Hydraulic fracturing is an important means for the development of tight oil and gas reservoirs. Laboratory rock mechanics experiments can be used to better understand the mechanism of hydraulic fracture. Therefore, in this study we carried out hydraulic fracturing experiments on Triassic Yanchang Formation tight sandstone from the Ordos Basin, China. Sparse tomography was used to obtain ultrasonic velocity images of the sample during hydraulic fracturing. Then, combining the changes in rock mechanics parameters, acoustic emission activities, and their spatial position, we analyzed the hydraulic fracturing process of tight sandstone under high differential stress in detail. The experimental results illuminate the fracture evolution processes of hydraulic fracturing. The competition between stress-induced dilatancy and fluid flow was observed during water injection. Moreover, the results prove that the “seismic pump” mode occurs in the dry region, while the “dilation hardening” and “seismic pump” modes occur simultaneously in the partially saturated region; that is to say, the hydraulic conditions dominate the failure mode of the rock.
Ultrasonic tomography, which is widely used in the study of the fracturing process of rocks, often exhibits low resolution due to insufficient ray coverage, particularly while evaluating the three-dimensional (3D) fractures. To resolve this issue, we adopted sparseness regularisation in tomography to reconstruct the ultrasonic velocity of rocks. Both numerical and laboratory experiments demonstrate that tomography with sparseness regularisation generates velocity images with clear fracture morphology than that with Tikhonov regularisation. Dynamic monitoring of the fracturing process of a granite slab with two-dimensional (2D) velocity images can reveal the accurate development of the fracturing process. The experiment on the internal structure of tight sandstone after hydraulic fracturing reveals demarcated low-velocity regions in the 3D ultrasonic velocity images of tomography with sparseness regularisation. These low-velocity regions correspond to the positions of the fractures when compared to the X-ray scanning images. Thus, tomography with sparseness regularisation can improve the resolution of ultrasonic velocity images, which can be used to accurately describe the fracture development and strain localisation during rock deformation.
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