EXPERIMENTAL STUDY ON PROPAGATION MECHANISM OF FRACTURE NETWORK IN SHALE RESERVOIR UNDER COUPLED STRESS ENVIRONMENT

The exploitation of unconventional oil and gas resources such as shale oil and shale gas can alleviate the current severe global energy shortage. However, the development of shale oil and gas resources mostly requires fracturing operations to stimulate production. In this study, a supercritical carbon dioxide fracturing experimental system was built by considering complex stress conditions, and then the fracture initiation, propagation and fracturing fluid intrusion characteristics were analyzed. The research shows that during the fracturing process, the fractures will expand first and then gradually deflect toward the direction of the maximum horizontal principal stress, and the injection pressure will first increase during the whole fracturing process until the fractures start to decrease and then gradually become stable. During the experiment, the fracture initiation pressure and propagation pressure were 81.67 MPa and 48.32 MPa, respectively. Moreover, during the fracturing process, the intrusion of the fracturing fluid around the fracture into the shale formation will gradually weaken with the expansion of the fracture.The study also found that as the fracturing fluid injection rate increased, fracture initiation and propagation became easier. When the fracturing fluid injection rate increases from 4 m(3)/min to 16 m(3)/min, the fracture initiation pressure and propagation length increase by 16.13 MPa and 4.97 m, respectively. Finally, it is found that the intrusion of fracturing fluid around the fracture also increases with the increase of fracturing fluid injection rate. Considering the fracturing effect and environmental factors, the design of fracturing fluid injection rate within the range of 4-10 m(3)/min is reasonable. This study can provide technical support and guarantee for efficient and environmentally friendly exploitation of shale oil and gas resources.