Experimental Study on Fracture Propagation Characteristics of CO₂ Pre-Fracturing in Shale Oil Reservoir

CO₂ pre-fracturing offers the dual advantages of enhancing oil recovery and generating complex fractures, demonstrating positive application effects in the field. However, there is a scarcity of research on the characteristics of fracture propagation associated with CO₂ pre-fracturing. This paper presents a comparative experimental study between CO₂ pre-fracturing and hydraulic fracturing, utilizing a true triaxial fracturing simulation system. The study examines the fracture propagation patterns of CO₂ pre-fracturing through the integration of acoustic emission (AE) monitoring and CT scanning techniques. The findings indicate that CO₂ pre-injection significantly reduces the breakdown pressure in the subsequent hydraulic fracturing phase. After the pre-injection of CO₂, the pressure curve during the hydraulic fracturing process exhibited a distinct zigzag pattern, with higher pressure fluctuation frequency, AE rate, and cumulative AE counts, indicating that the CO₂ pre-fracturing had generated a large number of micro-fractures. CT scan results before and after the fracturing process demonstrate that the fractures formed following CO₂ pre-fracturing have a larger fractal dimension, indicating a more complex fracture network. Additionally, the pre-injection of CO₂ activates the beddings within the formation, which subsequently influences the direction of fracture propagation during the following hydraulic fracturing. This study confirms that CO₂ pre-fracturing provides superior stimulation effects compared to conventional hydraulic fracturing methods. Consequently, the application of CO₂ pre-fracturing technology is recommended for the development of shale oil.

Shale oil is one of the most important unconventional oil and gas resources in China, and due to its tight reservoir and poor crude oil mobility, it needs to be stimulated by large-scale fracturing to realize the effective mobilization of crude oil. Shale oil wells are developed through large-scale fracturing, but problems such as rapid depletion of formation energy and rapid decline in production often occur during the production process. Some oilfields have conducted CO₂ pre-fracturing tests, effectively alleviating the above problems. The results of production data analysis show that after the implementation of CO₂ pre-fracturing, the oil well pressure maintenance level was greatly improved, the production decline rate was significantly reduced, and the oil recovery rate was effectively improved.