Hydraulic Fracture Reorientation Research Articles

Visualized Hydraulic Fracture Re-Orientation in Directional Hydraulic Fracturing by Laboratory Experiments in Gelatin Samples

Directional hydraulic fracturing (DHF) is popular with hydraulic fracturing operations in coal mining to create cave-hard roofs, in which radial initial notches are created around open borehole walls before injecting high-pressurized fluid. Despite extensive field application of DHF, the three-dimensional irregular hydraulic fracture (HF) geometry in DHF remains unclear, and the HF re-orientation mechanism requires comprehensive understanding. Here, we experimentally examined factors affecting HF re-orientation in DHF in transparent gelatin samples with a self-developed experimental device. We found that it is the ratio between the differential stress and gelatin elastic moduls that determines HF re-orientation rather than the absolute magnitudes of these two factors. Both shear failure and tensile failure occur during HF re-orientation. The HF tends to propagate asymmetrically, and the step-like HF geometry is likely to form in gelatin samples with low elastic moduli and under high differential stresses. HF re-orientation is not necessarily a near-borehole effect, and HFs can propagate along the notch direction for longer distances in stiffer gelatin samples under relatively low or moderate differential stresses. Finally, recommendations are provided for the effective utilization of DHF at coal mine sites.

Numerical Investigation of Influence of Fluid Rate, Fluid Viscosity, Perforation Angle and NF on HF Re-Orientation in Heterogeneous Rocks Using UDEC T-W Method

Numerical simulation is very useful for understanding the hydraulic fracture (HF) re-orientation mechanism from artificial weaknesses. In this paper, the UDEC T-W (Trigon–Weibull distribution) modeling method is adopted to simulate the hydraulic fracturing process in heterogeneous rocks. First, the reliability of this method is validated against previous laboratory experiments and numerical simulations. Then the effects of fluid rate, fluid viscosity, perforation angle and natural fracture (NF) on the HF re-orientation process in heterogeneous rocks are studied independently. The results show that the HF re-orientation process depends on the combined effect of these factors. The HF re-orientation distance increases significantly, the final HF re-orientation trajectory becomes more complex and the guiding effect of perforation on the HF propagation path is more evident with the increase of fluid rate, fluid viscosity, and perforation angle if the hydraulic fracturing is performed in relatively heterogeneous rocks, while the differential stress is the main influencing factor and is more likely to dictate the HF propagation path if the rocks become relatively homogeneous. However, increasing the fluid viscosity and fluid rate can attenuate the impact of differential stress and can promote HF propagation along the perforation direction. Besides, NFs are also the important factor affecting HF re-orientation and induce secondary HF re-orientation in some cases in heterogeneous rocks.

Simulating Hydraulic Fracture Re-orientation in Heterogeneous Rocks with an Improved Discrete Element Method

Understanding the hydraulic fracture (HF) re-orientation mechanism from artificial weaknesses is critical to screen-out prevention in the petroleum industry and caveability management in the mining industry. An improved discrete element method is proposed and incorporated in the Universal Distinct Element Code (UDEC) to simulate hydraulic fracturing in heterogeneous rocks, and its reliability is validated against previous laboratory experiments. The effects of rock heterogeneity and rock strength on HF re-orientation are unveiled to fill the research gaps in the existing knowledge. The results show that the proposed UDEC T-W (Trigon-Weibull distribution) model can well simulate HF propagation in rock samples of different homogeneity degrees and yields more realistic simulation results compared with the classic extended finite element method (XFEM). The HF re-orientation process depends on the combined effect of all the influencing factors. HFs tend to be directed by perforations if hydraulic fracturing is performed in relatively heterogeneous rocks, while the differential stress is more likely to dictate the HF propagation paths if rocks become relatively homogeneous. We also find that higher rock strength weakens the impact of the differential stress and favours the control of perforations over HF propagation. Finally, recommendations are provided for effective utilization of hydraulic fracturing at the mine site.

