Experimental Study on Microscopic Distribution Characteristics of Water-Sealed Gas in Pore–Fracture Carbonate Gas Reservoirs

Pore–fracture carbonate gas reservoirs exhibit complex pore–throat structures and significant heterogeneity. Formation water can easily migrate along fractures, leading to extensive water-sealed gas accumulation, which severely limits the recovery rate of such reservoirs. To further investigate the water invasion process and the distribution characteristics of water-sealed gas in these reservoirs, a microscopic visualization model was developed using core CT scanning and laser etching technologies, incorporating the actual fracture sizes. The effects of fracture size, fracture density, and permeability heterogeneity on the microscopic distribution of water-sealed gas were analyzed in detail. The results indicate the following findings: (1) Homogeneous fracture size leads to a smaller extent of water-sealed gas accumulation. The ultimate gas saturation in the homogeneous fracture model is 5.79% lower than that in the heterogeneous model. (2) An increase in fracture density helps reduce the volume of water-sealed gas. The ultimate gas saturation in the high-density fracture model is 2.58% lower than in the low-density model. (3) The presence of seepage channels at the reservoir boundary with significant permeability heterogeneity exacerbates the accumulation of water-sealed gas and accelerates water invasion.