Review on Coal Bed Methane Recovery Theory and Technology: Recent Progress and Perspectives

Coal bed methane (CBM) is a primary clean energy source found in coal seams. The recovery ratio of CBM is very low, especially with ground extraction, due to the strong adsorption of CH4 on the pores and fissures of coal and low permeability of the coal bed. On the basis of the theory of energy balance, a theory of enhanced CBM (ECBM) recovery by energy stimulation is proposed in this review. The desorption and transportation of CH4 in coal beds are analyzed from the perspective of energy consumption. Two pacesetting stimulation technologies are proposed to increase the permeability of coal beds: supercritical CO2 (SCCO2) fracturing and thermal fracturing through steam injection. The experimental study and the related results of SCCO2-enhanced CBM recovery are presented and analyzed. The results demonstrate that the permeability of SCCO2 in the tested four ranked coal specimens decreases in the form of a negative exponent function with the increase of effective stress. The transportation capacity of SCCO2 in high ranked coal beds deep underground is inferior to that in low ranked coal beds at shallow depth. The experiment of the total permeability of CO2 with CH4 in coking coal shows that the permeability decreases in the form of a logarithmic function as the content of CO2 increases. SCCO2 has higher transportation capacity than gaseous CO2 in coal beds, and the total permeability decreases with its content increasing in the synthetical system with CH4. The SCCO2-enhanced CBM recovery experiment demonstrates that the economic production time is different for the four ranked coal specimens given 50% of each content in production gas being defined as a threshold for economic production. The economic production for gas coal and anthracite takes a longer time than that in weakly caking coal of a lower ranked coal. The study demonstrates that different ranked coals have different inner structures and transportation properties. However, the original structures of different ranked coals can be modified by SCCO2 by changing the void volume or the structure of the pores and fissures. The challenges and perspectives for the CBM recovery theory and technology are also presented. The study is of importance with the novel dual efficient approach for clean energy CBM recovery ratio improvement and greenhouse gas sequestration underground in deep coal beds.