Propagation Laws of the Pressure Disturbance and Mathematical Model of Unstable Seepage in Water-Bearing Tight Sandstone Reservoirs

Abstract

Abstract Due to their complex pore-throat networks, low permeability, and high water saturation, water-bearing tight sandstone reservoirs exhibit pronounced heterogeneity in formation pressure distribution and unclear reserve mobilization patterns. These factors hinder accurate estimation of remaining gas and the design of balanced development strategies for such reservoirs. To address these issues, a long outcrop core sample from the main reservoir (eighth member of the Shihezi Formation) in the Sulige Gas Field was selected to examine gas seepage characteristics, pressure decline patterns, and dynamic boundary propagation behavior using a multipoint pressure measurement physical simulation system. Furthermore, a model describing the formation pressure distribution and dynamic boundary was established, enabling analysis of the unstable seepage pressure characteristics and the corresponding controlling factors in water-bearing tight sandstone gas reservoirs. The following research results were obtained. First, the reservoir pressure, gas deliverability, and drainage efficiency in water-bearing tight sandstone gas reservoirs showed considerable differences throughout various production stages, with water saturation, optimal production rate, reservoir permeability, and abandonment pressure being the key factors influencing both balanced recovery efficiency and overall development performance. Second, the decline in reservoir pressure was mitigated by the dynamic boundary effect of pressure propagation, which extends outward at a speed that decreases over time and is inversely proportional to permeability and water saturation. Third, by treating the investigation boundary as the outer edge of pressure disturbance and introducing a correction factor for the gas–water relative permeability relationship derived from the displacement pressure difference, the partial differential flow equation was solved using an integral method to derive the propagation behavior of the investigation boundary induced by gas well production, thereby establishing a model describing the formation pressure distribution and dynamic boundary for vertical wells with fractures in tight gas-bearing sandstone formations. In conclusion, the proposed model can effectively calculate reservoir pressure decline patterns under various reservoir and production conditions, predict drainage efficiency at different development stages, offer technical support to the theory of balanced recovery in complex water-bearing tight gas reservoirs, and provide valuable insights into the cost-effective development of analogous gas reservoirs.