Fracture propagation stimulated by hydraulic injection under stress field

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Hydraulic fracturing is a major scheme for improving the production of unconventional oil and gas as well as horizontal wells. Created fractures increase the permeability of reservoir formations, which are usually tight and of low permeability,through a network of fractures in the reservoir. Laboratory experiments indicate that hydraulic fractures would propagate in the direction of the maximum principal stress around the fracture tip. This indicates that in-situ stress could play an important role in the behavior of hydraulic fracture propagation in the field scale. It is, however, difficult to observe the fracture propagation directly due to the depth of the reservoir layer (>2km generally).There are two types of rock failure that are suggested to take place at the hydraulic fracturing, (i) tensile and (ii) shear fractures. In earthquake seismology, we know the latter is dominant in the generation of natural earthquakes. However, the ratio of tensile to shear fracture events induced by the fluid injection has not been well investigated yet due to the small magnitudes of failures. To tackle this problem, we adopted the extended finite element method (X-FEM) and added a new degree of freedom for the effects of the fluid inside fractures. It would bring an idea on how fracture propagates in a stable stress field no matter how the magnitude of each event becomes small. We developed a hydraulic fracturing simulation tool to explore the mechanism of fracture propagation triggered by the fluid injection. For the evaluation of the fracture propagation, we assumed a numerical simulation model in real scale and put external forces as an in-situ stress. We conducted two types of simulations, one homogeneous and the other inhomogeneous in the rock strength distribution. The homogeneous model showed that fractures propagate with both tensile and shear failures even if the injected fluid acts homogeneously outward at the fluid-solid interface. The inhomogeneous model showed that fractures no longer propagate simply in the direction to the maximum principal stress field. Our results indicate that both tensile and shear failures take place even in the homogeneous model probably due to the influence of stress field, and that the propagation of small fractures takes place randomly in the inhomogeneous model due to localized small-scale inhomogeneous stress field acting at the tip of fractures.