Numerical evaluation of shear and tensile stimulation volumes based on natural fracture failure mechanism in tight and shale reservoirs

摘要

Although many scholars have put forward the methods and models to predict the stimulated reservoir volume (SRV), the mathematical models do not reflect well the mechanism of SRV development. In addition, the effects of relative fracture treatment and reservoir parameters on different stimulation areas are not well understood. During the process of hydraulic fracture propagation, fracturing fluid leak-off from the main fracture due to the activation of natural fractures can elevate the reservoir pore pressure, resulting in shear slippage and tensile failure of the natural fractures and, finally, in microseismic events. Different stimulation regions, including tensile failure zone, shear failure zone, and swept region, may co-exist along the activated natural fracture. In this study, a new mathematical model was presented based on the shear slippage and tensile failure criterion of weakness plane, hydraulic fracture propagation model, mechanical conditions of natural fracture activation, fluid diffusivity equation, and using a shear dilation model to characterize the reservoir permeability variation after shear slippage of the natural fractures, so as to better describe the growth of SRV. The model was also verified by matching field microseismic monitoring data. Then, the effects of azimuth angle and horizontal principal stress difference on the shear and tensile failure pressure of natural fractures, permeability enhancement, and critical net pressure of main fracture to activate natural fractures were illustrated. The impacts of treatment fluid viscosity, natural fracture azimuth angle, and horizontal stress difference on the reservoir pore pressure, SRV shape distribution, different SRV sizes, and SRV bandwidth and length were also analyzed. The results indicated that increasing the horizontal stress difference decreased the tensile failure area but increased the shear slippage zone sharply. Both shear and tensile failure regions decreased on increasing the natural fracture azimuth angle from 30 degrees to 50 degrees. Increasing the fluid viscosity from 1 to 10 mPabolds expanded the size of the tensile failure zone but reduced the shear slip zone/bold.