The Impact of Oriented Perforations on Fracture Propagation and Complexity in Hydraulic Fracturing

To better understand the interaction between hydraulic fracture and oriented perforation, a fully coupled finite element method (FEM)-based hydraulic-geomechanical fracture model accommodating gas sorption and damage has been developed. Damage conforms to a maximum stress criterion in tension and to Mohr–Coulomb limits in shear with heterogeneity represented by a Weibull distribution. Fracturing fluid flow, rock deformation and damage, and fracture propagation are collectively represented to study the complexity of hydraulic fracture initiation with perforations present in the near-wellbore region. The model is rigorously validated against experimental observations replicating failure stresses and styles during uniaxial compression and then hydraulic fracturing. The influences of perforation angle, in situ stress state, initial pore pressure, and properties of the fracturing fluid are fully explored. The numerical results show good agreement with experimental observations and the main features of the hydraulic fracturing process in heterogeneous rock are successfully captured. A larger perforation azimuth (angle) from the direction of the maximum principal stress induces a relatively larger curvature of the fracture during hydraulic fracture reorientation. Hydraulic fractures do not always initiate at the oriented perforations and the fractures induced in hydraulic fracturing are not always even and regular. Hydraulic fractures would initiate both around the wellbore and the oriented perforations when the perforation angle is >75°. For the liquid-based hydraulic fracturing, the critical perforation angle increases from 70° to 80°, with an increase in liquid viscosity from 10−3 Pa·s to 1 Pa·s. While for the gas fracturing, the critical perforation angle remains 62° to 63°. This study is of great significance in further understanding the near-wellbore impacts on hydraulic fracture propagation and complexity.

Effect of discontinuity stress shadows on hydraulic fracture re-orientation

Hydraulic fracturing has been applied to the cave mining industry as a pre-conditioning method to improve rock mass caveability and fragmentation sizes. In theory, the hydraulic fracture orientation is dictated by and is perpendicular to the minimum in-situ stress orientation. Such orientations will not always result in the creation of a blocky rock mass to aid caveability. The understanding of hydraulic fracture re-orientation induced by the stress shadow effect is important for either avoiding the undesirable interaction between multiple transverse hydraulic fractures or taking advantage of this phenomenon to create prescribed hydraulic fractures that result in the creation of blocky rock masses. In this paper, the existing knowledge of the stress shadow effect around a pre-existing discontinuity is reviewed. A 3D numerical modelling code, Rock Failure Process Analysis (RFPA) is used to investigate factors influencing hydraulic fracture re-orientation. It is established that hydraulic fractures can be forced to propagate in desired directions if advantage is taken of the stress shadow effect in combination with knowledge of rock mass homogeneity and site far field stress conditions. Both the differential stresses between σ2 and σ3 and between σ1 and σ3 affect the hydraulic fracture re-orientation geometry. The minimum far field stress magnitude has no direct impact on hydraulic fracture re-orientation. Rock mass homogeneity significantly influences how far a hydraulic fracture propagates along its predefined orientation from its initiation point before re-orientation.

Numerical modeling of hydraulic fracture propagation and reorientation

Hydraulic fracture propagation, the prevailing method of reservoir stimulation, is controlled by the in situ reservoir stress state. This controls hydraulic fracture orientation and, for the most part, the created hydraulic fracture geometry. In this study, the boundary element method based on the displacement discontinuity formulation is presented to solve general problems of hydraulic fracturing propagation under biaxial loading. The crack tip element and a higher order displacement discontinuity method are used to study the hydraulic fracture propagation and reorientation. The maximum tangential stress criterion (or σ-criterion) is used to find the fracture path. The effect of fluid pressure and stress difference on the hydraulic fracture propagation is studied too. The developed numerical program (2DFPM) for modelling the reorientation process is verified by comparing its result with those of analytical and experimental tests, where a good agreement was observed. The numerical analysis showed that hydraulic fracture reorientation is effected remarkably by stress difference, fluid pressure and hydraulic fracture inclination.

Hydraulic fracture reorientation: Does it occur? Does it matter?

Hydraulic fracture growth, the dominant method of well stimulation, is controlled by the in-situ reservoir stress state. The in-situ stress state controls hydraulic fracture orientation and, for the most part, the created hydraulic fracture geometry. In-situ stress evolves over the geologic history of a reservoir, leaving footprints of this evolution such as natural fractures and anisotropic permeability. However, on a vastly different time scale of months or even hours, the reservoir stress state can be dramatically altered via fluid production and/or injection. These man-made poroelastic stress perturbations alter the stress magnitude and sometimes even the stress orientation, inducing a change in hydraulic fracture orientation that has been observed on refracture treatments. Fracture reorientation has also been observed on fracture treatments of new infill development wells in mature fields, and is regularly observed on cyclic injection operations in disposal wells.