| The development of shale gas is the focus of the Chinese future energy development,but the existing level of development has a certain gap with the advanced level.Shale gas has the characteristics of self-generation,self-storage,self-sealing,saturated gas,short migration distance,low porosity(3%-10%),low permeability(generally several millidarcy),which requires water injection to create cracks inside the shale.In this paper,the hydraulic fracturing crack propagation law is selected as a breakthrough point,and the hydraulic fracturing simulation experiment and numerical simulation are used to study the hydraulic fracturing crack propagation.The in situ detection of the hydrofracturing cracks is limited by the shortages of the technologies existed,implemented difficultly and costly.It is difficult to understand and accurately describe the expansion of shale fracturing cracks,and the actual fracturing effect of shale lacks scientific and accurate judgment.It is of great scientific significance and engineering to reveal the hydraulic fracturing mechanism of shale through laboratory research to accurately and quantitatively judge the hydraulic fracturing effect,effectively implement hydraulic fracturing technology,improve the recovery rate of unconventional oil and gas resources,and reduce mining cost.This thesis uses physical experiments and numerical simulation methods to study the crack propagation law of hydraulic fracturing,and reveals the mechanism of crack initiation and expansion of shale hydraulic fracturing.The research contents and results of this paper include:(1)The basic theory of shale hydraulic fracturing is summarized,the theory of tension failure and shear failure of hydraulic fracture initiation is studied,and the conditions of hydraulic fracture propagation through fracture mechanics failure theory is summarized;through the interaction of single hydraulic fracture and natural fracture,analyze the propagation mode of hydraulic fracture under different conditions.(2)Shale outcrop in southern Sichuan,selecting uniaxial compression failure anisotropy test is carried out on shale specimens with three bedding angles of 0,45 and 90 degrees.By analyzing the change of anisotropy corresponding to compressive strength,it is found that the strength difference between the same bedding angle is small,and the difference between different angles is large.The fracture failure mode of anisotropy is further analyzed.It is easy to destroy bedding surface,difficult to penetrate bedding,and obvious anisotropy exists.The brittleness index of mineral content is obtained by X-ray diffraction analysis of whole rock,and then the elastic modulus and Poisson's ratio of the sample are obtained by triaxial compression test of the shale sample.Then the brittleness index of the shale sample is calculated,and the fracturability of the shale sample is evaluated,and the shale is found.In the process of fracturing,it is easy to produce complex fracture mesh.(3)Hydraulic fracturing test of shale is carried out at laboratory scale.The in-situ stress of shale is simulated by true triaxial test.Fracturing simulation is carried out under the condition of pressure fluid injection system.Acoustic emission device monitors the acoustic emission phenomenon caused by internal fracture of sample and simulates the process of hydraulic fracturing in actual production.Are obtained the initiation time and the approximate pressure of hydraulic cracks.At the same time,the generation,expansion and entry of hydraulic cracks are analyzed.Finally,the process,time and approximate pressure of breaking through the bedding were verified by AE data.At the same time,the natural structural plane would affect the generation of cracks,steering and the formation of the final fracture network were analyzed.(4)The cohesive element of ABAQUS is used to establish a three-dimensional natural structural plane-bedding hydraulic fracturing model.The parameters of Chapter 3 and Chapter 4 are used to simulate the propagation path of hydraulic cracks in the model.The pressure at injection point changes with time and the crack width changes with time are analyzed to determine the initiation time and the initiation pressure of hydraulic cracks in the three-dimensional model of natural structural plane-bedding.?Through the analysis of displacement and stress on a single structural plane,of hydraulic crack propagation and the time crack initiation on each surface are obtained,and the crack propagation at different distances from the injection point is analyzed.At the same time,the influence of natural structural plane and bedding plane on hydraulic crack propagation is also analyzed. |
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