Injection Rate-Dependent Deflecting Propagation Rule of Hydraulic Fracture: Insights from the Rate-Dependent Fracture Process Zone of Mixed-Mode (I-II) Fracturing

Abstract

Mixed-mode (I-II) fracturing is a prominent mechanical characteristic of hydraulic fracture (HF) deflecting propagation. At present, understanding the effect of injection rates on HF deflecting propagation remains challenging and restricts the control of HF deflecting propagation bearing tensile and shear stresses with fluid injection rates. Our recently published experimental results show that the fracture process zone (FPZ) length of mixed-mode (I-II) fractures in rock-like materials increases with the rising quasistatic loading rate. Both the deformation in FPZ and the generation of real fracture surfaces are tensile. On this basis, the rate-dependent mixed-mode (I-II) cohesive fracture model was proposed under quasistatic loading, and a couple of theoretical outcomes were obtained. Under different injection rates, the deflecting HF propagates step-by-step under mixed-mode (I-II) fracturing, and the HF extension path is supposed to be straight in each step. With the increment of injection rate, the increased (tensile) FPZ length is the stable propagation distance of deflecting HF in each step and besides deteriorates the fracture resistance discontinuity of FPZ developing to be a real tensile fracture. Thus, the mixed-mode (I-II) fracture tends to propagate unstably driven by kinetic energy once FPZ develops completely under fast loading. Moreover, two injection rate-dependent (IRD) HF deflecting propagation modes were determined, i.e., the step-by-step stable-propagation and step-by-step unstable propagation modes. HF deflection occurs in the step alternation of fracture propagation. With the increasing fluid injection rate, the increased FPZ length and kinetic energy (from fracture resistance discontinuity) extend the stable and unstable HF propagation distance along the initial direction in an extension step, respectively. Therefore, fast fluid injection improves the HF deflecting propagation radius; i.e., it inhibits the HF deflecting propagation or promotes HF extension along the initially designed direction. The injection rate-dependent HF deflecting propagation modes (based on the proposed model) were validated by further processing of published true triaxial physical simulation tests of hydraulic fracturing. The ordinal response of Fiber Bragg grating sensors embedded along the fracture propagation path, and the continuous fluctuant injecting pressures validate the step-by-step propagation of the hydraulic fracture. The test-measured deflecting HF trajectory indicates that high fluid injection rates remarkably increase the HF deflecting radius, which is consistent with the theoretical analysis in this work. The above findings can provide theoretical bases for controlling the HF deflecting propagation in the surrounding rock of mines and oil-gas reservoirs.

Factors Controlling the Hydraulic Fracture Trajectory in Unconventional Reservoirs

“…When the hydraulic fracture extends along the approximate horizontal principal stress direction, the influ-ence of the maximum horizontal principal stress on the hydraulic fracture trajectory increases gradually. Xing et al [20] proposed the influence of injection rate on fracture deflection; the increase in injection rate is similar to the increase in injection pressure in cases 3-5. But their research did not involve the effect of stress difference, perforation angle, fracturing fluid viscosity, etc.…”

“…Experiment results have shown that with the loading rate increasing, the length of fracture process zone increased from about 5 mm to 17 mm. Fast fluid injection inhibits the deflecting of NF (natural fracture), allowing HF to propagate along the initially designed direction [20]. Yang et al used dynamic caustics in conjunction with high-speed photography to investigate the interaction of running fractures by obliquely incident blast stress waves, which revealed that the moving fracture intended to deflect away from the blast wave source when the fracture and stress wave have the same direction; however, it will propagate toward the blast wave source when they have opposite direction [21].…”

An Improved Model for Evaluating the Brittleness of Shale Oil Reservoirs Based on Dynamic Elastic Properties: A Case Study of Lucaogou Formation, Jimusar Sag

“…Rocks. The fracture process zone is a nonlinear development zone in front of the macrofracture, which could be regarded as a plastic zone roughly [42][43][44][45]. In the hydraulic fracturing of reservoirs, the fracture process zone will be generated ahead of the hydraulic fracture, as shown in Figure 10.…”