A Water Influx Calculation Model Incorporating Dissolved Gas Evolution and Residual Gas Expansion and Its Application to the W Gas Field

Water influx calculations in high-permeability sandstone gas reservoirs often overlook the contributions of dissolved gas evolution and residual gas expansion within the aquifer. To address this limitation, a novel material-balance-based water influx model is developed for confined gas reservoirs. The model comprehensively integrates six physical mechanisms: gas-zone pore volume contraction, irreducible water expansion, aquifer water expansion, aquifer pore contraction, residual gas expansion, and dissolved gas evolution. A dimensionless water influx intensity type-curve and an analytical expression for component-wise water influx are derived, enabling quantitative decomposition of the contribution from each driving mechanism. The proposed method is applied to 11 production wells in the W Gas Field. Water influx is identified using three approaches—production performance analysis, the apparent reservoir pressure method, and the multi-factor type-curve method—and the calculated water influx volumes are systematically compared. Results show that the production performance method can only detect wells with a strong water influx response. The apparent reservoir pressure method yields physically unreasonable negative water influx values in confined gas reservoirs, indicating limited applicability. In contrast, the proposed model agrees well with the type-curve method, with relative errors below 10%. Component-wise calculations reveal that during early development, aquifer water expansion and aquifer pore contraction together account for over 60% of the total water influx, dominating the process. As reservoir pressure declines continuously, the contribution of dissolved gas evolution increases substantially, exceeding 30% in the middle-to-late development stage and becoming a non-negligible factor. Based on the component-wise water influx results, aquifer drive energy and activity are evaluated using the water-drive index and the water influx constant. Gas Group IV and the Lower Gas Group I exhibit “strong water drive + active” characteristics. Furthermore, gas–water contact rise height and advancement distance are calculated using both the volumetric method and a pore volume iteration method that accounts for actual structural geometry. The volumetric method systematically overestimates results by 15–30%, whereas the structural geometry-based method provides higher accuracy. According to the rise magnitude, the study area is classified into high-, moderate-, and low-risk water breakthrough zones. Wells A3H and A8 in Gas Group IV exhibit the highest risk, with rise magnitudes of 30.19% and 37.75%, respectively. This study provides a theoretical foundation and technical support for water influx performance prediction and differentiated water-control development in analogous high-permeability sandstone gas reservoirs.