Abstract
Shale gas reservoirs are tight reservoirs with ultralow porosity and ultralow permeability, and their matrix pores are mostly nanoscale. In addition, matrix particles and organic pore surfaces adsorb shale gas. These problems cause the production per well of shale gas to be lower than that of conventional natural gas. The use of hydraulic fracturing technology to exploit shale gas can achieve a good production increase effect. However, using this technology has some limitations caused by technical characteristics and geological conditions. Therefore, new technologies for shale gas exploitation need to be explored. In this study, we propose a method to improve the flow characteristics of shale gas by using ultrasonic waves to increase shale gas production and perform experimental tests to research the actual effect of this method. The lithology, mineral composition, pore structure, specific surface area, and pore size distribution of shale samples are tested. Then, the attenuation characteristics of ultrasonic waves propagating in shale are analyzed. Finally, the effect of ultrasonic waves on the adsorption, desorption, and seepage of shale gas is explored. Results show that the Langmuir adsorption isotherm can describe the adsorption characteristics of shale gas under the action of ultrasonic waves. The gas adsorption constant decreases with increasing ultrasonic wave power. The ultrasonic waves accelerate the gas desorption rate, significantly increase the desorption volume, and prolong the time taken to reach desorption equilibrium. They also increase the permeability of shale gas, and the growth is proportional to the power of the ultrasonic waves. These results indicate that the permeability of shale gas has a power function relationship with the effective stress under ultrasonic waves.