The effects of sorbing and non-sorbing gases on ultrasonic wave propagation in fractured coal

The acoustic wave propagation in geomaterials at different frequencies (ultrasonic, sonic, and seismic) has been extensively studied in the past to relate geomaterials' physical parameters to wave features. The ultrasonic analysis, commonly used for laboratory scale measurements, has been mainly employed to investigate wave propagation in non-sorbing rocks. The ultrasonic response of gas sorptive sediments particularly coal and its involved physical processes have however attracted limited attention to date. In this study, we use a combination of ultrasonic transmission and X-ray micro-computed tomography (XRCT) along with numerical simulation to investigate the mechanisms influencing the wave propagation in fractured coals. The simultaneous ultrasonic transmission and XRCT imaging are performed on stressed coal specimens saturated with non-sorbing (He) and sorbing (CO₂) gases at different pore pressures. We further simulate ultrasonic wave propagation on segmented XRCT images and compare the numerical results with those of the experiments. A further numerical simulation is conducted on synthetic coal geometries to investigate the cleat and matrix property variation on wave propagation. Analysis of experimental and numerical results reveals that the closure of fractures (cleats) by matrix swelling (induced by CO₂ adsorption) predominantly controls the change in wave velocities and the effect of change in matrix properties by gas adsorption on wave propagation is rather trivial. The numerical analysis on synthetic coal geometries further confirms that the effect of changes in coal matrix properties (density and stiffness) by gas adsorption on wave velocity is insignificant. In addition, the results show that the fracture aperture and density influence the wave velocities more significantly at low fracture porosity indicating that wave velocities are more sensitive to fractures when swelling is present (reduction in fracture porosity). These observations have significant implications in the characterization and monitoring of coal seam gas production and gas drainage.