Influence of deep coal pore and fracture structure on occurrence of coalbed methane: a case study of Daning-Jixian Block in eastern margin of Ordos Basin

The pore-fracture structure of deep coal reservoir is of great significance to the evaluation and exploration of deep coalbed methane resource potential. this work takes the 5 coal and rock samples from the DJ57 Benxi Formation in Daning-Jixian Block, east margin of Ordos Basin are selected as the research object. On the basis of coal rock and coal quality parameter testing, gas adsorption method, high pressure mercury injection method and micron CT scanning and other testing methods are adopted to conduct full-scale quantitative characterization of nano-scale pore and micron scale cracks in the pore and fissure structure of coal reservoir, and comprehensively evaluate the pore and fissure structure characteristics of different scales. Combined with permeability and methane isothermal adsorption experiments, the influence of micro-porosity and fissure on the occurrence and migration of coalbed methane in deep coal reservoirs was discussed. The results show that the multi-scale quantitative characterization of deep coal reservoir pores and fractures is based on a variety of pore characterization methods, and the pore and fracture volume distribution type is mainly U-shaped, showing a bimodal coexistence of micro-pores and micro-fractures, mainly concentrated in in 0.3-1.5 nm and > 100 μm. Among them, micropores (<2 nm), mesoporous (2−50 nm), macroporous (50 nm−10 μm) and microfractures (>10 μm) accounted for 80.18%,6.70%,1.65% and 11.47% of the total pore crack volume, respectively. With the development of micropores, the adsorbed gas tends to increase, and micropores can provide a large number of adsorption sites for deep coalbed methane adsorption and occurrence. With the development of micro-fractures, the free gas volume tends to increase. Micro-fractures can provide a large amount of storage space and provide space conditions for deep coalbed methane enrichment. In addition, microcracks are interconnected in three-dimensional space, forming a network structure with strong connectivity. With the development of micro-fractures, the permeability of coal reservoir increases, and micro-fractures enhance the seepage capacity of coalbed methane. The adsorption capacity and development potential of deep coalbed methane are controlled by the development characteristics of nano-scale pores and micro-scale fractures respectively.