Chapter 05 - Pore Pressure

8/17/2019 Chapter 05 - Pore Pressure

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Chapter FiveChapter Five: Pore Pressure: Pore Pressure

Topics

Pressure Gradients Explained

Hydrostatic Pore Pressure

Stress Generated Pore Pressure

Geology Implications ofOverpressure

Pressure Calculation Methods


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© 2005 GeoMechanics International2

What is Pore Pressure?


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Absolute Pressure vs. Depth

WaterTable

Effective

Stress


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Equivalent Mud Weight Plot

Temsah Field, Nile Delta, Egypt

Mazzoni,T. et al., 1997



Pressure Gradients

Water/

Brine

Pressure

Saline

Fresh

Fresh 0.43 psi/ft

Saline up to 0.54 psi/ft

HydrocarbonGas

Oil

Oil 0.29-0.41 psi/ft

Gas 0.04-0.13 psi/ft

Pressure

8.3-10.4ppgGas 0.8-2.5ppg

Oil 5.5-7.9ppg



Pressure Gradients for Different Fluid Types

0.43-0.54 psi/ft

0.29-0.41 psi/ft

0.04-0.13 psi/ft

OWC

GOC

How would the

pressures look if they

were replotted inequivalent mud

weight?



Overpressured

Seals andCompartments

Nile Delta, Egypt

The model shown assumes that

adjacent seal (shale) and

compartment (sand) pressures arein equilibrium. Is this correct?



Lateral Pore Pressure Variations

c)

Norwegian North Sea

Grollimund et al., 2000How could lateral seals develop as opposed tovertical seal?



Overpressure Generation Mechanisms

Stress Generated

Undercompaction

Tectonic compression

Thermally Generated

Aquathermal

Diagenetic

Fluid Redistribution inPermeable Zones

Buoyancy

Centroid Lateral transfer

Hydraulic head

Osmosis


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Underpressure

GenerationMechanisms

Swarbrick, R.E. and Osborne, M.J., 1998


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Multiple Overpressure Generation Mechanisms

Bowers, G.L., 2001


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Pressure Redistributed within Permeable Zones

Mouchet, J.P. and Mitchell, A., 1989


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Fertl, W.H., 1976

Weeks Island, La

10.0ppg

9.1ppg

8.6ppg

Normallypressured

shales

Pressure Redistributed within Permeable Zones

What is causing the

overpressure at the top

of the reservoir?


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A Dynamic Overpressure System

Pore pressures increase or decrease in response

to burial or tectonic stresses

Thermally induced diagenetic changes can lead

to over- or under-pressures at depth

Pressures are leaked and redistributed through

permeable zones at rates dependent on the

system permeability

Very Difficult to Model and Predict


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A Dynamic Overpressure System

Holm, G.M., 1996

What mechanism

could cause the

episodic breach & fluid

loss?


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Reservoir Depletion

Brown, 1987

What causes the

increase in pressure in

the lower Tor

formation?


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Hydrostatic (Normal)Hydrostatic (Normal)

Pore PressuresPore Pressures


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Hydrostatic (‘Normal’) Pore Pressure

A hydrostatic pressure is one in communication with

the surface free water level

• Offshore – Sea level

• Onshore – Water Table

Hydrostatic Pressures depend on fluid density – The density of formation water varies with the concentration ofdissolved solids, mostly salt

– Salinity varies as a function of;

• Original water history

• Temperature

• Diagenesis

• Proximity to Salt bodies

• Osmosis


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Stress Generated PressuresStress Generated Pressures


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Compaction

Expulsion of Water in Response to Stress



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Typical Compaction Curves

So why are the curves

so different?


These areall shales


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Undercompaction

Undercompaction occurs when sediment dewatering is

inhibited during burial. This happens if either;

– new sediments are deposited before fluid has had chance todrain, i.e. rapid burial

– or seals and barriers to fluid flow form

During undercompaction pore fluids are trapped,porosity is retained, and further compaction is slowed

or prevented

As burial continues pore fluids support part of the

weight of the overlying sediments and the fluid

becomes overpressured


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Porosity and Depth

mud

clay

mud-stone

shale

0 0.25 0.50 0.75 1.0

clay & shale,

“normal” line

sands &

sandstones

effect of overpressureson porosity

depth

porosity

slate (deep)

+T


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Compaction and Undercompaction

Swarbrick, 2001

‘T t i ’ St


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‘Tectonic’ Stress

‘Tectonic’ stress is a stress often higher than

the overburden. In these cases mean stressshould be considered for calculating pore

pressure

Overburden Stress Sv

Mean Stress (σv + σHmax + σhmin)/3

St R li f (U l di )


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Loading and Unloading

Experiment

Stress Relief (Unloading)

Moos & Gretchen, 1998

Loading and Unloading Stress Paths


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Loading and Unloading Stress Paths

Unloading can occur due to;

stress relief (with constant pore pressure)

In situ pore pressure increase (with constant

stress)

Sayers,C.M. et al., 2001

What geological or oilfield

processes could cause

stress relief ?

What geological or oilfield

processes could cause a

pore pressure increase in

situ?


