Study On The Law Of Expansion Of Hydraulic Crack At Stratum Interface

With the increasing demand for oil and gas resources, the objective of hydraulically fractured have been changed from sandstone to sand shale interbed, shale and coal seams. Under the influence of interface and rock differences, the hydraulic fracture propagation is very complex. The shapes of hydraulic fracture near the interface include stopping extension, growing through the interface and propagating along the interface. The fracturing technology of the interface developed reservoirs is different to the normal reservoirs. The research of the shape of hydraulic fracture near the interface has guiding significance for the fracture design.So far many scholars research much about the shape of hydraulic fracture near the interface, which based on the continuous medium mechanics theory and it’s against to forecast the shape of hydraulic fracture near the interface accurately. Considering these factors, First the paper based on the interface mechanics theory and complex function theory, which considered the influence of interface and rock differences, established a stress field model for the problem of the hydraulic fracture near the interface; second the paper based on the the crack tip plastic zone theory, established a modified formula for the crack tip plastic zone; next the paper based on the the rock damage criterion, established the possible destruction styles for the problem of the hydraulic fracture near the interface.The following main achievements and understandings were made:(1) After the simplification of stress field model for the problem of the hydraulic fracture near the interface based on complex function theory, the comparison of which to the stress field model for model I Griffith crack under uniaxial tension was made, which shows the accuracy and reliability of the model.(2) For the problem of the hydraulic fracture un-contact the interface, the effect of the shape of cross-section can be neglected, for the problem of the hydraulic fracture contact the interface, the effect of the shape of cross-section can not be neglected.(3) The possible destruction styles for hydraulic fracture near the interface includes tensile failure of one side of interface, shear failure of interface, tensile failure of interface and mixed failure.(4) The stress distribution in formation have the characteristics of stress reversal, symmetry and antisymmetry. The relative stress magnitude and the stress variation slope on the sides of interface influenced by rock mechanics parameters and shape parameters.(5) For the problem of the hydraulic fracture un-contact the interface, the stability of the interface influenced by rock mechanics, in situ stresses, shape parameters, the distance between the hydraulic fracture and the interface and fracture parameters.(6) For the problem of the hydraulic fracture contact the interface, the shape of hydraulic fracture near the interface influenced by rock mechanics, in situ stresses, shape parameters and fracture parameters.The research of the shape of hydraulic fracture near the interface has guiding significance for the fracture design of sandstone to sand shale interbed, shale and coal seams, which can raise fracture operation successful ratio and affectivity.