Quantitative Analysis of Anisotropy Effect on Hydrofracturing Efficiency and Process in Shale Using X-Ray Computed Tomography and Acoustic Emission

Hydrofracturing technology has become successful in enhancing the permeability of shale gas reservoirs. However, given the geological complexity in the subsurface, considerable challenges remain in quantitatively analyzing the hydrofracture geometry and understanding the anisotropic fracture process. To investigate, we conducted hydrofracturing tests through a horizontal borehole in shale core samples to simulate a horizontal fracturing stimulation. Based on the X-ray computed tomography scanning and fracture geometry reconstruction results, a new quantification method was established to evaluate the effect of bedding inclination and stress contrast on hydrofracturing efficiency. We concluded that the complex fracture network with higher stimulation index could be created at bedding inclinations of 0°–30° and stress contrast of 10 MPa. In this scenario, the fracturing process presented distinctive stages of fracture initiation, propagation, thoroughgoing failure, and stable seepage. The main fracture propagation in arrester mode would require higher fluid pressure, generate a complex fracture pathway, and a larger amount of acoustic emission amplitude, energy, and events. Also, the time from fracture initiation to physical breakdown was relatively long due to the branching of the main fracture into the weak beddings. With the increase in bedding inclination, the fracturing mode gradually changed from arrester to short transverse. The fracturing process was becoming instant, which was accompanied by a lower breakdown pressure, short acoustic emission response, and simple fracture geometry. The understanding of anisotropic fracture process and evaluation method are of interest in shale reservoir fracturing engineering and academic applications.