POROSITY AND PERMEABILITY OF SHALY SANDSTONES

Porosity and permeability are important properties of petroleum reservoir sandstones. Prediction of these properties for shaly sandstones is more difficult than for clean, quartzose sandstones. Measured porosities generally include clay micropores that do not contribute to available void space required for effective porosity and resulting effective permeability. More accurate estimates are needed for evaluation of petroleum reservoirs. Effective porosity is the interconnected pore volume that contributes to fluid flow in a reservoir. For shaly sandstones, the non-effective porosity is predominated by clay-related micropores. We find that previous estimates of clay microporosity permit more accurate predictions of effective porosity and permeability for shaly sandstones.

In this study, a single data set from the literature is used to illustrate the relationship between effective porosity and permeability. The data set consists of 14 samples, which were drawn deliberately from a larger sample population of 44 petroleum reservoir sandstones. These samples cover a range of porosity and air permeability ranging from 8.45 to 26.5% and 0.031 to 1173 millidarcy, respectively. We proposed an empirical approach to estimate effective porosity based on clay type and distribution. The results indicate that effective porosities can be applied to predict permeability for a wide range of shaly reservoir sandstones. The prediction of permeability by the widely used Kozeny-Carman equation can also be improved by effective porosity. The basis for the correlation between permeability and effective porosity is the strong relationship between the latter and median pore-throat diameter.