1.4- reservoir rock
DESCRIPTION
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RR > Reservoir Rock
Sedimentary Rock Cycle, Rock Types, Igneous and Metamorphic rocks,
sedimentary rocks, Clastics, Carbonates, Porosity and Permeability,
Capillary Pressure
1.4- Reservoir Rock
Dr. M. Watfa
1.4 : Reservoir Rock
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Sedimentary
Rock
Process
Source Rocks
Igneous Metamorphic Sedimentary
Weathering
Mechanical and Chemical
Deposition Clastics Carbonates Evaporites
Compaction Dissolution Precipitation
Diagenesis
Sedimentary Rock Layers
Sedimentary Rock Cycle
1.4 : Reservoir Rock
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The start and end of all rocks is the magma in the mantle.
This is cooled to create igneous rocks.
These can be broken down into sediments.
The sediments are turned into sedimentary rocks.
These can be buried deeper with heat and pressure, turning into metamorphic rocks.
If these are then heated we return to the magma.
Inside this major cycle are sub-cycles. Igneous rocks can be heated to give metamorphic rocks.
Any rocks can be broken into sediments to give sedimentary rocks.
Sedimentary Rock Cycle
1.4 : Reservoir Rock
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Rocks and Rock
Types:
Sedimentary
Characteristics
This chart shows the
relative abundance of
most sedimentary
rocks.
Sedimentary Rock Types- Relative abundance
Sedimentary Rock Cycle
1.4 : Reservoir Rock
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Carbonate
Fraction
130 billions barrels of oil
300 billions barrels of oil
World Oil Reserves
1.4 : Reservoir Rock
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Rocks and Rock Types:
There are three main types of rock which are classified as:
igneous,
metamorphic
sedimentary
Igneous rocks: Formed from molten material deep in the earth’s crust. This includes granite
Metamorphic rocks- Modified by high pressure and temperature, such as gneiss.
Igneous and metamorphic rocks are called basement rocks. Only when highly fractured can these rocks serve as a reservoir.
Sedimentary Rocks: Eroded, transported and deposited.
Rock Types
1.4 : Reservoir Rock
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These are the most important for the oil industry as it contains most of the source rocks and cap rocks and virtually all reservoirs.
Sedimentary rocks come from the debris of older rocks and are split into two categories
Clastic
and Non-Clastic.
Clastic rocks - formed from the materials of older rocks by the actions of erosion, transportation and deposition.
Non-Clastic rocks - Formed from chemical or biological origin and then deposition.
Sedimentary Rocks
Rock Types
1.4 : Reservoir Rock
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Rock Types: Sedimentary
Clastic
– Boulders/Cobbles, Granules(>2mm)
– Sand (0.06 – 2.0 mm)
– Silt (0.004 – 0.04 mm)
– Clay (<0.004 mm)
Carbonate
– Limestone / Dolomite
Evaporite
– Salt / Gypsum
Rock Types
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The depositional environment
can be
Shallow or deep water.
Marine (sea) and lake or
continental.
This environment determines
many of the reservoir
characteristics
Sedimentary Rocks-
Depositional Environments
Rock Types
1.4 : Reservoir Rock
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The structure of a reservoir can
also be determined by
deposition; a river, a delta, a
reef and so on.
This can also lead to
permeability and producibility. of
these properties are often
changed by further events.
The depositional characteristics
of the rocks lead to some of
their properties and that of the
reservoir itself.
The reservoir rock types are
either clastic or non-clastic.
The type of porosity (especially
in carbonates) is determined by
the environment plus
subsequent events.
Sedimentary Rocks-
Depositional Environments
Rock Types
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The environment is not
static.
Folding and faulting change
the structure.
Diagenesis (Dissolution and
fracturing) can change the
porosity & permeability.
Sedimentary Rocks-
Depositional Environments
Rock Types
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Sediments settle to the
bottom of the sedimentary
basin.
As the sediments
accumulate the
temperature and
pressure increase
This process expels water
from the sediments.
