Water Occurrence Differences between Primary Adsorbed Water and Artificial Fracturing-Invaded Water in Coal Reservoirs and Their Control Mechanisms on Water-Blocking Damage

Water occurrence in coal pores is a key factor inducing water-blocking damage and restricting efficient coalbed methane (CBM) production. Current studies mainly focus on the damage caused by artificial fracturing fluid invasion, while the primary adsorbed water formed under the original reservoir humidity environment and its water-blocking effect remain insufficiently understood. Coal samples from the No.3 coal seam of Dongfeng Coal Mine in the Qinshui Basin were selected for this study. Controlled experiments of equilibrium water vapor adsorption (simulating the original reservoir humidity) and pressurized water saturation (simulating fracturing fluid invasion) were carried out. Low-field nuclear magnetic resonance (LF-NMR) was used to characterize the distribution of pore water under the two occurrence conditions, and gradient centrifugation experiments were employed to quantitatively evaluate water mobility. The differences in water occurrence and their control mechanisms on water-blocking damage were systematically compared. The results show that under equilibrium water vapor adsorption, water mainly occurs in micropores and transition pores; higher environmental humidity leads to larger occupied pore sizes and higher water contents at adsorption equilibrium. Under pressurized water saturation, water predominantly occupies transition pores; increasing saturation pressure broadens the range of water-occupied pore sizes, while transforming pore structure and enhancing pore connectivity. Compared with pressurized water-saturated samples, equilibrium water-adsorbed samples have a lower overall water content. However, water vapor can penetrate into micro- and nano-scale pores and even isolated pores, resulting in significantly weaker water mobility and greater difficulty in drainage. The water-blocking damage caused by primary adsorbed water in low-humidity reservoirs is far higher than that caused by high-pressure fracturing fluid invasion. This finding remedies the cognitive deficiency of overemphasizing artificial water injection damage while neglecting primary humidity-related water-blocking. The results can provide microscopic theoretical support for accurate evaluation of water-blocking damage and the optimization of efficient damage removal technologies for coal reservoirs.