According to Terzaghi's principle, the effective stress σ' is related to the total stress σ and the pore pressure Pp by the relationship σ' = σ - Pp. In a typical reservoir depletion scenario, the pore pressure decreases under constant total stress, and as a result, the effective stress increases. The increase of the effective stress has a negative impact on permeability due to rock compaction, while the decrease in pore pressure enhances the apparent permeability because of the gas slippage effect. In tight gas reservoirs, since the slippage effect is no longer negligible, the pressure dependency of the permeability is very complicated.
In this paper, we used three samples from the Montney tight gas siltstone reservoir in western Canada, whose absolute permeabilities range from in the order of 10-4 mD to 10-3 mD. Permeability measurements under different pressure conditions using both the unsteady state pressure decay and the pressure pulse decay methods were carried out.
Based on the measured data, permeability was modeled as a function of reservoir pressure. Using this function, we carried out reservoir simulations and examined the impact of pressure dependence of permeability on the gas recovery process from a tight gas reservoir.