Sedimentary Rocks-
Depositional Environments:
Sedimentations
Rock Types
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Sedimentary muds become sedimentary
rocks.
Calcareous muds become limestone.
Sands become sandstone.
Another effect involves both the grains in the
matrix and the fluids reacting to create new
minerals changing the matrix and porosity.
Fluids can also change creating a new set of
minerals.
This whole process is called Diagenesis.
Sedimentary Rocks-
Depositional Environments:
Sedimentations
Rock Types
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Comprise 95% of the Earth's
crust.
Originated from the
solidification of molten material
from deep inside the Earth.
There are two types:
Volcanic - glassy in texture
due to fast cooling.
Plutonic - slow-cooling,
crystalline rocks.
Igneous Rocks
Granite
Igneous & Metamorphic Rocks
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Igneous rocks can be part of
reservoirs.
Oil could migrate up due to
geometric location
Fractured granites form
reservoirs in some parts of the
world.
Volcanic tuffs are mixed with
sand in some reservoirs.
Igneous Rocks and
Reservoirs
Granite
Igneous & Metamorphic Rocks
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Metamorphic Rocks are formed by
the action of temperature and / or
pressure on sedimentary or
igneous rocks.
Examples of Metamorphic Rocks
Marble: formed from limestone
Hornfels: from shale or tuff
Gneiss (pronounced- NICE): similar to granite but formed by metamorphosis
Metamorphic Rocks
Gneiss
Schist
Igneous & Metamorphic Rocks
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Clastics
Consist largely of quartz (silicon oxide SiO2)
Clastic rocks – formed from rock debris
Sand grains cemented to form rock
Commonly contain other silicate minerals: clays, micas, feldspars
Quartz has low reactivity due to very low solubility in brine
Ref: T. Jones / SCR
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Clastic Depositional
Environments
Alluvial Fan
Lacustrine
Eolian
Fluvial
Delta
Shelf
Marine
Sandstone
Clastics
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Clastic Rock Clastic rocks are:
sands,
silts and
shales.
The difference is in
the size of the
grains.
Clastics
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Sediments are transported
to the basins by rivers.
A common depositional
environment is the delta
where the river empties into
the sea.
A good example of this is
the Mississippi and the Niger
Delta.
Clastic Rock- Depositional
Environment - DELTA
Clastics
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Some types of deposition occur in rivers and sand bars.
The river forms a channel where sands are deposited in layers. Rivers carry sediment down from the mountains which is then deposited in the river bed and on the flood plains at either side.
Changes in the environment can cause these sands to be overlain with a shale, trapping the reservoir rock.
Clastic Rock- Depositional
Environment - RIVER
Clastics
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High Sphericity
Low Sphericity
Very Angular Angular
Sub- Angular
Sub- Rounded Rounded
Well- Rounded
(Geologists like their sandstones well rounded and with high sphericity)
Roundness and Sphericity of Clastic Grains
Clastics
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Very Well Sorted
Well Sorted
Moderately Sorted
Poorly Sorted
Very Poorly Sorted
Grain-Size Sorting in Sandstone
Clastics
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Change of Composition Change of Size
Change of Shape Change of Orientation
Change of Packing
Sand
Shale
Eolian
Fluvial
Slow Current
Fast Current
River
Beach
Types of Textural Changes Sensed by the Naked Eye as Bedding
Clastics
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NMR and FMI in Sandstone
X100-x110: Shaly interval. Low
T2 and hence small pores.
X110-x120: mainly clean
sandstone with small shale as
shown by small T2 values.
X120-x140: FMI shows thin
shale streaks. NMR shows more
low values of T2 confirming the
presence of shale. This is also
confirmed by the high resolution
NMR spikes on the FFV and
BFV.
X140-TD: Tight formation with
low T2 and small pores.
