Development Of The Coupled Gas-Liquid-Solid Numerical Method For Rock Hydraulic Fracturing

Rock fracturing in the coupled gas-liquid-solid conditions is involved in various areas such as the petroleum engineering,mining engineering,hydraulic engineering,and renewable energy industries.With the gradual consuming of earth resources,human mining activities continue to advance deep into the earth,and the large-scale hydraulic fracturing technology is widely used in the development and utilization of various deep underground resources.Meanwhile,the hydraulic fracturing is also a common factor in the safety assessment of large hydropower structures.However,as the hydraulic fracturing involves the dynamic fracturing and failure of solids(rock masses),the fluid flows(usually characterized by multiphase flow)in the dynamic evolving rock fracture network,and the interaction between them,it is a typical complex fluid-solid coupling problem.Both conventional analytical and experimental methods have certain limitations in solving it.In this thesis,based on the previous research,a numerical method that can simultaneously consider the rock nonlinear fracturing and failure,anisotropic characteristics and coupling of multiphase flows and solid is established.The main contents of the thesis are as follows:(1)The nonlinear discrete constitutive model for describing the whole tensile failure of rock.Firstly,the mechanism of the tensile failure of rock is investigated in detail by using the three-dimensional distinct lattice spring model(DLSM).Through a series of studies,it is concluded that the non-uniformity of the mesoscopic intensity of rock has an effect on the pre-peak response of its macroscopic tensile failure,and the spatial position of the initial defect of rock can control its post-peak response.Besides,it is proved that the nonlinear discrete constitutive model is necessary for the DLSM to describe the whole process of the rock tensile failure(pre-peak characteristics and post-peak characteristics).Based on the characteristics of the DLSM and the traditional cohesive zone model,three new nonlinear discrete constitutive models are established and their characteristics are analyzed.The numerical results show that the post-peak portion of the discrete constitutive model of rock can simultaneously control the pre-peak and post-peak responses of its macroscopic tensile failure;however,the effect on the pre-peak response is scale-independent,while the effect on the post-peak response is scale-dependent.In this thesis,based on these conclusions and further comparison of numerical analysis results,a parameter selection method for the discrete constitutive model is proposed with considering the scale effect,and its effectiveness is validated through experimental results.(2)The discrete constitutive model for the quasi-brittle crack propagation of rock and its parameter selection method considering size effect are proposed.Based on the physical experimental results of the quasi-brittle crack propagation of rock-like materials,the DLSM is further developed to solve the uasi-brittle crack propagation by developing a new constitutive model.The necessity of solving the quasi-brittle problem by the DLSM with the nonlinear discrete constitutive model is clarified,then,a new nonlinear discrete constitutive model considering the effect of the particle size is established,and the closed relationship between the discrete model and macroscopic parameters is derived.The new quasi-brittle crack propagation model proposed in this thesis is verified in detail by extensively comparing its simulation results with the physical experimental results of the existing quasi-brittle crack propagation.(3)A digital discrete numerical model characterization method for rock's anisotropy is developed.An anisotropic modeling method based on the discontinuity representation is first proposed,in which the weak interlayer and microcracks is modelled by adding discontinuous surfaces.The input of the weak surface parameters adopts the basic mechanical parameters of the conventional joints,and the damage uses the basic Coulomb friction law.Considering that the rock anisotropy,in reality,can be caused by many factors such as weak surface and inclusion,an anisotropic characterization method based on the digital image is further proposed.The numerical results show that the model based on the digital image can reproduce the anisotropy of rock from various factors such as elastic modulus,uniaxial tensile strength and crack resistance.(4)A new gas-liquid-solid coupled numerical model for rock fracturing is established.By further coupling the single component multiple phases(SCMP)-lattice Boltzmann method(LBM)with the DLSM,a new numerical model considering gas-liquid-solid coupling is established.The coupling model and its implementation are verified,including the classical multiphase flow problem,the Laplace law of the surface tension and the three-phase contact angle problem.The rock fracturing process under the high-speed droplet impact is also simulated.The numerical results of the coupled model are also compared with the existing physical experiments.It is verified that the coupled numerical model is able to solve fluid-solid coupling problems.On this basis,the newly developed model is used to study the influence of heterogeneity,anisotropy and multi-phase pores on the hydraulic fracturing,and also includs a hydraulic fracturing analysis of a high concrete dam under water pressure.Finally,the main contributions,conclusions as well as the pros and cons of the model are analyzed and summarized.On this basis,the possible research directions in the future are proposed.