1 3 formation pressures

Formation Pressures• Lecture Objectives; – At the end of this course YOU will be able to: • Define various formation pressures including: – Hydrostatic pressure,– Overburden pressure,– P

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Formation Pressures

– Hydrostatic pressure, – Pore pressure,

– Overburden pressure, – Formation fracture gradient, – Causes of abnormal pressure, – Geopressure predication.

Formation Pressures

• Lecture Objectives;

– At the end of this course YOU will be able to:

• Define various formation pressures including:

– Hydrostatic pressure,– Overburden pressure,– Pore pressure,

– Formation fracture gradient.

• Use various techniques and methods to compute them,

• Plan well pressure profile applying safety margin,

• Understand how these pressures were generated,

• Describe some of the prediction techniques.

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Introduction

• The Study of Formation Pressures ( Overburden,

Following:

– Casing Design, – Avoid Stuck Pipe, – Well Control,

– ROP, – Mud Weight Design, – Overpressure Zones.

Fr act ur

e G ra die

nt

P or

e G ra die nt

Ov erb urd en G rad ien t

vertical height or depth of fluid column,

– Mathematically it is expressed as:

D f

g

HP = Hydrostatic pressure (psi),

g = Gravitational acceleration,

f = Average fluid density (ppg),

D = True vertical depth (ft)

– By converting wellbore pressure to gradient relative to to

a fixed datum (seabed or msl);

• It is possible to compare pore, fracture, overburden

pressure, MW and ECD on the same basis,

• Also gradient assists in plotting of these pressures.

Pore Pressure

• Definition:

• It is defined as the pressure acting on the fluids in the pore

spaces of the rock It is related to fluid salinity

• Normal Pore Pressure:

– Normal pore pressure is equal to the hydrostatic pressure of a column of formation fluid extending from the surface to the subsurface formation,

– The magnitude of normal pore pressure varies with concentration of dissolved salts, type of fluid, gas present and temperature gradient.

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Pore Pressure

• Definitions;

– Abnormal Pore Pressure :

• It is defined as any pore pressure that is greater than the

hydrostatic pressure of the formation water occupying the pore space

• The causes of abnormal pressure is attributed to the

combination of various geological, geochemical, geothermal and mechanical events

Pore Pressure

• Definitions;

– Subnormal Pore Pressure:

• It is defined as any pore pressure that is less than the

corresponding pore fluid hydrostatic pressure at a given depth,

• Occurs less frequent than abnormal pressures Might have

natural causes related to the stratigraphic, tectonic and geochemical history of an area

Subnormal Pore Pressure

Abnormal Pore Pressure

0.465 psi/ft (80,000 PPM)

Normal Pore Pressure

Overburden Pressure

Definitions;

• It is defined as the pressure exerted by the total weight of overlying formations above the point of interest,

– It is function of:

• Bulk density,

• Porosity,

• Connate fluids.

• It can also be defined as the hydrostatic pressure

exerted by all materials overlying the depth of interest.

Overburden Pressure

– Overburden pressure and gradient are given by:

ov = Overburden pressure (psi),

b = Formation bulk density (ppg).

D

052

ovg = overburden gradient (psi/ft),

 = porosity expressed as fraction,

Fracture Gradient

• Definition;

– It is defined as the pressure at which formation break down

occurs,

– Accurate prediction of fracture gradient is essential to

optimize well design,

– At well planing stage it can be estimated from the offset

well data

– If no data is available, then other empirical methods are

used for example:

• Matthews & Kelly (1967),

• Eaton (1969),

• Daines (1982).

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• Example of Computing Equations:

Fracture Gradient

– Eaton Method:

ov = Overburden pressure.( psi),

 = Poisson's Ratio, dimensionless.

D D

Fracture Gradient

– Breakdown Mechanism:

• Definition;

The Resulting Fracture in The Rock

Minimum Principle Stress

Intermediate Principle Stress

• To approximate fracture gradient, (Leak off Test),

• To investigate wellbore capability to withstand pressure

below the shoe,

• To collect regional information on the formation strength for

optimization of well design on future wells

– Cement pumping units should always be used in preference

to the mud pumps

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Fracture Gradient

• Formation Integrity Test (FIT):

– Leak-off Test Procedure:

• Drill 5 to 10 ft below the last casing shoe,

• Circulate to condition mud (MW in = MW out),

• POOH to last casing shoe,

• Connect cement unit to either drillstring or Kill line,

• Test surface lines,

• Close Pipe Rams,

• Start pumping at slow rate (i.e 1/4 BPM) (A),

• While pumping, observe the pressure build-up until it deviates (B),

• Record pressure (C),

• Bleed-off and record return.

