Research On Hydraulic Characteristics And Fracture Propagation Mechanism Of Reservoir Shale Based On Particle Discrete Element

With the development of the global economy,energy consumption continues to rise,the contradiction between the shortage of non-renewable resources,environmental pollution,and ecological destruction has intensified.Because shale gas has the advantages of high efficiency,cleanliness,and abundant reserves,it is a new type of unconventional oil and gas resource.The status of the company will become increasingly prominent.However,the recovery rate of shale gas in my country is significantly lower than that in North America.The main reason is that my country's reservoir shale bedding is obvious,natural fractures are developed,the occurrence environment is complex,and the strong disturbance of the fracturing process induces complex and variable propagation paths of hydraulic fractures.The formation law of the crack network is difficult to accurately grasp.Today's society is developing rapidly,and China's economic construction is developing rapidly.Socialism with Chinese characteristics has entered a "new era",and people have put forward higher requirements for a better ecological environment.As one of the most ideal new resources to replace conventional oil and gas resources,shale gas is bound to become an indispensable promoter of the next stage of the goal of a better life for the people.In this paper,the Longmaxi Formation shale collected on site is used as the research object.Based on the experiment,a numerical model of the reservoir shale is established based on the particle discrete element software PFFC2 D,and numerical simulations of triaxial compression,multi-field coupling and hydraulic fracturing are carried out respectively.Analyze and study the mechanical response characteristics of reservoir shale under load,the process of permeation evolution and the law of fracture propagation from the meso-scale.The main research contents are as follows:(1)By carrying out the triaxial mechanical test of the Longmaxi shale,the mechanical response of the shale under different load combinations was obtained,and the shale model based on the particle flow discrete element was established,the meso parameters of the model were calibrated and the reliability of the numerical model was verified.Furthermore,based on the calibrated numerical model,the mechanical properties of shale with different bedding angles under different confining pressures are deeply studied.The results show that with the increase of confining pressure,the compressive strength and elastic modulus of shale increase accordingly.,Poisson's ratio will decrease and the rate of decrease will gradually decrease;the elastic modulus of shale gradually increases with the increase of the bedding angle,while the compressive strength and Poisson's ratio are basically "U" with the bedding angle.Type changes,shale anisotropy is obvious.(2)Further improve the particle flow discrete element fluid-structure coupling algorithm developed by Cundall et al.,and propose a fluid-structure coupling analysis technique that uses geometric figures to define flow pipes instead of contact-defined flow pipes to solve the problem of flow pipe failure due to contact damage in the basic algorithm,Can truly describe the preferential conductivity of shale fractured into fractures.Furthermore,based on the permeability test of reservoir shale,the accuracy of the improved fluid-solid coupling model was verified.(3)Research on the permeability characteristics of reservoir shale with different confining pressure combinations based on the improved fluid-solid coupling model,and the results show that the reservoir shale bedding dip has an important influence on the initial permeability.When the confining pressure is constant,The initial permeability of reservoir shale gradually increases with the increase of the bedding angle,and when the confining pressure is small,the degree of this influence will be strengthened;when the confining pressure is constant,the bedding dip angles are 60°,75°,and 90° When the osmotic pressure increases,the initial permeability increases with the increase of osmotic pressure;when the osmotic pressure is constant and the bedding dip is about 60°,the initial permeability decreases with the increase of confining pressure;in addition,the permeability of different confining pressure combinations The fracture morphology of the shale in the pressured reservoir is significantly different.Under high confining pressure and high permeability,the shale of the reservoir is completely destroyed and has many micro-fractures,mainly X-type shear failure or tensile-shear mixed failure.Under low confining pressure and low permeability pressure,the failure of reservoir shale mostly forms shear failure along the bedding plane.(4)Based on the improved particle fluid-solid coupling model described above,the determination of the flow opening during hydraulic fracturing is discussed,and a mesoscale numerical model of reservoir shale hydraulic fracturing considering the effect of fluid-solid coupling is constructed and its feasibility is verified.Further research on the hydraulic fracturing process of reservoir shale with multiple factors has been carried out.The results show that: for intact shale,with the increase of water injection rate,the initiation and injection pressure of shale gradually decreases,and the reduction rate gradually decreases;The difference between stress and crack propagation is significant.When the two horizontal principal stresses are equal,three hydraulic fractures with equal angles are easily formed.As the difference between the two horizontal principal stresses increases,the hydraulic fractures gradually evolve toward the maximum principal stress.It extends in the direction of the model and extends to the boundary of the model;different water pressures have different effects on fracture propagation.Under larger water injection pressures,the transfer of water flow lags behind the propagation of fractures.The propagation of fractures mainly depends on the transfer of matrix stress.Under lower water injection pressure,the flow of water closely follows the expansion of the cracks,and the expansion of the cracks mainly comes from the action of water pressure.For shale with bedding,the impact of water injection rate on initiation pressure is the same as that of intact shale,but the overall initiation pressure is lower than that of intact shale;the difference in ground stress has a more impact on bedding shale than intact shale It is similar,but the difference is that the existence of bedding has a certain guiding or hindering effect on cracks.When the gap between the two horizontal ground stresses is large,the cracks that develop along the direction of the maximum horizontal ground stress are likely to penetrate the bedding and expand.When the local stress is not much different,the propagation of the fracture is easily guided by the bedding;under different water injection pressures,the water flow transfer will stagnate at the bedding,and the fracture will be guided by the bedding.