Laboratory Measurement of Porosity

Bulk volume is first determined by displacement of liquid, or by accurately
measuring a shaped sample and computing its volume.
Then any of the following methods are used to measure either the pore volume or grain volume.

1. Summation of Pore Fluids – involves independent determination of gas, oil and pore water volumes from a fresh core sample. The pore volume is determined by adding up the three independent volumes.

2. Washburn-Bunting Method – measures pore volume by vacuum extraction and collection of the gas (usually air) contained in the pores.

3. Liquid Resaturation – pores of a prepared sample are filled with a liquid of known density and the weight increase of the sample is divided by the liquid density.

4. Boyle’s Law Method – involves the compression of a gas into the pores or the expansion of gas from the pores of a prepared sample. Either pore volume or grain volume may be determined depending upon the porosimeter and procedure used.

5. Grain Density – measures total porosity. After the dry weight and bulk volume of the sample are determined, the sample is reduced to grain size and the grain volume is determined and subtracted from the bulk volume.

Another method of porosity determination is by petrographic analysis of thin sections of a rock sample. This is done by point counting of pores under a microscope. Impregnation of the sample in a vacuum with dyed resin facilitates pore identification.

A common source of porosity data are the well logs. Porosity may be calculated from the sonic, density, and neutron logs. These three logs are usually referred to as porosity logs.

Porosity may also be obtained from the resistivity logs.

Porosity ranges