Study on comprehensive evaluation of low permeability sandstone reservoirs ofcoal-derived gas in Huaibei Coalfield

Coal-derived gas is an important supplement to natural gas resources. The Huaibei Coalfield is rich in coal-derived gas resources and has great exploration potential. However, the study of low permeability sandstone reservoirs is limited, and there is also less reservoir evaluation. In order to effectively evaluate sandstone low-porosity and low-permeability reservoirs of Shanxi Formation, Lower Shihezi Formation and Upper Shihezi Formation in Huaibei Coalfield, the sedimentary facies, pore permeability characteristics, pore structure, pore types and diagenesis were comprehensively evaluated by means of pore permeability test, rock slice analysis and scanning electron microscope. The results show that from the perspective of pore permeability and pore structure, the Upper Shihezi Formation is the best, followed by the Shanxi Formation, and the Lower Shihezi Formation is poor. From the perspective of the degree and type of pore development, the Upper Shihezi Formation has the most developed pores, mainly primary pore and secondary dissolution pore, followed by Shanxi Formation, mainly secondary dissolution pore, while the Lower Shihezi Formation has poor pore development. Comprehensive evaluation shows that sandstone reservoirs in Upper Shihezi Formation have the best development. Type Ⅰ reservoirs account for 22.65%, Type Ⅱ reservoirs account for 50.32%, and Type Ⅲ reservoirs represent 27.03%;This was followed by Shanxi sandstone reservoirs, where Type I reservoirs account for 21.7%, Type Ⅱ reservoirs account for 42.05%, and Type Ⅲ reservoirs shares another 36.25%; The sandstone reservoir of Lower Shihezi Formation is poorer, with Type Ⅰ reservoir accounting for 2.91%, Type Ⅱ reservoir accounting for 38.85%, and Type Ⅲ reservoir accounting for 58.24%.