Influences of dynamic entrainer-blended supercritical CO2 fluid exposure on high-pressure methane adsorption on coals

Abstract

Injecting carbon dioxide (CO2) into unmineable coal reservoirs can reduce CO2 emissions and simultaneously enhance coalbed methane (CBM) recovery. The injected CO2 at the optimum coal seam depth is supercritical fluid, which may affect physicochemical properties and fluid adsorption/desorption performance of coals. Aiming to further elucidate the potential extractive effect of CO2 fluid, the influences of dynamic ethanol-blended supercritical CO2 fluid on physicochemical properties and methane (CH4) adsorption performance of coals are studied. The results indicate that ethanol as entrainer could strengthen extractive power of supercritical CO2 fluid to small organic compounds within coal matrix. Interaction with ethanol-blended supercritical CO2 fluid improves the micropore surface area and volume but decreases the oxygenic functional groups of Tianji coal (VRr = 0.72%) and Datong coal (VRr = 0.83%); however, Shenmu coal (VRr = 0.88%) and Yangquan coal (VRr = 2.62%) give opposite results. They account for increased CH4 adsorption amount of Tianji coal and Datong coal, and decreased CH4 adsorption amount of Shenmu coal and Yangquan coal. The isosteric heat of CH4 adsorption on coals exposed to ethanol-blended supercritical CO2 fluid decreases, further implying that the interaction is viable to weaken adsorbability of coals to CH4. Adsorption kinetics study confirms that ethanol-blended supercritical CO2 fluid exposure causes a rise in both Dmacro/Rmacro and Dmicro/Rmicro of CH4 within coals. Briefly, ethanol as candidate entrainer for supercritical CO2 fluid is expected to enhance CBM recovery and optimize the current CO2-ECBM process.