NMRI online observation of coal fracture and pore structure evolution under confining pressure and axial compressive loads: A novel approach

Quantitative characterization of the spatial distribution, content and heterogeneity of fracture and pore structure (FPS) in coal reservoirs under confining pressures and axial compressive loads is significant for the engineering of coal bed methane. A novel online observation approach that combines nuclear magnetic resonance imaging with triaxial loading techniques is employed to achieve the visualization and full-scale quantitative characterization of the evolution of FPS in coals in the laboratory. The relationship between the stress states and FPS evolution was formulated. The results show that the spatial distribution of the FPS evolution process of coal samples can be divided into four stages: initial pore and fracture compaction closure, pore and fracture stable growth, pore and fracture unstable growth, and failure stages. As the deviatoric stress increases, the content of the adsorption pores, the heterogeneity of the adsorption space, and the gas adsorption capacity of coal samples gradually increase. In contrast, the seepage pore and fracture content as well as the permeability of coal samples decrease first and then increase. The heterogeneity of the seepage space of coal samples initially increases and then decreases. The maximum compression of seepage space and increase of adsorption space are 4.742% and 14.743%, respectively.