Clastics
1.4 : Reservoir Rock
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Carbonates encompass limestones (largely calcite CaCO3) and dolomites (largely CaMg(CO3)2 )
Formed by carbonate precipitation and aggregation of animal shells
Often associated with evaporite minerals
High reactivity due to relatively high solubility in brines (0.15 grams/litre in 1 molar sodium chloride solution
Wide range of pore sizes, from vugs (~ cm) to micropores (< 1 mm)
Carbonates
Ref: T. Jones / SCR
1.4 : Reservoir Rock
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Carbonates form a large
proportion of all permeable
sedimentary rocks ( ≈ 14%).
They consist of:
Limestone.
Dolomite.
Carbonates usually have an
irregular pore structure.
Often, a formation has a mixture
of Limestone and dolomite
Limestone
Dolomite
Carbonates
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Carbonate Types
Chalk is a special form of limestone
and is formed from the skeletons of
small creatures (cocoliths).
Dolomite is formed by the replacement
of some of the calcium by a lesser
volume of magnesium in limestone.
Magnesium is smaller than calcium,
hence the matrix becomes smaller
and more porosity is created.
Limestone Ca CO3
Dolomite Ca CO3 Mg CO3
Evaporites such as Salt (NaCl) and
Anhydrite (CaSO4) can also form in
these environments.
A dolomite is formed when one
magnesium ( Mg) molecule replaces a
Calcium (Ca) molecule
Carbonates
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Porosity Types- Carbonates Interparticle: Pores between particles or grains
Intraparticle: Pores within individual particles
Moldic Pores formed by dissolution of an individual grain crystal in the
rock
Fracture: Formed by a planar break in the rock
Vug Large pores formed by indiscriminate dissolution of cements and
grains
Carbonates
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Dunham Carbonate Rock Classification
Depositional Texture Recognizable Depositional Texture
Not Recognizable
Mudstone Wackestone Packstone Grainstone Boundstone Crystalline Carbonate
Grain Supported
Lacks Mud, Grain-
Supported
Components Not Bound Together During Deposition
Mud Supported
Contains Mud (clay and silt size particles
<10 % Grains
>10 % Grains
Original Components Bound Together
During Deposition
(modified from Dunham, 1962)
Carbonates
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Carbonate Porosity - Example
Thin section micrograph - plane-polarized light
Smackover Formation, Alabama (Photograph by D.C. Kopaska-Merkel)
Moldic
Pores
• Due to dissolution
and collapse of ooids
(allochemical particles)
• Isolated pores
• Low effective porosity
• Low permeability
Blue areas are pores.
Calcite
Dolomite
Moldic
Pore
Carbonates
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Cross section
showing complex
facies relations in a
carbonate reef
setting. Reservoir
quality varies with
facies.
Carbonate Depositional Environment
Reef System
Carbonates
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Carbonate Rock Distribution
Reef; Shelf Carbonate, Deep Carbonate; Carbonate Oil Province
Carbonates
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Sedimentary Rock Characteristics
Porosity
– The percentage of pore volume or void space
that can contain fluids
Permeability
– The measure of how easily fluid moves through
a rock, typically measured in Darcies or
millidarcies
Sorting
– Range of sedimentary grain sizes that occurs in
sedimentary rock
Matrix (lithology) - major constituent of the rock
Porosity and Permeability
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How permeability is formed will
depend on many factors.
For the same porosity, a wide
range of permeabilities can
develop.
These various controls can be
grouped under various facies
categories
Effects of various controls on
Porosity & Permeability
Porosity and Permeability
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1- Definition of Porosity
Sand
Anhydrite
Shale
The Matrix could be complex Lime-Dol
Porosity and Permeability
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A rock can be made up of small grains or large grains but have the same porosity.
Porosity depends on grain packing, not the grain size.
In a clastic rock the grain size ( same size grains ) does not affect the porosity.
A sand, a silt and a shale can have the same porosity .
Differences come in permeability where the grain size has a direct effect, large grains meaning higher permeability.
Porosity in sandstones: Grain Size
Different grain size- same porosity
Porosity and Permeability
1.4 : Reservoir Rock
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The porosity of a sandstone depends on the packing arrangement of its grains. The system can be examined using spheres.
In a Rhombohedral packing, the pore space accounts for 26% of the total volume.
