Fracturing is the necessary means of tight oil development and the most common fracturing fluid is slickwater. However, the loess plateau of ordos basin in China is seriously short of water resources. Therefore, the tight oil development in this area by hydraulic fracturing is extremely expensive and bad for the environment. In this paper, a new method using CO2 as pre-fracturing fluid is applied in hydraulic fracturing. This method can give full play to the dual advantages of supercritical CO2 characteristics and mixed water fracturing technology while saving water resources at the same time. On the other hand, this method can reduce reservoir damage, change rock microstructure, and significantly increase oil production after full interaction with formation fluid, which is a development method with broad application prospect.

In this work, the main mechanism, the the system energy enhancement, and flowback rate of CO2 as pre-fracturing fluid are investigated. Firstly, the microscopic mechanism of CO2 fracturing was studied, and the effects of CO2 on pores and rock minerals were analyzed by NMR test, XRD analysis and SEM experiment. Secondly, the high pressure chamber reaction experiment was conducted to study the distribution and existence state of supercritical CO2 in the multiphase fluid during the full-cycle fracturing process. Finally, four injection modes of CO2 injection experiments were designed to compare the pressure increase, production enhancement, and flowback rate of CO2 and slickwater, so as to optimize the optimal CO2 injection mode and the optimal injection amount of CO2 slug.

The results show that supercritical CO2 can increase the porosity of rocks by dissolving calcite and clay minerals (illite and chlorite) that gather around the pores according to the changes of pore size and rock composition before and after CO2 action. Supercritical CO2 injection increases the saturation pressure, expansion coefficient, volume coefficient, density, and compressibility of crude oil, which are the main mechanisms of fracturing energy increase and production enhancement. After analyzing the four different injection modes tests, the optimal one is to first inject CO2 and then inject slickwater for fracturing. The CO2 slug has the optimal value which is 0.5 pore volume in this study.

In this study, supercritical CO2 is used as the pre-fracturing fluid, providing a new idea for the stimulation of tight oil. Experimental studies have proved the pressure increase, production enhancement, and flowback potential of CO2 pre-fracturing. The application of this method is of great significance to the protection of water resources and the improvement of fracturing effect.

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