Properties of Fluid-Containing Rocks

Porosity

The porosity, 4, is equal to the void volume of the rock divided by the bulk volume and is expressed as a percent or fraction of the total bulk volume of the rock. Oil-bearing sandstones have porosities which often range from 15% to 30%. Porosities of limestones and dolomites are usually lower. Differentiation must be made between absolute and effective porosity. Absolute porosity is defined as the ratio of the total pore volume of the rock to the total bulk volume of the rock whereas effective porosity is defined as the ratio of the interconnected pore volume of the rock to the total bulk volume of the rock.

Factors affecting porosity are compactness, character and amount of cementation, shape and arrangement of grains, and uniformity of grain size or distribution. In problems involving porosity calculations it is convenient to remember that a porosity of one percent is equivalent to the presence of 77.6 barrels of pore space in a total volume of one acre-foot of sand.

Pore Volume

The pore volume of a reservoir is the volume of the void space, that is, the porosity fraction times the bulk volume. In conventional units, the pore volume, V,, in reservoir barrels is:

permeability

The permeability of a rock is a measure of the ease with which fluids flow through the rock. It is denoted by the symbol k and commonly expressed in units of darcies. Typical sandstones in the United States have permeabilities ranging from 0.001 to a darcy or more, and for convenience the practical unit of permeability is the millidarcy which equals 0.001 darcy. Some other useful conversion factors are given in Table 5-6.

Absolute Permeabllity

If a porous system is completely saturated with a single fluid, the permeability is a rock property and not a property of the flowing fluid (with the exception of gases at low pressure). This permeability at 100% saturation of a single fluid is termed the absolute permeability