Flow-Geomechanics Theory And Numerical Simulation Method Of Site Stability For CO₂ Sequestration In Coalbeds

CO₂ is a main Green house gas that contributes the most to the Greenhouse effect of the Earth. CO₂-ECBM is an attractive technology that can store CO₂ massively underground and meanwhile enhance coalbed methane recovery. The flowing law of the multi-component fluids and the sequestration site stability in CO₂-ECBM are key contents of the feasibility study of this technology. However, there are few studies in this field in China as CO₂-ECBM still just starts here. In this thesis, the author took a systematic research in view of the geomechanics of coal and gases flow in CO₂ sequestration on the basis of existing related researches. The study results can be divided into four parts in the main, they are respectively the solid-gas coupling mathematical theory and models, lab tests, FE program developing and its preliminary application. They are as following.(1) A solid–gas coupling mathematical model considering multi-component fluid and solid interactions was developed after the analysis to the key geomechanics and flow in CO₂-ECBM. In this model, pore and permeability are changing as a result of the deformation and adsorption induced swelling of the coal matrix . So new evolution equations of pore and permeability that can describe this affection were put forward. (Chapter 3.)(2) Split tests and conventional triaxial loading-unloading tests of coal samples were conducted. The results show that the tensile strength is about 1% of the CTC strength thus can be neglected. For triaxial tests, when the confining pressure is relative low, the stress-strain curve is nice linear elasticity. When the confining pressure is relatively high, the stress-strain relation is nonlinear and obvious plasticity appears at quite early load stage. So the hardening constitutive models should be adopted at high confining pressures. (Chapter 4.)(3) The description of the plastic characteristics of coal. Firstly a new method that defined the initial yield point on each stress–strain curve was put forward creatively. Then a new computational method for strain path dependent hardening parameters using the tiaxial test data was presented. On these two bases, the cohesion and the internal friction angle of coal during the hardening process were calculated and positively correlated to the plasticity. Hardening curves in q-p space were fitted using the CTC tests and the fitting effect of different functions were compared. The fitting results show that the slopes of the q-p curves are increasing with the plasticity hardening. (Chapter 4.)(4) On the basis of the description of the hardening characteristics of coal, a new plastic hardening model called pseudo strength parameters-based plastic hardening model was presented. This model can properly discrib the increasing characteristic of the slopes of the q-p curves with the plasticity hardening. It was embedded into the FE program COALSEEP 1.0 developed by the author. Then it was used to simulate the CTC test of coal sample. The simulation result is quite consistent with the experiment. (Chapter 4.)(5) Adsorptive-elasticity theory and its parameter estimation. Miao's Humidity Stress Field Theory was proved strictly and then was used as the adsorptive-elastic constitutive model of adsorption induced swelling media. The swelling coefficient of this model is a key parameter. An estimate method for this parameter of coal was presented, which not only supplied a convenient parameter estimating method but also bridged this mechanical parameter with the adsorptive parameters. This is beneficial to getting a deeper and more comprehensive understanding of coal properties. (Chapter 5.)(6) The thermodynamic restrictions of plastic yield and failure functions were studied. The dissipation potential functions corresponding to a class of yield /failure functions were got by solving the Dissipation Differential Equation constructed by the author. Thus the thermodynamic consistency of these yield /failure functions can be verified from this dissipation potential expediently. This bridges the dissipation and plasticity functions and supplies us with a new way for the rational verification of traditional plastic yield/failure functions. ( Chapter 5.)(7) The adsorptive-elatoplastic constitutive frame of porous media was studied and a constitutive model of coal belonging to this frame was presented. Based on a phenomenological viewpoint, the adsorption was a independent state variable of coal was supposed.Then adsorption was introduced to the yield function if the fluid affect the yield function. The tensor form of the incremental adsorptive-elatoplastic constitutive relationship and the loading-unloading criterion were presented. Substitute the pseudo strength parameters based plastic hardening model to this constitutive relationship, an adsorption-elatoplastic constitutive model was got for coal. (Chapter 5.)(8) The study of the solution algorithm of the coupling mathematical model and its FE program developing. In view of the process of CO₂-ECBM,a solving strategy for the coupling mathematical model and the weak form of each field equation were presented . Especially,the mole ratio field of CH4 was solved with the characteristic finite element method. A new algorithm of Vector Judging Method to judge the spatial relation between point and polyhedron was presented. This algorithm which was called by the program of characteristic finite element method. Then a coupling analysis program of COALSEEP1.0 was developed using FEPG.on the above bases.These constituted the study content of the simulating method in this thesis. (Chapter 6.)(9) Numerical simulation study of the flow of multicomponent fluids and the injecting induced deformation of reservoir and ground surface. Three simulating cases were carried. The first was the seepage law of multicomponents fluids at the working condition of five-point method. The second was the injecting induced deformation of reservoir and ground surface at the working condition of one well injecting and the other pumping. The third was influence of CO₂-induced swelling on casing stability in CO₂-ECBM. (Chapter 7.)...