Supercritical carbon dioxide (SC-CO2) jet fracturing is a promising alternative for shale gas fracturing instead of water. However, most studies pay more attention to the fracture generation and ignore the flow characteristic of SC-CO2 jet fracturing in limited perforation scenarios. To accurately explore the flow field in a limited perforation tunnel, a numerical model of a SC-CO2 jet in a limited perforation tunnel before fracture initiation is established based on the corresponding engineering background. The comparison between the numerical simulation and experiments has proved that the model is viable for this type of analysis. By using the numerical method, the flow field of the SC-CO2 jet fracturing is analyzed, and influencing factors are discussed later. The verification and validation show that the numerical model is both reliable and accurate. With the dramatic fluctuating of turbulent mixing in a fully developed region, there is an apparent increase in the CO2 density and total pressure during limited perforation. When the z increases from 10 times r0 to 145 times r0, the velocity on the perforation wall surface would decrease below 0 m/s, resulting in backflow in the perforation tunnel. The structure of the nozzle, including the outlet length and outlet diameters, significantly affects the axial velocity and boosting pressure in the perforation tunnel. The highest total pressure exists when the nozzle length-to-radius ratio is 2. The maximum velocity of the jet core drops from 138.7 to 78 m/s, and the “hydraulic isolating ring” starts disappearing when the radius changes from 1 to 1.5 mm. It is necessary to increase the aperture ratio as much as possible to ensure pressurization but not over 1. Based on a similar theory high-speed photography results clearly show that the SC-CO2 develops to fully jetting in only 0.07 s and a strong mixing exists in the annular region between the jet core and the surroundings, according with the numerical simulation. This study should be helpful for scholars to comprehensively understand the interaction between the SC-CO2 jet and perforation, which is beneficial for studying SC-CO2 fracturing.

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Summary In deep well drilling, rock breaking has some problems, such as low rock breaking efficiency, serious thermal wear of cutters, short service life, and high cost. It is noticed that the application of CO2 drilling fluid in the oil and gas underbalanced drilling is an efficient approach to achieve the reduction of CO2 emissions. Thus, based on the rock-breaking advantages of CO2 jetting, a new rock-breaking method of combing high-pressure CO2 jet and polycrystalline diamond cutter (PDC) is proposed in our study. The cooling mechanism and influencing during rock breaking by using the high-pressure CO2 jet-PDC are conducted. With the test system during composite rock breaking of the high-pressure CO2 jet-PDC, the composite rock-breaking experiment of the high-pressure CO2 jet-PDC was carried out. In the experiment, the comparison of the CO2 jet, N2 jet, water jet, and without a jet was conducted and analyzed. And based on the numerical simulation analysis, the intense cooling mechanism was expounded. In the process during composite rock breaking of the high-pressure CO2 jet-PDC, the intense cooling mechanism was mainly attributed to three main reasons: the thermal effect of the jet flow, the expansion endothermic effect of the jetted flow, and the phase transformation cooling effect of the CO2 jet. The effects of rock samples, jet temperatures, jet flow pressures, and rock temperatures on the cutting temperature were experimentally explored, and finally, the intense cooling rules of composite rock breaking were obtained. The experimental results showed that the CO2 jet had a stronger cooling effect on granite than that of the sandstone. In a certain range, jet pressure was positively correlated with the cutting temperature, whereas jet temperature and heating time were negatively correlated with cutting temperature. The study provides the theoretical support for the CO2 application as a drilling medium in underbalanced drilling.

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