RESUMEN
To improve the efficiency of hard rock breaking by a pulsed water jet (PWJ), a hydraulically controlled piston-pressurized PWJ (HCPPPWJ) device has been developed, by which the large amplitude pressurization of the jet could be realized through the motion coupling of the piston and the valve core inside the device without requiring additional control or ultra-high-pressure components. Under the continuous injection of low-pressure hydraulic oil, the device has a stable pressurization effect and controllable pulse pressure and pulse frequency. The jet pressure varies periodically with the alternation of high and low pressures; in the rising stage of the pulse pressure, the jet morphology presents an umbrella-like thin-layer structure, which ensures an effective initial impact force of the jet in contact with the target. With the addition of high-frequency stress waves and water wedge pressure, local flaky exfoliation was observed when the granite surface was eroded, and the maximum radius and volume of the erosion pit were greater than those in the case of employing a continuous water jet. Compared with the interrupted PWJ, the HCPPPWJ efficiently utilizes the jet energy during the erosion process, and the specific energy is lower. The results prove that the HCPPPWJ device is an advanced tool in the field of hard rock breaking.
RESUMEN
To better understand how supercritical carbon dioxide (CO2) enhances shale gas production, it is necessary to study the interaction of supercritical CO2 with shale and its impact on shale microstructure. The different mechanisms by which supercritical CO2 changes the shale pore structure were studied by X-ray diffraction analyses, scanning electron microscopy (SEM), nuclear magnetic resonance spectroscopy, and low-pressure nitrogen gas adsorption tests on shale samples before and after treatment with different pressures and gases (CO2 and Ar). The results showed that after treatment with CO2, the mineral content of shale changed significantly, and in particular, the proportions of calcite and dolomite decreased. The mineral content of shale changed the most after treatment with supercritical CO2, and the microscopic pores were most observable by SEM. In a gaseous CO2 environment, the effect of CO2 adsorption on shale pores is greater than the effects of gas pressure and dissolution reactions. However, in a supercritical CO2 environment, the changes in shale pore structures are mainly controlled by extraction and dissolution reactions. When shale is exposed to supercritical CO2, the fractal dimensions of adsorption pores and seepage pores decrease, indicating that the specific surface area and roughness of adsorption pores decrease. This implies that the adsorption capacity decreases, and that the complexity of the seepage pores declines, which is conducive for gas migration.