Swirling-round jet could highly improve rock erosion efficiency. Besides, supercritical carbon dioxide (SC-CO2) fluid is characterized by low rock breaking threshold pressure and high rock breaking rate. This paper proposes a new type of jet, called swirling-round SC-CO2 jet. Rock breaking mechanism of swirling-round SC-CO2 jet is studied by means of rock-erosion experiment and scanning electron microscope (SEM) observation. Standard cylinder sandstone cores were used for rock erosion experiment. Then, slices were cut off from these rock samples for SEM observation. Results show that the jet penetration efficiency of swirling-round SC-CO2 jet is higher than that of round SC-CO2 jet. SEM observation presents: numerous crystal particles are snapped; inter-crystalline rupture occurs, generating large number of micro fractures; shear failure of crystal particles appear widely. Therefore, the major rock breaking mechanisms of swirling-round SC-CO2 jet are tensile and shear failure, while the breaking mechanism of round SC-CO2 jet is tensile failure. For one thing, swirling-round SC-CO2 breaks rock by combining axial impinging, radial tension and circumferential shear. For another, with characteristics of low viscosity, compressibility, strong diffusivity and permeability, numerous micro-fractures are caused by SC-CO2, and the existed fractures are expanded, led to fractures joining together and massive rocks split off.

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