With a Cubic packing arrangement, the pore space fills 47% of the total volume. In practice, the theoretical value is rarely reached because:
the grains are not perfectly round, a
the grains are not of uniform size.
Porosity in sandstones: sorting
Porosity and Permeability
1.4 : Reservoir Rock
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The environment can also involve
subsequent alterations of the rock
such as Chemical changes.
Diagenesis is the chemical alteration
of a rock after burial.
An example is the replacement of
some of the calcium atoms in
limestone by magnesium to form
dolomite.
Mechanical changes - fracturing in a
tectonically-active region.
Porosity in Carbonates: Diagenesis and secondary porosity
Porosity and Permeability
1.4 : Reservoir Rock
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Mouldic porosity: Pores created by
the dissolution of shells, etc.
Interparticle porosity:
Each grain is separated, giving a
similar pore space arrangement as
sandstone.
Intergranular porosity:
Pore space is created inside the
individual grains which are
interconnected.
Intercrystalline porosity:
Produced by spaces between
carbonate crystals.
Porosity in Carbonates: Diagenesis and secondary porosity
Porosity and Permeability
1.4 : Reservoir Rock
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Vuggy porosity:
Created by the
dissolution of fragments,
but unconnected.
Fracture porosity:
Pore spacing created by
the cracking of the rock
fabric.
Channel porosity:
Similar to fracture
porosity but larger.
Porosity in Carbonates: Diagenesis and secondary porosity
Porosity and Permeability
1.4 : Reservoir Rock
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Intergranular porosity is called
"primary porosity".
Porosity created after deposition is
called "secondary porosity".
The latter is in two forms:
Fractures
Vugs.
Definition of Porosity Carbonate Porosity Types
Porosity and Permeability
1.4 : Reservoir Rock
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Fractures are caused when a
rigid rock is strained beyond its
elastic limit - it cracks.
The forces causing it to break
are in a constant direction,
hence all the fractures are also
aligned.
Fractures are an important
source of permeability in low
porosity carbonate reservoirs.
Fractures
Porosity in Carbonates: Fractures
Porosity and Permeability
1.4 : Reservoir Rock
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Vugs are defined as non-
connected pore space.
They do not contribute to the
producible fluid total.
Vugs are caused by the
dissolution of soluble material
such as shell fragments after
the rock has been formed.
They usually have irregular
shapes.
Porosity in Carbonates: Fractures
Porosity and Permeability
1.4 : Reservoir Rock
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Carbonate
Dissolution
Cavity:
Carbonates
have dissolution
cavities- but not
as large as this
cave.
Vugs
Courtesy Schlumberger
Porosity and Permeability
1.4 : Reservoir Rock
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The rate of flow of a liquid
through a formation depends on:
The pressure drop.
The viscosity of the fluid.
The permeability.
The pressure drop is a
reservoir property.
The viscosity is a fluid
property
The permeability is a measure of the ease at which a fluid can flow through a formation.
Relationships exist between permeability and porosity for given formations, although they are not universal.
A rock must have porosity to have any permeability.
The unit of measurement is the Darcy.
Reservoir permeability is usually quoted in millidarcies (mD).
Permeability Definition
Porosity and Permeability
1.4 : Reservoir Rock
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The flow of fluid of viscosity m
through a porous medium was first
investigated in 1856 by Henri
Darcy.
He related the flow of water
through a unit volume of sand to
the pressure gradient across it.
In the experiment the flow rate can
be changed by altering the
parameters as follows:
Permeability Definition
Darcy Experiment Q = f(P1-P2); Q = f (1/L); Q = f( A), Q= f (1/µ)
Q = Constant . A . (P1-P2)/ (µ . L)
Q = K . A . (P1-P2)/ (µ . L)
P1
P2
Area: A
L
Porosity and Permeability
1.4 : Reservoir Rock
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K = permeability, in Darcies.
L = length of the section of rock,
in centimeters.
Q = flow rate in centimeters /
sec.
P1, P2 = pressures in bars.
