Theoretical analysis of threshold pressure in tight porous media under stress

Abstract

The threshold pressure is critical to characterizing multi-phase nonlinear flow through tight porous media under effective stress. Due to the complex and heterogeneous pore structures, the essential controls on the threshold pressure of tight porous media are not determined. In this study, based on the fractal theory, a theoretical model for the threshold pressure of gas–water two-phase flow through tight porous media is proposed. The derived model considers the effective stress, pore structures, gas–water capillary pressure, and boundary layer. The predicted threshold pressure from the developed model is in good agreement with the available experimental results, which validates the model. Moreover, based on the derived model, the effects of relevant parameters (e.g., gas–water surface tension, contact angle, initial porosity, and elastic modulus) on the threshold pressure are studied. Under a given effective stress, threshold pressure decreases as the initial porosity (or elastic modulus) increases. However, threshold pressure increases with the increase in gas–water surface tension (or contact angle). In addition, a positive relationship exists between threshold pressure and water saturation in tight porous media. From a practical standpoint, this model is of great significance in predicting threshold pressure and researching on the gas–water two-phase flow mechanism in tight sandstone gas reservoirs.