Experimental investigation of CO2-CH4 core flooding in large intact bituminous coal cores using bespoke hydrostatic core holder

This paper presents the CO2-CH4 core flooding and permeability studies conducted with large intact coal core samples. A bespoke core holder was designed and commissioned to conduct the core flooding experiments. The CO2 and CH4 core flooding experiments were performed to understand the CH4 displacement efficiency of CO2 from the coal cores and the permeabilities of CO2 and CH4 during the core flooding. The CO2-CH4 gas displacement experiments were performed on intact coal cores of 9.9 cm in diameter with varying lengths to make up 60 cm-long cores that were extracted from coal blocks using a diamond core-drilling machine. The CO2 and CH4 core flooding experiments were conducted using the core holder at maximum of 1450 psi confining pressures and at 298.15 K. A constant gas flow rate of 1 mL/min was maintained to flood the core with an outlet backpressure set at 25 to 30 ± 2 psi. The core holder is equipped with two differential pressure taps to measure the pressure change along the core length to measure the differing permeability along the core lengths which is difficult to observe in the smaller size intact samples. Both coals, EMB and ZM, showed favourable CH4 displacement efficiencies of CO2 about 96.5% and 99.7%, respectively. The dependency of the core lengths on the permeability measurements is more pronounced in the results obtained in the current study. The results indicated a decreasing trend in CO2 permeability and increasing CH4 permeability during CO2 injection. The observation indicated CO2 adsorption and CH4 displacement. Overall, the core flooding experiments improved the current understanding of CO2-CH4 core flooding by showing the core size dependence in permeability measurement experiments. In contrast to the single species CO2 flow experiments, the permeability variation during the CO2-CH4 flooding and the CH4 sweeping efficiency of CO2 provided insights into the CO2-ECBM operational design.