Compressibility of Different Pore and Fracture Structures and Its Relationship with Heterogeneity and Minerals in Low-Rank Coal Reservoirs: An Experimental Study Based on Nuclear Magnetic Resonance and Micro-CT

Accurate characterization of the compressibility of pores and fractures in coal reservoirs is of great significance for predicting permeability and for optimizing the production of coalbed methane (CBM). However, for low-rank coal (R-o,R-max < 0.65%), compressibility and the influences of different pore structures and minerals on compressibility have not been studied thoroughly. To solve this problem, four low-rank coal samples were collected from the south of Junggar Basin. Low-field nuclear magnetic resonance is used to study the compressibility of pore and fracture in low-rank coal and its relationship with the heterogeneity. In addition, the effects of fracture structure and minerals on compressibility were studied by combining with micro-CT. The results show that the effects on the compression space of the low-rank coal with the increase in effective stress mainly come from the fracture and seepage pore volume, and the compressibility becomes weaker. The low-rank coal reservoirs with well-developed fractures and seepage pores show better compressibility. The compressibility of the pores and fractures in low-rank coal is noticeably affected by the mineral content, mineral shape, and mineral arrangement. When the fracture development is similar, the coal reservoirs with a high mineral content and with many fractures filled by minerals show poor compressibility. There is a negative linear correlation of the compressibility of pores and fractures with the fractal dimension of the seepage space in low-rank coal. Moreover, the closing rate of the pores and microfractures in low-rank coal with significant heterogeneity in the seepage space is faster under confining pressure. The research results are beneficial to the optimization of CBM production measures in low-rank coal reservoirs.