Reassessing the Water Invasion Intensity Indicator Curve and Endpoint Equation in Water-Drive Gas Reservoirs

The traditional view that the production indicator curve of water-drive gas reservoirs exhibits an upward trend is not entirely consistent with production practices.Additionally, the classical method of calibrating gas recovery using the water invasion intensity indicator curve in conjunction with the endpoint equation has shown limited applicability.To address these issues, a material balance-based dynamic prediction approach that accounts for water production was employed in this study.Both the Carter-Tracy unsteady-state model and the Schithuis steady-state model were used to calculate water invasion volumes, followed by a sensitivity analysis of the factors influencing the production indicator curve of water-drive gas reservoirs.A reassessment of the water invasion intensity indicator curve, the endpoint equation, and gas recovery in water-drive gas reservoirs was conducted, and the findings were validated using field production data from a typical reservoir.The results indicate that (a) When water production is considered, the overall production indicator curve of water-drive gas reservoirs exhibits a smooth convex shape, intersecting the cumulative gas production axis at the dynamic reserves point.The early-stage characteristics may appear as an upward trend, an approximately linear segment, or a downward bend.(b) The water invasion intensity indicator curve is only applicable for short-term predictions in the early development stage.It is more suitable for strongly water-driven gas reservoirs under steady-state conditions.The endpoint equation may vary depending on different aquifer conditions and development scenarios.(c) The larger the aquifer radius, the higher the aquifer permeability (i.e., the greater the water invasion index), the greater the compressibility coefficients of the rock and formation water, the lower the gas production rate, the deeper the gas reservoir burial depth, the more pronounced the convexity of the dimensionless production indicator curve, the higher the abandonment pressure, and the lower the gas recovery.These findings provide insights into the production indicator curve and recovery of water-drive gas reservoirs, which align with production practices, offering valuable guidance for development patterns, recovery calibration, and enhanced recovery techniques.