Porosity and Fluid Saturations

Porosity is the best known physical characteristic of an oil reservoir.It determines the volume of oil or gas present,and all recovery computations must be based on knowledge of its value.
Porosity of a material is defined as that fraction of the bulk volume of this material that is not occupied by the solid framework of the material.In oil reservoirs,the porosity represents the percentage of the total space that is available for occupancy by eighter liquids or gases.It determines the storage capacity of the sand and is generally expressed on a percentage basis or as a fraction or a decimal.
One may distinguish two types of porosity,namely,absolute and effective:
Absolute porosity is the percentage of total void space with respect to the bulk volume regardless of the interconnection of the pore voids.
Effective porosity is the percentage of interconnected void space with respect to the bulk volume.
Grain volume methods:In these methods the consolidated sample is solvent extracted and dried;the bulk volume is determined eighter by the displacement of a liquid which does not penetrate the sample or by saturating the sample and volumetrically displacing a suitable liquid with the saturated sample.The grain volume,or volume of the solid framework of the sample,maybe measured by the volumetric displacement of a gas or a liquid,while the pore volume may be measured by determining the amount of liguid neccesary to saturate the sample.
It is obvious that the percentage of porosity may be calculated from such data by use of eighter of the two following relationships:
Per cent porosity=100x(bulk volume – grain volume/bulk volume)
Per cent porosity=100x(pore volume/bulk volume)
Bulk volume determination:The bulk volume of the extracted and dried samples may be determined by volumetric displacement of mercury.

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