Characterization of ultrasonic induced damage on multi-scale pore/fracture in coal using gas sorption and ?-CT 3D reconstruction

Ultrasonic physical stimulation is a promising approach of coalbed methane enhancement recovery. This study is devoted to quantitatively characterizing the damage of multi-scale pores and fractures in coal induced by power ultrasonic stimulation using the gas sorption and CT image-based 3D reconstruction. The gas sorption tests show that the ultrasound treatment (3 kW, 25 Hz, 2 h) increased the maximum N2 sorption quantities (by 75.44 %- 111.8 %), the average pore diameter by 12.15 %-20.81 %, and the pore surface area (by 30.99 %-56.02 %) in the tested coal. The mesopore volumes increase by 16.46 %-79.15 %, and the macropore volumes increase by 2.38 %-14.82 % after the ultrasound treatment, which reveals that the ultrasound implemented in this study is more efficient for structure modification in pore with scale of 2-50 nm. The CT image-based characterization shows that fracture volume increased by 8.11 %-11.81 %, and the fracture surface increased by 13.92 % -21.63 %, and the average equivalent diameter increased by 6.76 %-10.74 %, and the porosity increased by 8.1 %- 11.86 % for the tested coal subjected to ultrasound treatment. Based on the pore network model, the fracture connectivity was quantitatively analyzed, and it shows that pore throat number increased by 17.62 %-45.76 %, and the average equal radius of pore throat increased by 3.86 %-4.83 %. The proportion of connected fractures in the total fractures was increased by a maximum of 12.78 % in the tested coal after ultrasound treatment. The results show that ultrasound stimulation can significantly improve the multi-scale structure of pores and frac-tures of coal, which will promote gas desorption, diffusive and Darcian flow flux in coal seam, and enhance coalbed methane production.

Automated summary

This study quantitatively characterizes the damage of multi-scale pores and fractures in coal induced by power ultrasonic stimulation using gas sorption and CT image-based 3D reconstruction. The results show that ultrasound treatment can significantly improve the multi-scale structure of pores and fractures in coal, thereby enhancing coalbed methane production.