Stress Sensitivity of Coal: A Digital Core Analysis

Identifying the variation of fracture structure and coal permeability with effective stress is important for protecting coalbed methane (CBM) reservoirs and formulating a reasonable drainage system. This study characterizes the coal fracture and permeability variations with effective stress using three-dimensional (3-D) micro-CT and digital core techniques. Dual-resolution scanning is adopted to characterize the variation of three types of fractures with effective stress and to clarify the stress sensitivity of the porosity of each type of fracture. Type-A fractures (>10 9 μm 3 ) are most sensitive to effective stress and tend to close as effective stress increases. Moreover, the porosity of this fracture has a negative exponential relationship with effective stress. Type-B fractures (10 7 –10 9 μm 3 ) are moderately sensitive to effective stress and have the highest contribution of total porosity at high effective stress. Type-C fractures (<10 7 μm 3 ) are least sensitive to effective stress, and the porosity of this fracture changes slightly. The seepage simulation in the two directions with digital core techniques confirms that the permeability of coal samples presents an exponential decrease with effective stress and shows an anisotropic behavior with space. Combined with the quantitative analysis of variation of pore–fracture models, the compression model of coal under effective stress can be established, which can reveal the reason that the permeability loss rate in the y -axis is more than that in the z -axis. The sensitivity of porosity and permeability is stronger under low stress and weaker under high stress. Therefore, digital core technology can quantitatively analyze the stress sensitivity of coal fracture structure and permeability based on 3-D models. This method is more intuitive to study the stress sensitivity of coal compared with traditional methods.