Resarch on the Differences in Physical Properties and Water Lock Damage Mechanisms Between Deep Coal Seams and Tight Gas Reservoirs Based on Low-Field Nuclear Magnetic Resonance Technology

Deep coal seams and tight sandstone gas reservoirs are vulnerable to water lock damage due to their inherent low porosity and permeability, resulting in reduced gas phase permeability and diminished productivity. This study utilizes low-field nuclear magnetic resonance (NMR) technology in conjunction with both macroscopic and microscopic experimental methods to compare the water lock characteristics between the deep coal seams (8 + 9# coal seams) and tight sandstone (He1) in the Linxing Block. Experiments such as X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), NMR T2 spectrum analysis, and evaluations of water lock damage were conducted to investigate the mechanisms by which mineral composition, pore-throat characteristics, and liquid phase retention distribution influence the water lock effects. The results indicate that deep coal seams contain significantly more clay minerals than tight sandstone, characterized by pores primarily composed of micropores (0.04–0.06 μm) and exhibiting weak connectivity. In contrast, tight sandstone displays a relatively uniform pore-throat distribution where permeability is predominantly attributed to pores ranging from 0.01 to 0.4 μm. NMR T2 spectra demonstrate that liquid retention in deep coal seams is primarily concentrated in small and medium pores, resulting in a bound water saturation of 79% and a water lock damage rate of 87.64%. In contrast, tight sandstone exhibits a bound water saturation of 46% along with a damage rate of 76.77%. The study concludes that deep coal seams are more susceptible to water lock damage because their micropore-dominated structure and elevated capillary forces impede fluid backflow. These findings offer theoretical insights for the development of targeted technologies to mitigate water lock effects and optimize reservoir stimulation strategies.