Evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification

Coal reservoir usually requires stimulation to enhance coalbed methane (CBM) production. As a novel stimulation method, microwave heating contributes to methane desorption and coalbed fracturing. However, the microwave-induced fractures are easily closed under in-situ stresses. Acidic stimulation is a process of demineralisation in the coal reservoir using hydrochloric, acetic, or formic acids, resulting in opening and interconnecting pores and fractures to flow gas. However, it is difficult for acids to enter microfissures under normal pressures. Microwave-asisted acidification may offer an alternative. In this work, the evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification was evaluated using low-temperature nitrogen adsorption and Scanning electron microscope. Compared to microwave heating or acidification, microwave-assisted acidification greatly increases the pore volume of coal and extends the length and width and connectivity of fractures. In addition, the isolated pores were interconnected and the methane migration was facilitated. This work demonstrates the potential of microwave-assisted acidification for CBM recovery.