Research On The Rule Of Crack Propagation During Hydraulic Fracturing In Shale Gas Reservoir

Energy is an essential material basis for human survival and development.Natural gas and non fossil fuels has been regard as the main energy development direction in the world.Shale gas is unconventional natural gas of occurrence in the organic matter of mud shale.Shale gas reserves of China ranks first in the world.Shale gas reservoirs are tight and low permeability and contain a lot of beddings and natural fractures,which means that the reservoirs have obvious heterogeneity and anisotropy.Only by hydraulic fracturing can the shale gas commercial production be obtained.The fracture propagation theory of hydraulic fracturing in conventional reservoir is no longer suitable for shale reservoirs:In order to reveal the rule of crack propagation during hydraulic fracturing in heterogeneous anisotropy shale gas reservoir,a study was conducted with jointly methods of experiment,theory and numerical simulation.The main research works and achievements are as follows:(1)A true tri-axial hydraulic fracturing experimental system was developed independently,which can be used for hydraulic fracturing experiments with a function of multiple methods to identify cracks.Experimental studies on anisotropic mechanical properties of shale and hydraulic fracturing in shale reservoirs with original shale rocks and similar material models were conducted.The stress-strain relationships of shale rock in the vertical and parallel bedding direction and strength index under uniaxial loading were obtained.It also pulled the rule of hydraulic fracture propagation affected by beddings and stress in shale reservoirs out:The fracture propagation direction is jointly effected by geostress,bedding orientation,tensile strength of bedding and shale matrix.(2)The stress state of the crack tip was analyzed using the complex function theory.The propagation criteria were obtained by comparing the needed injection water pressure of fracture extending along original direction and turning to the bedding direction.Introducing a critical strength ratio index of bedding and shale matrix get the analytical rule of hydraulic fracture propagation in shale gas reservoirs effected by geostress,bedding orientation,strength of bedding and shale matrix:The hydraulic facture is more easily turning to bedding when the dip angle of bedding,the geostress difference,the tensile strength ratio of bedding and shale matrix decreases.(3)Considering the induced stress model is corrected by ground stress field effect,on this basis,the formation mechanism of fracture network in shale reservoir is studied under the condition of both the induced stress and the fracturing fluid leak-off after hydraulic fracture crossing bedding:The tensile stress concentration exists at the fracture tip and then the fracture tip is tensile failure.The bedding and natural crack is easily sheared failure after it is pass through by hydraulic fracture,which can form the secondary fracture.(4)An anisotropic composite damage model was established based on the extended finite element method.Composite damage program and heterogeneous modeling program of shale were developed.The rule of crack propagation during hydraulic fracturing in heterogeneous anisotropic shale reservoirs was obtained by numerical simulation:The crack propagation direction during hydraulic fracturing in heterogeneous anisotropic shale reservoirs is jointly determined by geostress,bedding orientation,tensile strength of bedding and shale matrix.The fracture is more easily crossing bedding when the geostress difference,the dip angle of bedding,the tensile strength ratio of bedding and shale matrix increases.Simulation results are agreed with experimental and analytical rule.(5)The three-dimensional parameterized modeling and automated analysis processing of multi crack propagation was achieved through secondary development program.On this basis,the numerical simulation of crack propagation impacted by induced stress in horizontal well multi cluster fracturing.The numerical simulation results shown as follow:The induced stress is compressive stress perpendicular to the fracture plane and it is tensile stress along the fracture plane besides fracture.However,the induced stress is tensile stress perpendicular to the fracture plane and it is compressive stress along the fracture plane at fracture tip.The induced stress is shear stress besides fracture tip.The induced stress of multi fractures superimposes on each other under the influence of induced stress,cracks appear incompatible deflection,and fracture plane no longer remain flat.The induced stress value of middle fractures increases because of stress superposition,which makes the middle fractures propagation suppressed by the edge fractures.