Response of multi-scale mass transport to oxidation treatment in subbituminous coalbed methane reservoir

Due to the characteristics of low porosity and permeability, poor stability of coals and formation damage caused by fracturing process, most fracturing effects are not ideal. Therefore, the oxidation treatment to stimulate the production of coalbed methane wells is proposed. This paper took subbituminous coal as the research object and H2O2 as oxidants. X-ray diffraction analysis, scanning electron microscopy, low-temperature N2 adsorption, infrared spectroscopy, exothermic heating test and wetting contact angle measurement experiments showed that the reducing components such as organic matter and pyrite were easily consumed by oxidation, and the generated H+, small molecule fatty acids and other inorganic minerals are further dissolved. After oxidation treatment, a large number of micro-fractures and dissolution pores were generated in coal, and the associated hydroxyl and carboxyl groups on the surface of coal increased. The water wettability became weak. The permeability of both raw and treated samples the of coal samples was measured. The results show that under the same oxidant concentration and oxidation time, the smaller the initial permeability of coal samples, the greater enhancement of permeability after oxidation. The average permeability of coal samples increased by 1.589 times (3 wt % H2O2), 1.781 times (6 wt % H2O2), 2.497 times (9 wt % H2O2), 4.073 times (12 wt % H2O2) and 5.795 times (15 wt % H2O2) after oxidation for 240 h. The methane adsorption-desorption experiment showed that the desorption rate (Vd/Va) of treated sample at each pressure step was decreased by 23.19% (on average). Furthermore, the oxidation reaction can rise the temperature of coal, the mass transport channels were widened due to the pore and fracture system was dissolved by the oxidant. This possibly explains the enhancement of gas desorption capacity and mass transport rate in coal.