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Thermally Generated Mechanisms

Aquathermal expansion of fluids

• PVT relationship

– If Temperature increases and volume doesnot change, then pressure will rise (pressure

cooker effect)

• Requires a very good seal – May explain very high pressure in rafts

within Salt

Hydrocarbon Generation


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Hydrocarbon Generation

Generation and Cracking of Hydrocarbons

• Phase changes from kerogen to oil to gas

• Results in an in-situ increase in fluid or gas volume

• Spatially, high overpressure is often associated

with sediments rich in organic matter actively

generating hydrocarbons, e.g., Kimmeridge Clay,Bakken Shale

Mi l Di i


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Mineral Diagenesis

• Mineral phase changes occur mainly in response

to increased temperature, e.g. – Gypsum to Anhydrite

– Smectite to illite

• Solution and precipitation

– In response to increasing temperature

– May also be controlled by pressure andavailability of ions

– More common in permeable zones

C i Th l M h i


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Comparison Thermal Mechanisms

Swarbrick et al , 2002


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Geological Implications of OverpressureGeological Implications of Overpressure

Artesian Pressures


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Artesian Pressures

Elevated formation pore pressure from increased hydrostatic head

Are these hydrostatic

(‘normal’) pressures?

Artesian Pressures


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Lowered formation pore pressure from reduced hydrostatic head

Artesian Pressures


Hydrocarbon Buoyancy Effects


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Hydrocarbon Buoyancy Effects

Hydrocarbon Columns Create Local OverpressureBowers, G.L., 2001

Hydrocarbon Buoyancy - Iran


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Hydrocarbon Buoyancy - Iran

Fertl, W.H., 1976

If reservoir and shale

pressure can be so

different, what does

thissay about the methodswe typically use for pore

pressure prediction?

Centroid Effect


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Centroid Effect

Bowers, G.L., 2001

How large is the centroid

effect if the reservoir is

normally pressured?

What happens if

hydrocarbons are added

to the reservoir?

Centroid and Compartmentalisation


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Centroid and Compartmentalisation

How could

you

determine

compartment

alization pre-

drill?

Lateral Transfer


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Lateral Transfer

Caillet,G. et al., 1998

N. Sea Chalk FieldsCaillet,G. et al., 1997

Lateral Transfer


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Lateral Transfer

North Sea Viking Graben

Pressure communication

through many reservoirs

over a large area

Localized oil and/or gas

hydrostatic columns within

each reservoir

What would

the shale

pressures be

in this area?


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Pore Pressure Calculation MethodsPore Pressure Calculation Methods

Trend-line Methods


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Trend line Methods

Require the establishment of a normal trend-line

Normally pressured compaction curve (NCT)

Applies only to ‘clean’ shales Many different methods

Early popular methods include Ratio, Equivalent

Depth, & Eaton Many other methods, many proprietary

Applicable to many types of data

d-exponent, sonic, velocity, resistivity, density, etc

Regional overlays can be constructed and applied

to new wells and even new regions

Normal Compaction Trend


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Normal Compaction Trend1000

1500

2000

2500

3000

3500

0.1 1 10

Dc Exponent

T V D ( m )

NCT

Top

Overpressure

Normally compacted

(normally pressured)

down to 3000m Normal compaction trend

(NCT) fitted in blue

A deviation from the trend

in the same rock type

indicates overpressure

Can be applied to any

formation porosity

informationWhat pitfalls might youexpect plotting the NCT?

Ratio Method


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The difference between observed values and the normaltrend-line extrapolated to the same depth is proportional tothe increase in pressure.

for sonic logs =

for density logs =

for resistivity logs =

Where : ΔTn = the value of the normal trend-line at a given depth, P =the pressure value to be calculated, Phyd = normal hydrostatic porepressure, ΔT log = log-value value for each curve corresponding to therequired pressure value.

Ratio Method

Ratio Method


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Ratio Method

Very simple

calculation

Does not take into

effect overburden

stress differences


Equivalent Depth Method


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)( Bd GG ZA ZBGG Ad eql B Aeql −−=

Where : deql A = equilibrium density at A, deqlB = equilibrium density

at B, ZB = equivalent depth, ZA = depth of undercompacted clay,

GG A= overburden gradient at A, GGB= overburden gradient at B

• Every point in and undercompacted clay (A) isassociated with a normally compacted point (B)

• The compaction at point A and B is identical, but the

overburden stress has increased, so:



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q p

Calculates pressures from

the depth of an equivalent

value on normalcompaction trend-line

Very simple calculation

that takes into effect localoverburden stress

Only applies to

overpressures generatedby undercompaction

Mouchet, J.P. and Mitchell, A., 1989In reality, what methods could the

engineer use to fudge the results using

this method?

Eaton Method


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Calculates a pore pressure based on therelationship between the observed

parameter/normal trend-line ratio and the

overburden gradient

for resistivity

for sonic

where : P = formation pressure, S = overburden, Rsh = resistivity of

shale, ΔT = sonic transit times, log = observed values of the log at the

given depth, n = value of normal at the given depth, hyd = normal

hydrostatic pressure

( )1.2

nsh

logsh

hyd

R

R PSP ⎟⎟

⎠

⎞⎜⎜

⎝

⎛ −−= S

( )3.0

log

nhyd

T

TPSP

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

Δ

Δ−−= S

Eaton

exponent

Comparing Trend-Line Method Results


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g/cc g/cc

What’s Wrong with Trend-Lines?


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Swarbrick, R.E., 2001What else is wrong with a trend-line

approach?

What’s Wrong with Trend-Lines?


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Bell, 2002

Could there be a better and moreconsistent way to generate a trend-line?

Thermally Generated Overpressures


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Bowers, G.L., 2001

Why do you think that undercompactionwas left relatively unchallanged for so

long?

Thermally Generated Pressures


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High pressures often associated with low

porosities are typical

Caused by thermally generated fluid

expansion that unloads the rock

The traditional undercompaction

(effective stress) – porosity relationships

breakdownWe need to use effective stress –

unloading relationship

Chapter 05 - Pore Pressure

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