A dynamic material balance model incorporating water-sealed gas for fractured gas reservoirs with edge-bottom water

Water invasion is a key factor limiting the efficient development of fractured gas reservoirs with edge-bottom water. In such reservoirs, water preferentially invades high-permeability fractures, increasing water saturation in the fracture system, reducing gas relative permeability, and progressively weakening local flow conductivity. Consequently, part of the remaining gas may shift from a movable state to a dynamically water-sealed state. Conventional material balance methods generally treat water invasion primarily as an energy-support process and cannot explicitly describe the coupled evolution of cumulative water influx and water-sealed gas. To address this limitation, we propose a dynamic material balance equation model in which water-sealed gas is treated as an independent state variable. The model couples net water-influx support, water-sealed-gas formation, and pressure response within a unified material-balance framework. We verified the model using reservoir simulation data and a field case from the Karamay gas field, China. Results show that the proposed model simultaneously estimates cumulative water influx and dynamic water-sealed gas volume. For cumulative water influx, the model achieves a mean absolute percentage error (MAPE) of 4.39%, representing a reduction of 17.18 percentage points relative to the water-sealed-gas correction method and 28.53 percentage points relative to the conventional material balance method. For water-sealed gas, the model captures the middle-to-late-stage accumulation trend and terminal magnitude, with terminal relative errors generally within 10%. Scenario analysis further indicates that aquifer properties primarily control water influx support, high-permeability fracture properties primarily control water-sealed gas accumulation, and production constraints mainly affect pressure response and the timing of water-sealed gas formation. The proposed model provides a quantitative approach for dynamic reserve evaluation and water-sealing diagnosis in fractured gas reservoirs with edge-bottom water.