Research on the evaluation of deep coal seam water lock damage and optimization of water lock release agents based on low field nuclear magnetic resonance technology

Deep coal seams are classified as ultra-low permeability gas reservoirs, characterized by inherently low permeability and micro-scale pore-throat structures. During production, these reservoirs are highly vulnerable to external fluid intrusion, resulting in water-locking damage. To tackle water-locking damage associated with the low permeability of deep coal seams, this study elucidates the microscopic mechanisms of water-unlocking agents, addressing the prior lack of microscopic quantitative characterization. By integrating Low-Field Nuclear Magnetic Resonance (LF-NMR) with physical simulation experiments, we quantitatively analyze fluid retention patterns across various pore-throat scales. Five water-unlocking agents (nonionic, anionic, cationic, zwitterionic, and fluorocarbon) were selected, and their efficacy was evaluated via surface tension, contact angle, and permeability recovery tests. Results indicate that, at a bound water saturation of 53.30%, the water-locking damage rate reaches 87.04%, indicating severe water-locking. LF-NMR analysis shows that fluid retention within small pore-throats (0.001~2.5 ms) accounts for 86.51% of total retention, serving as the primary contributor to water-locking damage. Upon addition of water-unlocking agents, surface tension decreases while contact angles increase. The fluorocarbon-based agent (Type V) exhibited the best performance, reducing surface tension to 19.60 mN/m, increasing contact angle to 48.85°, and achieving a water-locking prevention contribution rate of 81.20%. Following Type V agent treatment, fluid retention in small pore-throats decreased by 31.51%, and permeability recovery reached 28.49%. Small pore-throats serve as the primary seepage channels. As fluid occupancy in smaller pores decreases, gas-phase seepage channels expand, resulting in increased macroscopic permeability. This study accomplishes microscopic quantitative characterization of water-locking damage via LF-NMR, demonstrating that water-unlocking agents alleviate capillary forces to unblock small pore-throat channels, thereby offering novel strategies for efficient deep coal seam development.