APPLICATION OF THE ULTRA-FAST TIME RESOLUTION METHOD TO DESCRIBE THE INFLUENCE OF SC-CO ₂ ON CRACK PROPAGATION BEHAVIOR

In recent years, more and more large oil fields have begun to use supercritical carbon dioxide (SC-CO[Formula: see text] instead of water as a fracturing agent for fracturing and stimulating shale reservoirs. However, only a few methods can accurately describe the influence of the SC-CO 2 treatment during the fracture process. And the fracturing behavior of SC-CO 2 -treated shale in very short time intervals has not been fully investigated. Therefore, shale samples with different bedding inclinations ([Formula: see text]) were treated by SC-CO 2 for 48[Formula: see text]h. Then, a series of three-point bending experiments were conducted on the shale samples before and after SC-CO 2 treatment. These samples were taken from the Cretaceous Qingshankou formation in Gulong Depression, Songliao Basin, NE China. The entire fracture process was recorded by the pulsed laser ultrafast time-resolution method device with a time resolution of up to 15[Formula: see text]ps. The experimental results showed that the key mechanical parameters obviously changed after SC-CO 2 treatment. The influence of SC-CO 2 gradually increases with the increase of bedding inclinations. Additionally, the fractal method was used to analyze the morphology of crack propagation. The fractal analysis results showed that the effects of SC-CO 2 treatment on shale samples with different bedding inclinations vary greatly. Moreover, the fractal dimension of shale with a [Formula: see text] bedding inclination changed the most, indicating that shale samples with the [Formula: see text] bedding inclination are most affected by SC-CO 2 treatment. From the perspective of crack propagation behavior, after SC-CO 2 treatment, the crack propagation type of shale samples with the [Formula: see text] bedding angle changed from first propagating along the bedding and then penetrating the shale matrix to propagating along the bedding inclination until the sample fails. This has a guiding significance for SC-CO 2 fracturing in oil extraction engineering.