Study On Optimization Of Artificial Fracture And Seepage Law Of Shale Gas Reservoir

Due to the low porosity,low permeability and develop less natural fractures in shale gas reservoir,it's necessary to obtain the ideal production capacity by fracturing.In this paper,taking Chang 7 of Yanchang Formation as the research object,based on the characteristics of shale gas reservior,hydraulic fracturing technology and gas occurrence state,a discrete fracture model is built for multi-scale gas reservoir and explore shale gas seepage law.Optimezing the fracture parameters with the help of numerical simulation,and the optimal communication relationship between artificial fratures,matrix and natural fractures is explored.Finally,the optimal relation and accuracy of model establishment are vertified by the experiment of indoor fracture permeability.The results show that the main pore types of shale gas reservoir of Chang 7 are intercrystal pore,emposieu,dissolution pore,organic inner pore,matrix edge pore and micro-frature.Fracture types include high angle fracture,low angle fracture.The average pore radius distribution range is broad 0~40?m.The minimum pore and fracture scale reach nanometer and micron grade.Average porosity and permeability are1.8% ? 7.8 × 10-6?m2,low porosity and extremely low permeability,there is a significant negative correlation between the two groups with poor correlation,and the pore structure is poor and reservoir heterogeneity is strong.According to the pore characteristics and the gas occurrence state,desorption of shale gas occurred on the surface of martrix,diffusion occurred from the micro pore to micro cracks,the flow of gas in the vicinity of wellbore,micro cracks and artificial fractureis is darcy flow.The numerical simulation result shows that the optimal communication between the artificial fracture and the natural crack is 75°.Based on this optimization,the optimization of the parameters of a given artificial fracture is obtained: when the artificial fracture length is 230 m,the crack width is 7mm,the crack spacing is from 120 to140 m,and the number of cracks is 3,artificial fracture,natural cracks and matrix communicate best.At the same time,optimize the parameters of matrix system and natural fracture system and draw that when natural fracture length is 70 m,the number of fractures is 4or 5,the crack spacing is 60 m,the ratio of transforming reservoir along the width of reservoir is 400:220,permeability is 1.686×10-2m D,artificial fracture,natural cracks and matrixcommunicate best.In order to verify the accuracy of the numerical simulation,permeability test of orientation of fractures is conducted by HXDL-2C type proppant fracture evaluation system: the angle between artificial fracture and natural fracture is 75°,the crack orientation is optimum,and it shows that the numerical simulation result agrees well with the experimental results.Numerical simulation results and discrete crack model built is accurate.The results of this study have some theoretical guidance value for fracturing design.