Effect of the Structural Deformation Degree of Coal Reservoirs on Methane Adsorption–Desorption and Production Capacity

Studies show that complex structural deformation reduces the productivity in high-value coalbed methane blocks. Dynamic variation of methane adsorption, desorption, and diffusion in coal reservoirs under different degrees of deformation needs to be studied. In this article, structural coal deformation and its impact on coalbed methane (CBM) production using five samples collected from a western Guizhou mine were studied. Coal deformation degrees were studied by using hand specimens and microscopic observations, while pore-fracture systems of deformation degrees were studied by using low-temperature N₂/CO₂ adsorption and high-pressure mercury intrusion tests. Then, the effect of pressure drop rate and structural deformation degree of coal reservoirs on methane adsorption-desorption and production capacity was studied. The results are as follows. First, as the degree of deformation increases, the pore morphology of 2-10 nm in different coal samples transforms from parallel plate-like (primary/fragmented/spotted coal) to bottle-shaped (mylonite coal), and the specific surface area of these pores gradually increases. Second, NMR simulation experiments show that adsorbed and free methane content and adsorption pressure satisfy the Langmuir and linear pressure relationships, respectively, and a faster pressure drop rate is beneficial for methane desorption. Primary coal exhibits greater desorption sensitivity than deformed coal. Then, permeability, porosity, coal thickness, and gas content positively influence production, where steeper pressure drop funnels (via rapid depressurization) elevate water and gas yields.