Some Studies on the Porosity and Permeability of Rocks

Abstract

The behavior of fluids in the voids of a rock is fundamental to the study ofmany of the problems of oil-field development and production. For it is byvirtue of these openings between grains that oil and. gas are able to movethrough the rock, to be contained by the rock, and to pass from the rock intothe well. The capacity to contain fluids is measured by the porosity of the rock, and theease of fluid movement is governed by the permeability. When the engineerwishes to estimate the amounts of oil and gas in an underground reservoir he isoften concerned with the porosities of the rocks and, when dealing withproblems relating to rate of production, he is more likely to be interested inthe permeabilities of the rocks. Rocks are permeable because they are porous, but they may be porous withoutbeing permeable. Clays are very porous, and yet they are impermeable from apractical standpoint. Attempts have been made to correlate permeability with porosity for certainrock members but, where a definite relationship between these two properties isfound to hold, it is purely fortuitous and local. Porosity and permeability donot necessarily bear any quantitative relationship to one another. The porosity of a substance is the percentage of void volume to bulk volume.The petroleum engineer is usually interested only in the intergranular voidsthat are accessible to fluids. In measuring porosity he does not wish toinclude sealed openings. The measurement he desires is one of "availableporosity," otherwise called" apparent porosity." The porosity of a natural aggregate of solid grains, such as a sandstone orclay, is governed by:Grain-size distribution,angularity of grain,colloidal content,degree of compaction. It is not difficult, forunconsolidated rocks, to measure the effect of anyone of these when it alone isvaried, and then to predict what the effects on porosity will be for suchsingle variables.