Study On The Rock-breaking Mechanism Of Supercritical Carbon Dioxide Jet

At present, the exploration and development degree of China’s conventional oil and gasresources is already quite high, the exploration and development of unconventional oil andgas resources such as low permeability reservoirs, tight sands gas, shale gas, coalbed methane,heavy oil and natural gas hydrate has become one of the important directions of China’senergy development strategy. The drilling engineering technology, together with theexploration, development technologies constitute the three pillars of the oil upstream industry,and drilling engineering technology is the core business, it is a breakthroug means ofexploration and development technologies.The function of drilling engineering has beentransformed from constructing a traditional oil and gas channel to improving the success rateof exploration, oil recovery of development and oil and gas production. However, it isdifficult to effectively explore and develop the unconventional oil and gas while protectingthe oil and gas reservoirs. Therefore, it is urgent to develop new methods and techniques toimprove the rate of penetration, protect oil and gas layer and improve oil recovery and overalldevelopment benefit. Research find that supercritical carbon dioxide has so many uniqueexcellent properties such as high density and strong solubility close to the liquid, lowviscosity and strong diffusion close to gas that it can be applied to solve a large number ofproblems faced in the development process of unconventional reservoir. Using supercriticalcarbon dioxide jet to directly rock-breaking drill can efficiently drill as well as protect thereservoir, which is an energy saving and emission reduction rock-breaking drilling technologyand has a broad application prospect.This paper first derived the density, viscosity and heat capacity equations of carbondioxide, programed the fluids properties, and developed a supercritical carbon dioxide drillingwellbore flow characteristics analysis system combined with the actual drilling conditions toanalyze the fluids properties distribution of carbon dioxide in the wellbore, which provides atheoretical basis for follow-up device development and numerical simulation. Second, thesimilarity theory was optimized combined with the drilling and completion conditions and supercritical carbon dioxide fluid properties to overall design and determine the main functionand basic process of the test device, then the supercritical carbon dioxide drilling andcompletion test device is developed. Third, the rock-breaking mathematical and physicalmodel of supercritical carbon dioxide jet was established and the variation of fluid propertiesin the jet rock-breaking process was analyzed, results found that the smaller the nozzle andthe greater the jet initial pressure are, the greater decrease of carbon dioxide temperature andpressure in the jet rock-breaking process will occur, and the phase change is more severe.Fourth, the variation of rock strength in the rock-breaking process of supercritical carbondioxide jet has been studied and found that the rock strength is significantly reduced after jetinjection. The greater the jet pressure, the higher the temperature and the longer the jet timeare, the larger the decrease of rock strength will be, which verifies the conclusions ofsupercritical carbon dioxide jet rock-breaking experiments, that is, the supercritical carbondioxide jet can significantly reduce the rock strength so as to improve the rock-breakingefficiency. Finally, the rock-breaking drilling potential of supercritical carbon dioxide jet wasexplored and the rock-breaking experiments of direct flow and swirl jet with supercriticalcarbon dioxide were carried out using the supercritical carbon dioxide drilling and completiontest device, results found that the supercritical carbon dioxide jet has a large rock-breakingadvantage compared to the high pressure water jet, the rock-breaking mechanism ofsupercritical carbon dioxide jet is different with high-pressure water jet, rock-breakingperformance enhances sharply as the bottomhole ambient temperature rises.Based on the theoretical and experimental methods, results found that the rock-breakingprocess and mechanism of supercritical carbon dioxide jet are:①forming the initial crushingpit after the jet impacting rock;②expanding the microscopic damage after penetrationwedging into the rock;③forming the macroscopic damage under quasi-static pressure role.