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.