A Productivity Evaluation Model for Low‐Permeability Water‐Bearing Sandstone Gas Reservoirs Incorporating Gradient‐Controlled Relative Permeability Variation

Static gas–water relative permeability curves are widely used to evaluate productivity in low‐permeability water‐bearing sandstone gas reservoirs, but treating these curves as fixed can overestimate deliverability during the middle and late production stages. Field production and pressure data provided by the operating company indicate that the principal near‐wellbore displacement pressure gradient in the target reservoir is 12–28 MPa/m. Gas–water relative permeability experiments were therefore conducted under reservoir conditions at 10, 20, and 30 MPa/m using a Huagang Formation core from the Xihu Depression, with the three levels bracketing the field‐relevant range. Increasing displacement pressure gradient reduced residual gas saturation, increased endpoint water relative permeability, and expanded the gas–water coflow region. We established a gradient‐controlled Corey model and implemented it through a local gradient–based calling rule in a gas–water two‐phase nonlinear productivity model that also incorporates threshold pressure gradient and stress sensitivity. Sensitivity analysis showed that threshold pressure gradient shortened the stable production period, stress sensitivity progressively weakened reservoir gas supply, gradient‐controlled relative permeability variation accelerated the loss of effective gas‐phase flow capacity during the middle and late stages, and increasing movable water saturation further shortened stable production and aggravated late‐stage decline. Field applications to Wells A1–A4 showed that the model can characterize the main productivity‐evolution trends, with whole‐period cumulative production deviations of 1.89%, 9.99%, 9.26%, and 2.60%, respectively. The model is intended for rapid evaluation of single‐well productivity trends under comparable local reservoir conditions rather than detailed field‐scale reservoir simulation.