Fracture Gradient

• Formation Integrity Test (FIT):

– Leak-off Test Procedure:

B

A-B: Linear Increase

B -C: Mud Penetrating the formation

C: Leak-off pressure is reached

D

*

) pressure (

FIT m

FG

052 0



 

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Causes of Abnormal Pressure

– Undercompaction:

developed due to disruption of the balance between rate of sedimentation of the clays and the rate of expulsion of the pore fluids as the clay compact with burial,

pores permeability, abnormal pore pressure will result.

• Depositional Related Effects:

– Deposition of Evaporites:

• The occurrence of evaporite deposits can cause high

abnormal pressure which approach the overburden gradient,

• Halite is totally impermeable to fluids and behaves

plastically thereby exerting pressure equal to the overburden gradient loads in all direction

Causes of Abnormal Pressure

– Example - Clay Diagenesis:

• With increasing pressure and temperature, sediments

undergo a process of chemical and physical changes,

• The diagenetic changes occur in shale by which abnormal

pressure may be generated

Causes of Abnormal Pressure

• Tectonic Related Effects:

– Tectonic activity can result in the development of abnormal pore pressure as a result of variety of mechanisms, these mechanisms are outlined below :

– Folding:

• Folding is produced by tectonic compression of a

geological basin This compression results in high pressure

Causes of Abnormal Pressure

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• Tectonic Related Effects:

– Faulting:

any of the following:

– There is commonly an increase in the rate and volume of

sedimentation across a fault on a downthrown block,

– This may introduce a seal against permeable formation thereby

preventing fluid expulsion,

– Non-sealing fault my transmit fluids from deeper formation to a

shallower zone, resulting in abnormal pressures in the shallow zone.

Causes of Abnormal Pressure

• Tectonic Related Effects:

– Faulting:

Hydrostatic Pressure Sands

Abnormally Pressure Sands

Hydrostatic Pressure Sands

Causes of Abnormal Pressure

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• Tectonic Related Effects:

• Gas Units in Mud,

• Chloride Content Indication,

• Temperature Indication,

• Wireline Logs.

– The more compactions are the higher will be the velocity

of sound Such change in velocity is further converted to abnormal pressures,

– After the well has been drilled, a confirmation survey is run such as Vertical Seismic Profile (VSP).

Geopressure Predictions

D = Bit diameter (inches),

d = Drilling exponent, dimensionless

d c = Corrected drilling exponent, unitless

d 1 = Formation fluid density.(ppg),

d 2 = Mud weight (ppg).

D 10

W

12 log

N 60

R log

d

2

1 c

Geopressure Predictions

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• Sloughing Shale:

– Maybe the result of the following hole conditions;

movement.

sloughing shale are noted,

Geopressure Predictions

• Shale Density:

– Normal trend for the density of compacted shales is to

increase with depth,

– If this trend is reversed it is assumed that the pore pressure

is increased.

Norm

al v alues

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• Gas in Mud:

– Gas may enter the mud as a result of the following:

• Mud Properties:

– Measurement of mud properties in and out of the hole could provide first warning of gas or chloride content change,

– Gas will decrease mud weight, – Chloride content will cause flocculation in the mud system and hence will result in an increase in YP and PV And hence, attention should be given if such problem takes place.

Geopressure Predictions

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– Continuous measurement of flowline temperature will

possibly indicate an entrance into overpressure zone,

– As the formation is compacted, thermal conductivity is

increased which indicates that an overpressure zone has been entered,

• Not applicable in offshore (C.P or Riser)

– Other factors could cause an increase in temperature namely:

– Increase in circulation rate,– Change in solids content in mud,– Increase in bit torque.

Geopressure Predictions

Formation Pressures

• Now YOU should be able to:

– Define various formation pressures including:,

– Understand how these pressures were generated, – Describe some of the prediction techniques

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Formation Pressures

End of Lecture