Research on gas injection to increase coalbed methane production based on thermo-hydro-mechanical coupling

Abstract

Gas injection to increase coalbed methane production is an effective energy saving and emission reduction technology, with triple benefits of clean energy production, environmental protection and safety production. In this paper, a coupled thermo-hydro-mechanical flow (THM) model is developed, the effects of different injected gases on ECBM are considered, the effect of initial water saturation on CO2/N2-ECBM is quantified using the control variable method. Exploring changes in storage and production during the injection gas mixture enhanced CBM recovery (GM-ECBM). The results show that: The breakthrough time of CO2 injection was much later than N2 injection. The higher the initial water saturation, the lower the CH4 production and gas storage. CO2 injection will reduce the permeability of coal seam and damage it, while N2 injection can effectively restore the permeability. The higher the initial water saturation, the slower the permeability decrease during CO2-ECBM and the slower the permeability increase during N2-ECBM. Increasing the N2 injection concentration before the N2 breakthrough time can promote CH4 production, and increasing the CO2 injection concentration near the N2 breakthrough time will delay the N2 breakthrough time and provide more time for CH4 to migration in the pore space, which will increase the CH4 production. When injecting gas mixture into water-bearing coal seam, the higher the initial water saturation, the slower the coal seam permeability increases when N2 is disturbed, and the slower the coal seam permeability decreases when CO2 and N2 are co-disturbed, while both production and storage will decrease.