MECHANISM STUDY ON ROCK BREAKINGWITH SUPERCRITICAL CARBON DIOXIDE JET

Rock breaking with supercritical CO2 jet has the characteristics of low threshold pressure and high rate of penetration. It also contributes to enhance oil and gas recovery dramatically and thus has drawn much attention recently. In order to illustrate the mechanism of rock breaking by supercritical CO2 jet, the eroded rock surface is analyzed microscopically through scanning electron microscopy (SEM) observations. The results show that from the macroscopic view, the difference between rock breaking by supercritical CO2 jet and water jet is that rocks in the former case are peeled into large pieces, forming large areas of etching, while the latter typically form cylindrical holes and the debris of broken rocks is small. In the microscopic view, rupture of rock cementations and peeling of matrix grains are the main damage features of rock breaking by supercritical CO2 jet. Meanwhile, numerous micro-cracks are formed in the eroded region, which has significant characteristics of tensile failure. Rock breaking features with water jet mainly include the rock cementations and matrix breaking up. Many serrated fracture zones are produced in the cutting surface of rocks, which has obvious characteristics of shear failure. Besides, three unique mechanisms for rock breaking by supercritical CO2 jet, which is different from water jet, is that the effect of thermal expansion, water wedge and micro-blasting. This paper, for the first time, is capable of providing theoretical guidance for the supercritical CO2 drilling technology.