Differences in the pore characteristics and adsorption/desorption of coalbed methane of different coal macrolithotype in low-rank coal reservoirs

LI Weibo; LI Fei; SHI Liyan; ZHOU Shaowei; MA Dongmin; WANG Chuantao; CHEN Yue

doi:10.16186/j.cnki.1673-9787.2021110078

Abstract：Objectives The heterogeneity of coal reservoirs is strong,and the differences in material composi-tion and pore structure among different macrolithotypes lead to significant differences in the adsorption and desorption performance of coalbed methane and the characteristics of gas-water migration.Methods Taking the Yan'an Formation low rank coal in the Binchang mining area of Huanglong Coalfield as the research ob-ject,the components of vitrinite and dark coal were collected and separated.Comprehensive testing methods such as microscopic component determination,elemental analysis,mercury intrusion,low-temperature liquid nitrogen adsorption,and adsorption desorption experiments were used to study the differences in porosity de-velopment characteristics of low rank vitrinite and dark coal and their impact on methane adsorption/desorp-tion performance.Results The results showed,(1)The mass fraction of vitrinite,volatile matter and H,O,N and S elements of vitrain were higher than those of durain,while the mass fractions of inertinite,chitinite,minerals,ash,fixed carbon and C elements were lower than those of durain;(2)The porosity of the coal sample was 2.92%～10.29%,and the overall pores were relatively developed.The porosity of durain was slightly higher than that of vitrain.The porosity of durain was slightly higher than that of vitrinite,with thicker pore throats,more developed macropores,and better connectivity.The BET specific surface area and BJH total pore volume of vitrain coal were slightly larger than durain,and the micropores were more devel-oped,mainly consisting of semi-closed and ink bottle shaped pores;(3)Mirror coal had more developed mi-cro pores,larger specific surface area,and stronger adsorption capacity.Conclusions The methane adsorp-tion/desorption process in coal generally exhibits desorption hysteresis,with relatively good pore connectiv-ity in dark coal.The degree of desorption hysteresis was lower than that of vitrinite,and the theoretical de-sorption efficiency was higher than that of vitrinite.The research results can provide reference for under-standing the physical properties of low rank coalbed methane reservoirs in Binchang mining area.