Experimental Investigation on EOR and Flowback Rate of Using Supercritical CO₂ as Pre-Fracturing Fluid in Tight Oil Reservoir

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.