NUMERICAL SIMULATION AND MECHANISM OF DEFORMATION AND FRACTURE OF SHALE GAS CAP ROCK

Due to the special nature of the occurrence mechanism of shale gas, storage conditions and type of cap affectshale gas reservoir. Dense overburden rock have a great impact on shale gas capping, take a greater role in the decision of reservoir fracturing technology. This article is for theoretical analysis of cap rock fracture mechanism for shale gas reservoir. and establish the numerical simulation model for cap rock. The cap rock fracturing is related to water injection pressure, rock tensile strength and in-situ stress.the fracture mechanism of cap rock includes two stages. The fracturing initial stage, the reservoir deformation, cap rock occurs shear crack, fracturing fluid into cap rock. When water injection pressure is less than the minimum principal stress, shear fracture rock When water injection pressure is the sum of least principal stress and tensile strength of rock, rock occurs tensile failure, the process of fracture involves tensile and shear failure, fracture increase and expand, form a mesh system. Numerical simulation results show that the high pressure water can produce hydraulic destruction in the reservoir area. With water injection pressure rising, hydraulic damage area expanded and occurs superimposition effect, result in the hydraulic failure focus on upper part of the middle water injection. In the whole process of reservoir fracturing, 1 hour later, the cracks of the reservoir develop, the cap rock stability, rock pore water pressure and deformation is small, there are small cracks at the bottom of cap rock. After fracturing 2 hours, cracks at the bottom of cap rock increase, rock pore water pressure rise, and rock occurs deformation, deformation is most obvious in the middle. After fracturing 3 hours, cracks extend from the bottom of the roof to the upper, cracks rock in internal rapidly develop, rock pore water pressure and deformation value increasing. The results of numerical simulation is consistent with the results of mechanism analysis of fracture.