0.433

psi/ft Gulf Coast 0.465 psi/ft

Abnormal Pressure Gradients

Depth, ft

10,000

? ?

Pore Pressure, psig

0.433 psi/ft 8.33 lb/gal

0.465 psi/ft 9.00 lb/gal

Normal

Abormal

Density of mud required to control this pore

pressure

Fracture Gradient

Pore Pressure 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 help

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 ...

sob

p sz

sOB p sZ

?

Abnormal Pressure contd

- Detection of Abnormal Pore Pressures
- Prediction of Abnormal Pore Pressures
- D-Exponent
- DC-Exponent
- Example
- Importance of Shale Density

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, 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

Prediction and Detection of Abnormal Pressure

Zones

- 1. Before drilling
- Shallow seismic surveys
- Deep seismic surveys
- Comparison with nearby wells

Prediction and Detection of Abnormal Pressure

Zones

- 2. While drilling
- Drilling rate, gas in mud, etc. etc.
- D - Exponent
- DC - Exponent
- MWD - LWD
- Density of shale (cuttings)

Prediction and Detection of Abnormal Pressure

Zones

- 3. After drilling
- Resistivity log
- Conductivity log
- Sonic log
- Density log

.

.

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What is d-exponent?

Decreasing ROP

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 - Exponent

- If we assume that K 1
- and E 1
- Then

D - Exponent

- A modified version of this equation follows

Example

- Calculate the value of the d - exponent if the

drilling rate is 35 ft/hr, the rotary RPM is 100,

and the weight on the 12 1/4 bit is 60,000 lbs.

d 1.82

Example

- What happens to d if R doubles to 70 ft/hr?
- Note that an increase in R resulted in a decrease

in d. - Doubling R decreased d from 1.82 to

1.57

Example

- d may be Corrected for density as follows

Example 2

- Calculate d if R 20 ft/hr
- N 100 RPM
- W 25,000 lbf
- DB 9 7/8 in

d 1.63

Example 2

- If the normal pore pressure gradient in the area

is 0.433 psi/ft, and the actual mud weight is

11.2 /gal, what is dc?

dc 1.21

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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

Procedure for Determining Pore Pressure From dc -

Exponent

Normal Trend

Normal

Depth

Abnormal

dc - Exponent

Procedure for Determining Pore Pressure From dc -

Exponent

- If possible, quantify the magnitude of the

abnormal pore pressure using overlays, or Ben

Eatons Method

Pore Pressure Grad.

Normal Pore Pressure Grad.

Overburden Stress Grad.

In normally pressured shales, shale compaction

increases with depth

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.

Alternate Method Use variable density column.

See p. 270 in text

Pore Pressure from Resistivity

Depth

- Shale resistivity plots may be developed from

(i) logs or (ii) cuttings

10,000

What is the pore pressure at the point indicated

on the plot? Assume Gulf Coast. Depth10,000 ft

0.2 0.5 1 2 3

EATON

- From plot, Rn 1.55 ohms
- Robs 0.80 ohms
- From Eaton

0.7307 psi/ft 14.05 lb/gal

P 0.7307 10,000 7,307 psi