Mechanism of enhanced coalbed methane desorption regulated by fracturing fluid imbibition in deep coal seams

Low-salinity fracturing fluids tend to induce ion migration, alter wettability, and cause fluctuations in gas desorption efficiency when penetrating deep coal seams. Taking the No. 8 coal from the Daning–Jixian area in the Ordos Basin, NW China, as a representative example, this study employs physical simulation experiments to reveal the coupled control mechanism of salinity gradient on the ion–coal matrix–gas/water interfacial system and its key role in the imbibition-desorption process. The increasing ionic concentration improves the hydrophobicity of coal, with multivalent ions exhibiting particularly significant effects. The imbibition and ion diffusion occur in opposite directions, with imbibition equilibrium being achieved earlier than ionic equilibrium. Water-coal interactions induce both mineral dissolution and secondary precipitation. When a low-salinity fracturing fluid is injected into a high-salinity reservoir, the osmotic-pressure difference drives imbibition, promotes CH 4 desorption, but results in higher fluid loss. Conversely, injecting high-salinity fracturing fluid into a low-salinity reservoir creates a reverse osmotic gradient that suppresses leak-off while improving flowback efficiency. Based on these findings, a high–low salinity sequential injection strategy is proposed for deep coal seams: high-salinity fluid is first injected to form stable fracture networks, followed by low-salinity fluid to enlarge the imbibition zone and enhance CH 4 desorption and diffusion. Moderate well soaking is recommended to increase the imbibition volume, thereby achieving multiple positive effects such as maintaining reservoir pressure, preserving formation energy, and promoting imbibition-driven displacement.