Relationship between Reservoir Productivity and Pore Pressure Drop

Abstract

The significance of permeability sensitivity to changes in pore pressure (i.e., to change in effective overburden stress) has been examined by modifying Darcy law for radial single-phase steady state flow. The elaborate model accounts for pore pressure drop and permeability reduction due to reservoir compaction. Three Saudi oil reservoir rock samples as well as Berea sandstone were tested for their physical properties. All of these cores were free of microfractures. Furthermore, using 1% NaCl aqueous solution, relationship between the overburden pressure and absolute permeability of these samples were determined at several levels of confining pressure at which the permeability was calculated. These tests were performed to establish the effect of reservoir depressurization on reservoir rock permeability.The experimental work performed in this study showed that reservoir rocks of high initial porosity and permeability are highly affected by reservoir (pore) pressure drop and resulting increase in the effective stress. For example, the production rate from sandstone sample N3 decreased 25% of its initial value when the pore pressure decreased by 25% of its initial value, whereas the production rate from carbonate sample N4 decreased 8% for the same pore pressure decrease. Moreover, it was observed that the reservoir rock permeability under in-situ conditions strongly depends on its initial porosity and permeability values. This study suggests that a severe error in productivity predictions can result on assuming that the formation permeability at depth is independent of the effective stress. Because the increase in the effective stress decreases porosity which, in turn, changes permeability.