Experimental evaluation of ultrasound treatment induced pore structure and gas desorption behavior alterations of coal

Effective reservoir stimulation is the key for enhancing coalbed methane (CBM) recovery. Ultrasound treatment provides a novel alternative to traditional hydraulic fracturing for gas drainage from coal formations. An improved understanding of the ultrasound effect on coal structure and gas desorption and transport behavior through coal is essential for the potential field implementation. This study aims to investigate the pore/fracture structure alteration with ultrasound treatment for coal and its implication on gas desorption and diffusion behaviors. We used multiple techniques, including nuclear magnetic resonance (NMR), low pressure nitrogen adsorption (LPNA) and gas desorption/diffusion experiments, to characterize the pore structure alterations and its implication on the gas desorption and diffusion behaviors of coal. The NMR tests show that the pore volume and their overall interconnectivity in coal increase with ultrasound treatment. With the ultrasonic treatment (25 kHz, 18 kw, 2 h), the porosity increases by 9.6%-11.9%, and the permeability increases by 21.5%-40.7% for the tested coal. Based on the experimental data, ultrasonic treatment can effectively modify the coal pores within size range of 1 nm-100 nm, and the greater power of ultrasound leads to the greater effect on the pore alteration in coal. After being treated with ultrasound (25 kHz, 3-18 kw, 2 h), the average pore diameters of the tested coal increased by 5.05%-61.81%. It was found that the micropore dilation due to mechanical vibration and cavitation effects can effectively convert micropores into the meso-/macro-pores during the ultrasonic treatment. The results demonstrate that ultrasound treatment changes the coal structure and significantly increases the N2 accessible specific surface area and pore volume in coal. These results can jointly make positive impact on the gas transport during the gas drainage and CBM production.