Post on 03-Apr-2018
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Classifying Pore Systems
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This section shows how to classify pore
types and explains how pores and fluids
interact.Rocks can be classified on the basis of
theirpore geometry into 4 major pore
categories that divided into 10subcategories.
A pore system: is an aggregate of pores
and pore throats that shares a similar
morphology .
(see Figure 9-9)
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A rocks storage capacity is controlled by the
size and numberof pores.
A rocks permeability (flow capacity) iscontrolled by the size, shape, and number of
pore throats.
Critical elements of pore systems geometry
Pore system shapes.
Pore & Pore throat size. Pore connectivity.
Ratio of pore throats to pores.
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Archie and non-Archie rocks porosity types
Include
petrophysically simple petrophysically complex(Archie porosity) (non-Archie porosity)
Water saturation (Sw) of the rocks with Archie
porosity can be predicted from log analysisusing Archie equation without modification.
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Sw= [ (a / m)*(Rw / Rt) ](1/n)
Sw: water saturation. : porosity .
Rw: formation water resistivity.
Rt: observed bulk resistivity. a: a constant (often taken to be 1).
m: cementation factor (varies around 2).
n: saturation exponent (generally 2).
To predict Sw in rocks with non-Archie porosity,
we modify the Archie equation.
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Characteristics of Archie and non-Archie rocks
I. Pore system shapes:I. Archie:
I. Intergranular (found between rounded particles)
II. Intercrystaline (found between angular particles)II. NonArchie:
I. Mold-likeI. Interparticle
II. Moldic
III. Shelter
II. Vug-likeI. Boring/burrow
II. Growth-framework
III. Fenestral
IV. Vug/channel/cavern
III. Fracture-like
I. Fracture
II. Shrinkage
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II. Relationship of pore shape to rock particles:
I. Archie:
I. Negative image of particles making up matrix
II. NonArchie:
I. Relates only indirectly to particles making up matrix
III. Pore connectivity:
I. Archie:
I. Pore throats connect pores into regular networks
II. Non-Archie:
I. Pores are irregular distributed and can be either poorly orvery well connected
IV. Porosity reduction processes:
I. Archie:
I. Grain coating or pore filling by calcite, silica, or dolomite
II. Non-Archie:
I. Pore or pore throat filling by clays or other minerals
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Pore and pore throat size have 2 defining parameter
1. Absolute size
2. Aspect ratio
Absolute size of a pore throat is the radius of a circle drawnperpendicularto fluid flow and fitting within its narrowestpoint.
Absolute size of a pore is the radius of the largest sphere thatwill fit inside it.
Measuring of pore and pore throat sizes: The pore size determined by measuring the radius of the
sphere.
The pore throat determined by measuring the radius of the
disk. Pore size can be measured using SEM.
Pore throat size can be measured using capillarypressure-mercury injection tests.
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Aspect ratio is the ratio of pore size to pore
throat size.
Aspect ratio have small ranges in intergranular
and intercrystalline.
Disparate Archie rock types (quartz-cemented
sandstones,bioturbated sandstone, and
sucrosic dolomites have aspect ratios that
range between 5:1 and 10:1.
Non-Archie rock types have even larger
variations in aspect ratio.
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Connectivity and pore throat size
Connectivity increases with the size of porethroats and with increasing numberof pore
throats surrounding each pore.
The number of pore throats that connect witheach pore is the coordination number.
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How pore shape, pore throat size, and pore
throat abundance effect the flow dynamics of
the reservoir?
Visualize a room (pore shape) with a door (pore
throat) in eachwall. The number of people who
can fit into the room (flow dynamics) is a
product of the size ,shape, and number ofdoors.
Pore throats control permeability to
hydrocarbons in reservoir rocks.
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Characterizing pore systems by size
Pore systems are easily characterized by size
using pore throat size.
Pore throat sizes can be measured using
capillary pressure curves.
A capillary pressure curve is converted to a
distribution profile of pore throat size, and a
pore throat size that characterizes the rock isdetermined by picking a certain saturation level.
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Which saturation level should we use?
A statistical correlation between optimal flow
through rocks and the radius of the pore throats
when 35% of the pore space of a rock is
saturated by a nonwetting phase during a
capillary pressure test.
They call the size of pore throat at 35%
nonwetting phase saturation r35 ,also called port
size.
Pore systems can be subdivided into port typesby port sizes
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Port size range(r35),Port category
>10Mega
2-10Macro
0.5-2Meso
0.1-0.5Micro
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Non-ArchieArchie
FractureVuggyClay
Cemented
IntercrystallineIntergranular/
interparticle
PoreGeometry/
Port Type
Fracture
width>50
Connected vugs
in a crystallinematrix
Chlorite or
illitecemented
(porelining)
coarsesandstone
-coarsely
Crystallinecarbonate
-Quartz- orCarbonate-
cementedCoarse
sandstone
Clean, coarse
Sandstone orCarbonate
grainstone
Mega/
Macro
Fracture
width5-50
Poorly
connected vugsor vugs/oomolds
in a fine tomedium
crystallinematrix
-Chlorite
or illitecemented
(porelining)
fine tomedium
crystallinematrix
-very fine to
mediumcrystalline
carbonate-Quartz or
carbonatecemented fine
to mediumsandstone
Clean, coarse
silt to veryfine
sandstone orcarbonate
grainstone
Meso
Fracturewidth