Study On Shear Slip Mechanism Of Hydraulic Crack In Shale Stratum

Hydraulic fracturing is an important technology of stimulation in shale oil and gas resources. In the fracturing process, in addition to form hydraulic cracks in formation, there are some discontinuities such as natural fractures、joints、faults and so on, all these planes’s mechanical behavior has an important impact on the final fracture shape and stimulation effects. In the past, due to the tensile strength of rock mass is less than the shear strength generally, so tensile damage of fracture is well studied, while the shearing slip process is not taken seriously. However, in recent years, with the development of shale gas, researchers begin to realize the shear slippage has important effects on conductivity enhancement, while the quantitative research on this issue is few.Firstly, this paper discussed the mechanical conditions and described the physical process of shear slip, then aimed at two-dimensional planar cracks, using the theory of complex variables and the displacement discontinuity method, established the Ⅰ-Ⅱ multiple fracture stress and displacement field calculation model as well as natural fractures’slippage calculation model under different boundary conditions. Finally, based on the calculated results of slippage, analyzed the conductivity change of fractures caused by shear slip quantitatively.Theoretical analysis shows that, shear slip generally occurs in natural fractures or joints, the shear dilatancy pheonomenon which happens in the process of slippage has an important influence on conductivity enhancement; the slip distance is controlled by mechanical parameters and stratum property, crack length, shear stress were positively correlated to slip distance, crack surface friction coefficient, young’s modulus hold back the crack sliding; the natural fractures’ shear slip has an threshold pressure, the presence of natural fracture has no effect on the stress distribution if stress is lower than the threshold value, while the fractures begin to slip from the middle if the stress exceed the threshold pressure. Experimens show that after fracture slip, the conductivity will increase an order of magnitude, however, the fracture conductivity has no direct relationship with slip distance.The results of this paper has a certain guiding significance on fracturing shear slip mechanism research and fracturing design optimization of shale gas.