Porosity

Various types

One distinguishes several kinds of porosity:

occluded or closed porosity: it is the porosity of the pores nonaccessible by the external agents (unusable for the exploitation of the resource);
free porosity: in opposition to occluded or closed porosity;
trapped porosity: it is a free porosity not allowing the recovery of the trapped fluids;
useful porosity: it is the porosity which allows the recovery of the trapped phase (term mainly used by the tankers);
residual porosity: it is porosity due to the pores communicating step between them or with the external medium;
total porosity: it is the sum of useful porosity and residual porosity;
effective porosity: it is a term especially employed in hydrogeology. This porosity is that where water circulates and is recoverable.

The porous rocks can be rocks tanks, i.e. containing a fluid (Natural gas, Pétrole, Eau); this fluid can be arrived naturally (gas or oil natural reserves) or injected by the man underground storages).

Models of porosity

Statistical models

The statistical models consist in defining a function of points F (M), where M is a point depend on the coordinates of spaces.

One then allots value 1 to the function if the point M is in the vacuum, and value 0 if the point is in the solid.

These models make it possible to model in space the porosity of a material. However they give bad qualitative results.

Arrangements of spheres

Capillary beam - Model of Purcell

This model makes it possible to model porosity but also the permeability. It consists in defining a certain number of right capillaries which cross material. This model is satisfying conceptually but in practice it represents reality badly. Indeed, the capillaries are right and do not communicate between them.

Rose and Bruce improved this model by taking of account tortuosity Τ of the capillaries.

Model in network of Fatt (1956)

Model of Houpeurt and Ehrlich

Measure porosity

To measure porosity, one can determine three parameters:

Vt, which is the total volume of the sample;
Vs, which is the volume of the sample without its porosity;
Vp, which is the volume of the pores.

Direct methods of measurements at the laboratory

One distinguishes:

measurements on samples " frais" or " préservés" ;
measurements on samples " extraits" or " exposés".

Measure porosity on samples " frais" or préservés"

There exists only one method which is known as " of summation of the fluides". It implies to coat the sample (with paraffin for example) at the exit with the coring, so that the fluids present in porosity do not escape.

The volumes of air are measured using a porosimeter with mercury. Volumes of water and hydrocarbons are measured by split distillation at ordinary temperature.

Measure porosity on samples " extraits" or " exposés"

In laboratory, the samples must be in the same physical status before carrying out measurements, which forces to prepare them. It is first of all necessary to extract the fluids from the sample, with, for example:

a Soxhlet extractor;
a Dean-Stark extractor;
an extraction by centrifugation;
an extraction by flash distillation.

Measure Vt total volume

Measurement

Measure of Vt using a positive-displacement pump

Measure by thorough of Archimedes

Measure of Vs

Use of the pycnometer

Method of immersion

Use of a room of compression

Measure of Vp using a porosimeter with mercury

Determination of porosity by indirect methods: diagraphics

See too

specific Capillarity
Surface
Permeability

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