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

Abstract

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 fractures of coal, which will promote gas desorption, diffusive and Darcian flow flux in coal seam, and enhance coalbed methane production.