Simulation of Effects of Incident Angles and Frequencies of Ultrasonic Waves on Wave Energy Propagation and Variation of Cleat Width Induced by Ultrasonic Waves

Abstract

Ultrasonic wave stimulation shows potential in enhancing the permeability and production rate of low-permeability coal seam gas (CSG) reservoirs, however, the mechanisms of mechanical vibration induced by ultrasonic waves in coal is still unclear. In this paper, the effects of physical attributes of ultrasonic wave signals, including the incident angles and frequencies, on the propagation/attenuation of wave energy and cleat width variation are numerically analyzed. In the numerical modelling, the propagation direction of ultrasonic waves to the orientation of cleats varies from 0° to 90° by an 10° increasing range, and the frequency range of signals varies between 2 and 10 MHz with an increment of 2 MHz. To evaluate the changes of mechanical vibration near coal-cleats induced by the changes of physical attributes of ultrasonic waves, four parameters are discussed, including shear wave energy (SE), accumulation of wave energy (WE), cleat-width variation at each moment (D1(t)), and accumulation of cleat-width variation during a certain period (D2). Numerical simulation results show that: (1) wave signals with larger incident angles to the orientation of cleats contribute to larger SE, longer SE accumulation near cleat interfaces, and larger D1(t) and D2; (2) the ultrasonic waves at higher frequencies contribute to larger SE near cleat interfaces, which attenuates more easily during propagation. Meanwhile, the D1(t) becomes larger during the initial period when waves firstly arrive at the cleat. However, in the case of smaller frequencies, comparatively larger D1(t) lasts longer after waves leave from cleat, and D2 is larger during the overall stimulation.