CH4-CO2 gas exchange and supercritical CO2 based hydraulic fracturing as CBM production-accelerating techniques: A review

CH4-CO2 gas exchange and CO2 based hydraulic fracturing are common enhancement techniques adopted in coal-bed methane (CBM) extraction industry, with promising results. This review provides a comprehensive overview of the CH4-CO2 gas exchange method at macro and microscopic level, and the CO2-based hydraulic fracturing. The process of CH4 replacement by CO2 is comprehensively discussed considering two possible CBM storage mechanisms; 1) by adsorption and; 2) at hydrate state. The CH4-CO2 gas exchange by sorption/diffusion process has been proven to be kinetically and thermodynamically viable as CO2 can easily penetrate through ultra-micro-pores region and adsorb into the coal pores due to its smaller kinetic diameter, linear shape and higher affinity. The gas exchange occurs in the hydrate phase at special reservoir conditions has also been shown to be feasible kinetically, chemically and thermodynamically, given the fact that the reverse reaction (i. e. CO2 replaced by CH4) is different and rather inefficient. Therefore CBM extraction using CO2 replacement is confirmed to be an effective extraction technique. The use of CO2, particularly in its super-critical state, as a fracturing fluid has a number of advantages. Owing to its gas-like viscosity and compressibility, super-critical CO2 (S-CO2) easily infiltrates through tiny defects in rock formations, inducing thinner and more three-dimensionally extended cracks, while maintaining a low break-down pressure. On the other hand, the liquid-like density of S-CO2 exerts more pressure on the rock, so that it can easily exceed the minimum principal stress and rock tensile strength, resulting in easy crack initiation. However, such CO2 utilization methods, particularly those based on super-critical CO2 are still novel to the CBM extraction industry and should therefore be subjected to careful investigation prior to implementation.