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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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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
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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
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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?
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Overpressured
Seals andCompartments
Nile Delta, Egypt
The model shown assumes that
adjacent seal (shale) and
compartment (sand) pressures arein equilibrium. Is this correct?
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Lateral Pore Pressure Variations
c)
Norwegian North Sea
Grollimund et al., 2000How could lateral seals develop as opposed tovertical seal?
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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
Mouchet, J.P. and Mitchell, A., 1989
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Typical Compaction Curves
So why are the curves
so different?
Mouchet, J.P. and Mitchell, A., 1989
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
Mouchet, J.P. and Mitchell, A., 1989
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
Mouchet, J.P. and Mitchell, A., 1989
Equivalent Depth Method
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Equivalent Depth Method
)( 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:
Equivalent Depth Method
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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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Mouchet, J.P. and Mitchell, A., 1989
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