Study On Rock Failure Mechanism Of Fractured Reservoirs

At present,low-permeability and unconventional oil and gas resources are the focus of China's oil and gas exploration and development.Hydraulic fracturing is a key technology for its development.In diagenetic and late tectonic changes,reservoirs often produce criss-crossing natural fracture systems that often appear fractured.Due to the existence of natural fractures,the cracking methods of fractured reservoirs have become diversified,and the prediction of fracture pressures has often been biased.At present,scholars at home and abroad have studied the cracking of natural fractures in reservoirs to consider the pressure within the fracture as a uniform pressure.In fact,in the hydraulic fracturing process,the fluid in the fracture is non-uniformly distributed due to the fluid flow in the joint and the hysteresis effect of the fluid.And because of the rock stress around the well,the crack wall surface stress is also non-uniformly distributed.This will affect the crack tip stress field,which will affect the fracture mode and fracture pressure of natural fractures.Therefore,based on the theory of fracture mechanics and around the non-uniform stress of the crack surface,a complex variable function method is used to derive the calculation model of the stress intensity factor at the crack tip under non-uniform stress,and the law of stress intensity factor variation under non-uniform stress is further discussed.Based on the theory of elastic mechanics and fluid mechanics,the study of stress distribution of fracture wall surface and non-wetting area of crack tip was carried out.Combining the rock deformation model,the fluid flow model in the joint,and the composite fracture criterion,a natural fracture initiation model for fractured reservoirs under non-uniform stress was established.Finally,the displacement discontinuity method and the finite difference method were used to solve the model,and the effects of reservoir parameters,fluid parameters,construction parameters,and fracture parameters on the crack initiation and crack initiation modes of natural fractures were investigated.The following major research findings and understandings have been obtained through research:(1)Crack dip is the main factor affecting the crack initiation of natural cracks,When the natural fracture is in the middle angle joint,it is prone to shear fracture.When the natural fracture is a low-angle joint and a high-angle joint,it is prone to complex fracture.The shear fracture pressure shows the trend of decrease first and then increase with the increase of the fracture inclination angle;the composite fracture pressure decreases with the increase of the fracture inclination angle.(2)When the fracture begins to crack,the fracture pressure increases with the increase of the ground stress difference,internal friction coefficient and Biot coefficient;and decreases with increasing Poisson's ratio and crack length;the increase of the azimuth angle increases first and then decreases,and takes the maximum at 45°.(3)When the fracture occurs complex fracture,the construction parameters and fluid properties have little effect on crack initiation,and reservoir parameters and fracture parameters have a greater impact on the fracture pressure and cracking angle.In the reservoir parameters,the fracture pressure decreases with increasing Poisson's ratio,and the crack angle remains unchanged;both the fracture pressure and the cracking angle increase as the geostress difference increases.In the fracture parameters,the fracture crack pressure decreases with the increase of fracture length,and the crack angle increases with the increase of crack length;both the crack initiation pressure and the crack initiation angle increase with the increase of azimuth angle.The law of small changes and the maximum value is obtained when the azimuth angle is 45°.The fracture crack pressure decreases with the increase of the dip angle of the fracture,and the crack angle increases first and then decreases with increasing dip angle.The research results of this paper define the main controlling factors that affect fracture initiation of natural fractures in fractured reservoirs,and have certain guiding significance for the prediction of fracture pressure of fractured reservoirs and the design of fracturing construction.