The Coupled Impact of Wettability and Pore Structure on Gas and Water Production in Coal Reservoirs

Abstract

Efficient development of coalbed methane is crucial for optimizing energy structure, ensuring energy security, and achieving carbon emission reduction targets. This study investigated the combined influence of wettability and pore structure on gas and water production in coal seams. Coal samples from three regions were characterized using low-temperature CO₂ and N₂ adsorption, high-pressure mercury injection, and contact angle tests to determine their pore structure and wettability. Relative permeability experiments were conducted to elucidate the impact of these parameters on gas and water seepage. A virtual vertical well was established based on typical reservoir characteristics of the Qinshui Basin, China. Simulations using the determined wettability and pore structure parameters of the three coal samples were performed to evaluate the influence of these factors on gas and water production in coal reservoirs. The results demonstrated that, as wettability weakened, the water saturation at the isotonic point (S_wx) decreased, while the relative permeability at the isotonic point (K_r(S_wx)) increased. The bound water saturation (S_wc) decreased, while the gas phase relative permeability at bound water saturation (K_rg(S_wc)) increased. The gas–water seepage "triangle area" shifted leftward and expanded. The amount of relative permeability loss was lower. Furthermore, both daily gas and water production significantly increased with decreasing wettability. Compared to hydrophilic reservoirs, hydrophobic reservoirs exhibited higher and earlier water production peak, while the gas production peak was higher but occurred later. For reservoirs with well-developed small aperture pores (d < 100 nm), the gas–water production varied more significantly with wettability. By integrating the analysis of wettability and pore structure, reservoirs with developed small aperture pores and strong hydrophilicity were identified as promising targets for wettability modification. This research, using both experimental and simulation methods, provides insights into the effects of wettability and pore structure on fluid flow and production in coal reservoirs at both core and reservoir scales, providing a basis for improving CBM recovery through wettability modification.