A = Pore area, in cm2.
µ = viscosity in centipoise.
Permeability Definition
Parameters
P1
P2
Pore Area: A
L
K = Q. µ . L / { A . ( P1 - P2 ) }
Porosity and Permeability
1.4 : Reservoir Rock
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Production rate
Radial flow rate is most important
Require values for the following
– ko = Permeability
– h = Net Pay
– Pe = Reservoir
– Pw = Bottom hole pressure
– μ = Fluid viscosity
– Bo = Formation volume factor
– re/rw = Drainage & wellbore radii
Radial Flow Rate
qo= 7.08 ko h (Pe – Pw)
μ Bo ln (re / rw)
rw
re
Porosity and Permeability
1.4 : Reservoir Rock
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In formations with large grains,
the permeability is high and the
flow rate larger.
In a rock with small grains the
permeability is less and the flow
lower.
Grain size has no bearing on
porosity, but has a large effect
on permeability.
Permeability Definition
Permeability and Rocks
Porosity and Permeability
1.4 : Reservoir Rock
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50 m-Darcy 10 m-Darcy
> 25 Darcy
5 m
m M
arb
les
10cm Diameter cup
>5 Darcy 1
.5 m
m M
arb
les
( B
ea
ch
Sa
nd
) 500 m-Darcy
1 mm
Sam
e po
rosi
ty Φ
≈ 2
5 %
Porosity and Permeability
1.4 : Reservoir Rock
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Fractures Compaction
& Cementing
Compaction
& Leaching
Porosity
Per
mea
bili
ty
The relationship
between porosity and
permeability for
various carbonate
rocks.
Permeability in
Carbonates
After R. Nurmi- 1986
Courtesy Schlumberger
Porosity and Permeability
1.4 : Reservoir Rock
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Free Fluid Index In Carbonates
Classical example of carbonates
with bi-modal porosity.
A free fluid index cutoff of 100
msec was used based on core
centrifuge.
Below X415: Sw ≈100%. NMR
shows low T2 distribution and
mainly BFV.
This is confirmed by the low
permeability values.
This allowed perforations to be
made to be made as low as X410
without producing high water cut.
This essentially improves the
hydrocarbon recovery. Courtesy Schlumberger
Porosity and Permeability
1.4 : Reservoir Rock
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Surface Tension
Interaction between hydrocarbons and water in the
reservoir depends on the surface tension between them
Surface tension is the apparent film which separates two
immiscible fluids
A pressure difference exists across any curved interface
Capillary Pressure
1.4 : Reservoir Rock
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Apparent Surface Film caused by
Imbalance of Molecular Forces
Capillary Pressure
1.4 : Reservoir Rock
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Pressure in a bubble
r
P1
P2
P1 – P2 = 2 . σ / r in dynes
Where:
P2 = Pressure inside Bubble dynes / cm2
P2 = Pressure outside Bubble dynes / cm2
r = Bubble Radius cm
σ = surface tension dynes/cm
1 psi = 68,948 dynes / cm2
Capillary Pressure
1.4 : Reservoir Rock
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Water 72.6 dynes/cm
Benzene 28.9 dynes/cm
Cyclohexane 25.3 dynes/cm
Surface tensions between some common fluids and air at 20° C
Interfacial tension between water
and oil at 20° C 30 dynes/cm
Interfacial tension between a liquid
and its vapor decreases with
temperature increase until at the
critical point, surface tension is
zero and differentiation between
fluid/vapor phases ceases to exist.
Contact Angle as a
measure of wettability
Capillary Pressure
1.4 : Reservoir Rock
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Contact Angle as a Measure of Wetting
Capillary Pressure
1.4 : Reservoir Rock
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Capillary Rise Fluid Rise in a Capillary
Tube Bundle
Capillary Pressure
1.4 : Reservoir Rock
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Shape of the Capillary Pressure vs.
Saturation Curve
Capillary Pressure
1.4 : Reservoir Rock
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Capillary Pressure
Shape of
Capillary
Curve
and Grain
Size
Distribution