Experimental Studies And Simulations On Coupled Multiphysicses Of CO₂ Sequestration In Deep Coal Seam

CO2 contributes the most to the greenhouse effect of the earth. CO2 sequestration in deep unminable coal seam is a potential management option for greenhouse gas emissions, reducing the risk of CO2 migration to the surface and meanwhile enhancing coalbed methane recovery. The injected CO2 leads to coupled multiphysical interactions of binary CO2-CH4 gases with coal and affects porosity and permeability, and gas flow in coal seam, which are recognized as the bottleneck scientific issues of CO2 sequestration unminable coal seam. Based on the theories of Surface Chemistry, Adsorption Thermodynamics, Seepage Mechanics, and Finite Element Analysis, the thesis presents systematical experimental studies and simulations on coupled multiphysical processes of CO2 sequestration in deep unmineable coal seam. Main contents of the thesis are listed as follows:(1) Single gas adsorption isotherms of CO2 crossing the supercritical temperature (31.4℃), and binary gas adsorption and desorption isotherms on Jincheng anthracite sample from Qinshui Basin, Shanxi province, were experimentally observed. Adsorption of CO2 crossing the supercritical temperature is not monotonically decreasing functioned with temperature and determined by both physisorption of coal surface and phase change of CO2. Desorption hysteresis happens in the measurement of binary gas adsorption and desorption isotherms, which implies that coal matrix swelling with CO2 adsorption can reserve free gas within the sealed shaped pores. According to the separation information of binary gas composition both in free phase and adsorptive phase, CH4 tends to be desorbed preferentially, which however slows down with desorption proceeding. On the contrary, CO2 adsorbs preferentially on coal. But the velocity of CO2 adsorption slows down as well. Above findings are regarded as the fundamental theories for competitive adsorption of binary gas and CO2 sequestration.(2) The adsorption isotherms of CO2 and CH4 were fitted by Langmuir equation, BET equation, DA equation and DR equation separately. Volumetric errors for the measurement and modeling of CO2 adsorption were analysed. A modified adsorption isotherm equation was derived to account for the volumetric errors with a volumetric correction and significantly better matching was obtained by the modified DA equation and Langmuir equation. The volumetric correction is able to quantitatively represent the coal swelling induced by CO2 adsorption. (3) Using the theory of adsorption thermodynamics, the surface free energy change and isosteric heat of adsorption for CO2 and CH4 on the coal were comprehensively investigated for the further explanation of coal having more affinity to CO2 and competing adsorption behavior of CO2 and CH4 on coal surface. Adsorption potential of coal for CO2 and CH4 was calculated with isothermal adsorption data of CO2 and CH4 on coal at different temperatures ranging from 25 to 40℃. Feature curves of CO2 and CH4 adsorption on the coal were constructed. Based on such curves, the relationship of adsorption volume of CO2, pressure and temperature was obtained.(4)A new highly nonlinear numerical model was developed to determine the coupled multiphysicses of gas competitive adsorption, gas counter-diffusion, gas flow, coal deformation induced by coal-gas interactions regarding CO2 geological sequestration. The new numerical model combines new coupled deformation submodels of coal, new coal porosity and permeability submodels under the condition of variable total stress, new gas diffusion and convection submodels of CO2 and CH4 in coal.(5) The established coupled nonlinear numerical model was solved by using COMSOL Multiphysics. Simulation model was verified by the state-of-art experimental data. Based on this FE simulator, coupled binary gas-coal interactions during CO2 sequestration and ECBM with different coal nature and gas injection conditions were quantitively investigated. Numerical results indicate that CH4 is swept by the injected CO2. Competing influences between the pore pressure and the CH4-CO2 counter-diffusion induced volume change play a controlling role on the evolutions of pore pressure and coal permeability. Initially, in lower pressure period, the coal permeability keeps decreasing due to the coal swelling. With the injection continuing, it rebounds when the pore pressure dominates the coal deformation. Increasing injective pressure, initial permeability or decreasing elastic modulus of coal results in lower CO2 injectivity.(6) COMSOL FE simulator was extended to simulate the CO2 injection performance in Qinshui Basin field under in-situ size and conditions, to address in-situ spatial-temporal evolutions of pore pressure, binary gas composition ratios and coal permeability. Simulation results suggest that about 1.75×104t CO2 can be sequestrated in 300×300 m2 area of Qinshui Basin within 10 years. During this period, coalbed methane recovery can be promoted by 1.44 times.