Simulation Of Frcture Propagation In Shale Reservoir Under Random Natural Frctures

In recent years,the commercial development of shale gas in the United States has a profound impact on the world energy pattern.After decades of exploration and research,the exploration and development of shale gas in China has also entered a period of rapid growth.At the same time,as a kind of clean,high-efficient unconventional energy,the large-scale development of shale gas plays an positive role in optimizing the energy structure of our country and carrying out the development strategy of clean and low carbon.As a necessary means of shale gas development,the effect of hydraulic fracturing directly determines the production of shale gas.Because of the extremely low permeability of shale matrix,the shale matrix could only rely on the multi-cluster fracturing technology to realize three-dimensional fracturing treatment and thus produce industrial gas flow.Consequently,a specialized fracture propagation model needs to be established which can simulate the geometry and the extension law of fracture network.Furthermore,the fracturing design can be scientifically guided,and the theoretical support can be provided for the high-efficient development of shale gas.In the process of shale gas reservoir fracturing,with the large-scale fracturing fluid injected into the bottom hole,when the bottom hole pressure exceeds the in-situ stress near the well wall and reaches the rock tensile strength,the formation will produce hydraulic fractures.The intersection between hydraulic fractures and natural fractures in strata will activate natural fractures and increase the complexity of fractures.At the same time,fractures will produce induced stress to change the original stress field during the extension process.Therefore,the extension mechanism is further complicated.The following work is carried out in this paper,based on the research background of shale horizontal well segmented multi-cluster fracturing technology:(1)Based on the identification of natural fractures in the mine field,the basic parameters of natural fractures are quantitatively characterized by statistical method,and then the random natural fracture distribution model in shale reservoir is established.(2)Based on the theory of linear clastic mechanics and fracture mechanics,the interaction criterion between hydraulic fracture and natural fracture is established,and the extension modes of hydraulic fracture and natural fracture are analyzed,which provide theoretical support for the subsequent fracture intersection;(3)According to the displacement discontinuity method,the crack induced stress field is modified,and the three-dimensional correction is carried out on the basic theory of the constant displacement discontinuity method,and the infinite crack height is revised to the finite crack height.By discretization of hydraulic fractures along the fracture length into several discontinuous units,the calculation model of induced stress field was established;(4)Using the displacement discontinuity method to solve the rock deformation,continuing to use the flow model of PKN model to simulate fracturing fluid flow,finally the shale reservoir complex fracture propagation model was established without considering the filtration of fracturing fluid into shale matrix,and the model solved by the finite difference method,after that the single fracturing,Sequential fracturing and alternating fracturing were simulated.The fracture propagation model constructed in this paper provides a theoretical tool and method for simulating the fracture propagation under the random natural shale fracture distribution.It is of great significance to understand the fracture propagation law under the shale random natural fracture distribution.It has certain guiding function to the field fracturing optimization design.