Pressure-sensitive Percolation Mechanism And Productivity Prediction For Fractured Tight Reservoir

The tight reservoirs are produced by fracturing.The complex fracture network in reservoir is the main seepage channel.Complex fracture networks have strong pressure-sensitive features that can cause rapid decline in productivity.At present,the microscopic analysis of fracture deformation does not take into account the deformation process caused by the interaction between the fracture system and the matrix rock mass system.The existing research does not consider the elastic anisotropy of the fracture medium.For fractured elastic anisotropic reservoirs,there is a lack of productivity models that can simulate the whole production process of fractured horizontal wells.To solve the above problems,this dissertation focuses on the fracture deformation,the variation of permeability tensor and the productivity model of fractured horizontal wells.The theory of pressure sensitive deformation in fractured medium is expounded.The boundary conditions for the deformation process and deformation microscopic analysis of the fracture medium and the microstructure of the fracture are defined by carrying out the micro-mechanical analysis of the fracture system and matrix.The mathematical description of the boundary conditions is given,and the function of the fracture aperture and the medium permeability is established.The deformation process and permeability change of matrix and fracture system under the boundary conditions are clarified.The above results show that the effective stress variation around the matrix in the fractured medium is symmetrical about the center of the matrix.Matrix occurs symmetrically about the center of the matrix.Fracture deformation analysis,the physical process of fracture deformation in the reservoir to comply with the fixed boundary conditions.Under the fixed boundary conditions,the fluid pressure is reduced and the matrix on both sides of the fracture surface expands,so that the relative movement of the matrix center will not occur.The equivalent permeability tensor of different type of fracture networks is given and a mathematical model of permeability tensor for fractured pressure-sensitive medium is established.The pressure-sensitive permeability tensor characteristics of the fractured medium are analyzed,and the influence of the elastic modulus parameters of the matrix system and the fracture system on the permeability tensor is described.It is concluded that the magnitude of the permeability tensor decreases with the increase of the pressure drop and the direction of the permeability tensor will also rotate toward the fracture with weaker pressure-sensitive in the elastic anisotropic medium.The deformation of the matrix and the fracture system all affect the permeability tensor,and the deformation of the fracture system plays a leading role.A semi-analytical productivity model is established,which considers the rotation characteristics of permeability tensor,the coupling effect of pressure sensitivity of the fracture network and artificial fracture,and the irregular shape and uneven distribution of the artificial fractures as well as pressure sensitivity of complex fracture networks into consideration.After that,this paper presented the corresponding solution,carried out the productivity factor analysis and discussed the impact on the productivity.Research suggests that the comprehensive conductivity from large to small of three kinds of fractures is classified as wedge,necking and rectangular.The reasonable production work system depends on the pressure sensitivity coefficient of the major fracture.In the elastic anisotropic medium,coupling effect of fracture network and artificial fracture intensified the decrease of fracture network permeability.Meanwhile,the change of direction of fracture network permeability tensor enhanced the permeability anisotropy,and all above resulted in the decrease of cumulative productivity.This paper established a productivity model which can describe the non-uniform fractures networks and irregular major fracture and simulate the fractured well with complex boundary,based on the fractured elastic anisotropic pressure-sensitive permeability model.Besides,we developed a calculation program and carried out a numerical simulation and application of the fractured horizontal well.It is analyzed that the initial productivity is determined by the conductivity of the major fracture.The deformation of the major fracture leads to the rapidly decline of permeability,which leads to a great decrease of the initial productivity.With the pressure gradually spreads,the deformation of the fracture network leads to the rotation of principal direction of the permeability tensor,which leads to the continuous decrease of productivity in the middle and late stages.The subsequent amplitude of fracture deformation decreases slowly,and the permeability is low,and the productivity enters a stable change stage.This paper developed the pressure-sensitive percolation theory of the fractured medium and established a theoretical foundation for the productivity prediction of fractured horizontal wells in the fractured tight reservoirs.