classifying pore systems

7/29/2019 Classifying Pore Systems

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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

classifying pore systems

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