Intense Cooling during Composite Rock Breaking of High--Pressure CO2 Jet-Polycrystalline Diamond Cutter

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, N-2 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.