Dynamic Process of Hydraulic Fracturing Under Triaxial Compression

Rock fracturing at a certain depth in the subsurface is caused by the weight of overlying strata and elevated fluid pressure in confined formations. It produces tensile fractures, with fracture walls moving against fluid pressure. Given that an individual fracture can form in a short time, capturing the dynamic process of hydraulic fracturing poses a challenge but is crucial for understanding the various phases of interaction between the rock and the fluid. In this study, a triaxial rock testing system was used to simulate in situ stresses on standard cylindrical shale samples. Fluid injection through a central borehole induced hydraulic fracturing. The dynamic process of the hydraulic fracturing in shale, including initiation, propagation and partial closure of the fracture, was reflected through the circumferential expansion of the rock specimen. The chain extensometer with high-frequency data acquisition monitored circumferential expansion, enabling high-resolution fracture process capture. After fluid pressure buildup in the shale, fracture initiation occurred with a slight pressure drop, followed by propagation marked by a sharp pressure decline. The fracture aperture increased sharply to its maximum value, followed by a transition from opening to partial closure. The final aperture stabilized at a certain value, with the fluid pressure approximately equal to the confining pressure. Additionally, breakdown pressure decreased with increasing deviatoric stress and increased with higher injection rates. The pressurization rate and fracture growth rate are critical for understanding hydraulic fracturing processes. The circumferential extensometer effectively captured fracture aperture changes, offering valuable insights into the dynamic fracturing process.