Enhanced coalbed permeability and methane recovery via hydraulic slotting combined with liquid CO2 injection

Enhanced efficient recovery of coalbed methane (CBM) is essential to prevent mine disasters and utilize unconventional gas resources. In this study, hydraulic slotting (HS) combined with liquid CO2 (LCO2) injection was employed to enhance coalbed permeability and the efficiency of methane recovery from a high-gas and low-permeability coalbed. First, mercury intrusion porosimetry and scanning electron microscopy experiments were conducted to analyze the variations in the coal microstructure and the permeability after HS combined and LCO2 freezing-thawing was conducted under laboratory conditions. The results indicated that the basic parameters of pore structures increased significantly for the treated coal; for instance, porosity and permeability increased by 47.65 % and 65.31 %, respectively. This indicated that coal permeability increased significantly under multiple stresses induced via HS and LCO2 freezing thawing. Subsequently, a simulated experiment of LCO2 injection to enhance CH4 recovery from coal was performed; the results thus obtained revealed that LCO2 injection into a high-gas-content coalbed can enhance CBM recovery with an average efficiency of >90 %. Furthermore, an in-situ test indicated that the effective radii for HS and LCO2 injection were 2.5 m and 10 m, respectively. The results of a 90-d-long methane extraction revealed that the efficiency of CBM recovery increased by a factor of > 2.2 and that the residual gas content in the test coalbed was <8.0 m(3)/t after the use of HS combined with LCO2 injection. Additionally, the time required to meet methane extraction standards was reduced significantly. (C) 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.