Bottom-water invasion behavior during depletion of fractured tight gas reservoirs with low permeability

The DX gas reservoir is characterized by ultra-high pressure, extremely low porosity and permeability, well-developed fractures, and a strong bottom-water drive. During depletion, the decline in formation pressure intrudes rapid bottom-water invasion through fracture networks, resulting in early water breakthrough and significant trapping of natural gas within micro-fractures and pore spaces, thereby reducing overall recovery. To investigate the bottom-water invasion behavior in fractured tight gas reservoirs, physical simulation experiments were conducted under high-temperature and ultra-high-pressure conditions (136 ℃ and 106 MPa) using full-diameter fractured core samples. The effects of depletion rate and aquifer sizes on key indicators, including recovery factor, water-gas ratio, water-free production period, and gas production per unit pressure drop, were systematically analyzed. Results indicate that both increasing aquifer size and accelerating depletion rate lead to higher water production, increased water-gas ratio, shortened water-free period, and reduced gas recovery. Aquifer size exerts a significantly stronger influence than the depletion rate. Specifically, increasing the aquifer size from 4.5 to 20.0 times the hydrocarbon pore volume results in a nearly tenfold increase in cumulative water production, an rise in water breakthrough pressure by 18 MPa, and a value of 21.61% reduction in gas recovery. In contrast, doubling the depletion rate results in approximately a twofold increase in cumulative water production, a 6 MPa increase in breakthrough pressure, and a 10.97% reduction in recovery. These findings indicate that appropriate control of rational depletion rate and effective water management strategies are critical for improving economic performance of fractured tight gas reservoirs. This research provides a crucial theoretical and experimental basis for understanding and managing bottom-water invasion and offers significant practical guidance for optimizing development strategies in fractured gas reservoirs.