Investigation of the effect of low-temperature oxidation on extraction efficiency and capacity of coalbed methane

To improve the extraction efficiency and capacity of coalbed methane (CBM) to the greatest extent possible, this study explores the effects of the internal mechanism of low-temperature oxidation of CBM reservoirs during CBM extraction. The evolution of the porosity and the methane adsorption and desorption characteristics of the coal matrix during low-temperature oxidation were separately explored using a nuclear magnetic resonance (NMR) spectrometer and a high-pressure gas adsorption analyzer. Moreover, the internal evolution mechanism was determined using gas chromatography and the experimental data of the proximate analysis parameters. This study shows that with an increase in the degree and temperature of low-temperature oxidation of CBM reservoirs, the moisture and volatile matter inside the coal matrix continuously decrease. This causes the number of pores with different diameters, porosity, and permeability in the coal matrix to be greatly improved, while the width and quantity of the flow channels for CBM increase synchronously. As a result, the resistance to CBM extraction declines and its efficiency improves. With constant CBM extraction, the maximum methane adsorption capacity of the coal matrix decreases, whereas the methane desorption capacity increases, under low pressures (lower than 1.74 MPa) owing to changes in the structures and quantities of pores inside the coal matrix. As a result, the maximum extraction capacity for CBM is improved. Finally, to guarantee CBM extraction safety and maximize its extraction capacity, it is necessary to control the temperature of the borehole used for extracting CBM to approximately 80 °C.