Study On The Mechanism Of Hydrofracture Propagation Beyond Boundary In Shale Mudstone Stratigraphic

When exploiting shale gas resources in Chongqing,the shale-mudstone composite formation interface is often encountered,which makes the hydraulic fractures show different expansion forms at the formation interface.It is of great significance for the selection of hydraulic fracturing construction technology and the effective implementation of hydraulic fracturing design scheme in shale gas production to accurately understand the influence of formation interface on the law of hydraulic fracture propagation.In this paper,based on the National Natural Science Foundation of China(Grant No.51374257),the Research Fund for Talents of Guizhou University(Grant No.201901),and the Special Research Funds of Guizhou University(Grant No.201903),factors such as the mechanical properties of rock formation materials,stratum interface strength,in-situ stress,fluid pressure in hydraulic fractures and other factors are considered.In addition,the theory of linear elastic fracture mechanics elasticity,complex variable function and interface mechanics are used to derive and determine the stress field distribution law around the hydraulic fracture when the hydraulic fracture orthogonally approaches the formation interface and the hydraulic fracture orthogonally contacts the formation interface.Combined with the critical stress failure criterion of rock,the subsequent crack propagation behavior is predicted,revealing the propagation law of hydraulic cracks at the stratigraphic interface.The main research contents and results are as follows:(1)Establishing a theoretical model of hydraulic fractures approaching the stratigraphic interfaceUnder the assumption,the theoretical model of the hydraulic fracture approaching the stratigraphic interface was established,and the analytical solution of the stress field at the crack tip when the hydraulic fracture approaching the interface was obtained.The eigenvalue of the crack tip stress nonlinear region is discussed,and the crack initiation angle of the hydraulic fracture was determined.According to the criterion of hydraulic fracture propagating through or along the formation interface,the law of hydraulic fracture propagating near the formation interface was studied.(2)Establishing a theoretical model of hydraulic fractures contact the stratigraphic interfaceUnder the assumed conditions,the theoretical model of the hydraulic fracture orthogonal contact stratigraphic interface was established,the general solution of the stress field at the crack tip was solved,and the expression and meaning of the singular eigenvalue of the stress field on the stratigraphic interface are discussed.Combined with the fracture propagation form at the interface,the propagation law at the interface was studied.(3)Combining ABAQUS numerical simulation and experiment to verify the approach and contact theory modelIn order to verify the correctness of the theoretical model,the finite element software ABAQUS was used to simulate the initiation and propagation of hydraulic fractures in shale and mudstone stratigraphic.Under the consideration of parameters including in-situ stress,elastic modulus,Poisson’s ratio and stratum rock tensile strength,the propagation process of hydraulic fractures from shale to mudstone and hydraulic fractures from mudstone to shale was simulated.Further,the expansion process of hydraulic fracture from shale to mudstone and the expansion process of hydraulic fracture from mudstone to shale are simulated respectively,and it is obtained that the hydraulic fracture directly passes through the stratigraphic interface or opens stratigraphic interface.The reliability of the theoretical model was verified by comparing the simulation value of stress field with that of theoretical calculation.In addition,the applicability of the theoretical model was verified again by comparing the theoretical calculation results with the hydraulic fracturing test results of relevant scholars.(4)Parameter sensitivity analysisBased on the theoretical model,the MATLAB calculation program was used to analyze in detail the influencing factors of the stress field on the interface when the hydraulic cracks approached the stratum interface and contacted the stratum interface and the expansion form of the hydraulic cracks at the interface,which affected the propagation behavior of the hydraulic cracks on the stratum interface The main factors are: net fluid pressure,in-situ stress,rock elastic modulus,rock Poisson’s ratio and rock fracture toughness.The greater the net fluid pressure in the hydraulic fracture,the easier the hydraulic fracture will pass through the formation interface.The greater the difference in in-situ stress,the easier it is for hydraulic fractures to pass through the formation interface,and the tangential stress on the interface is independent of the value of the in-situ stress.Rock elastic modulus,Poisson’s ratio and Fracture Toughness have little effect on the normal stress at the interface,but have a greater effect on the tangential stress at the interface.The Mohr Coulomb criterion was difficult to meet,and changes in rock elastic modulus,Poisson’s ratio and Fracture Toughness have little effect on the propagation shape of hydraulic fractures at multi-field interfaces.