【作者】 周来；

【导师】 冯启言； 刘继山；

【作者基本信息】 中国矿业大学 ， 环境科学， 2009， 博士

【摘要】 二氧化碳是温室效应最主要的贡献者,利用深部不可开采煤层封存二氧化碳可同时实现CO2大规模减排和增加煤层气资源产量(CO2-ECBM),具有巨大的发展潜力。CO2注入煤层封存会引起CO2-CH4二元气体与煤体之间的多物理耦合作用,对煤储层的孔隙性和渗透性以及气体的流动产生重要影响,是深部不可采煤层封存CO2的瓶颈问题。本文基于表面化学、吸附热力学、渗流力学、有限元数值分析等理论和方法,对深部煤层封存CO2中的多物理过程耦合机制开展了较系统的实验与模拟研究,核心内容如下:(1)利用沁水盆地晋城一组无烟煤煤样进行了跨CO2临界温度(31.4℃)条件下煤对CO2吸附特性的实验及二元气体吸附/解吸实验。跨临界温度吸附时,CO2在煤中的吸附量与温度变化不呈单调递减关系,反映了煤物理吸附和CO2相变综合作用的结果。二元气体吸附和解吸时,解吸曲线表现出“滞留”现象,说明了CO2在吸附造成煤微孔闭合扣留残余气体的机制。分割二元气体的游离相和吸附相组分比例关系,揭示了CO2-CH4二元气体解吸时CH4优先解吸,解吸速度由快转慢;而吸附时,CO2会优先吸附,吸附速度由快转慢,上述结论是煤层中二元气体竞争吸附和CO2封存的重要依据。(2)分别运用Langmuir模型、BET模型、DA模型和DR模型对煤样吸附CO2和CH4的曲线进行了拟合,分析了CO2等温吸附实验中体积误差对实验结果的影响,运用体积修正项对传统的吸附量计算模型进行了修正,并应用修正的DA和Langmuir模型进行了重新拟合,结果表明修正模型的拟合精度大幅提高,同时,体积修正项也定量描述了二氧化碳吸附同时引起的煤体膨胀变形。(3)从煤的吸附热力学角度,利用煤表面自由能和等量吸附热评价了不同温压条件下CO2在煤层中优先吸附性以及CO2与CH4竞争吸附机制。计算了25~40℃温度下煤对CO2和CH4的吸附势,建立了煤吸附CO2和CH4的特征曲线,得到了CO2吸附量、温度、压力三者之间的关系式。(4)建立了一套包含气体竞争吸附、竞争扩散、气体渗流以及煤基块变形的多物理耦合过程的高度非线性数学模型,包含煤层耦合变形方程、适用于变应力边界的新的孔隙率和渗透率方程、二氧化碳和甲烷气体的对流扩散耦合方程以及上述三者的耦合方程。(5)应用COMSOL Multiphysics有限元数值分析系统求解了多物理耦合非线性数学模型,运用最新实验数据验证了模拟结果的正确性,基于COMSOL有限元数值模拟平台,研究了不同储层物性和注气条件下,CO2驱替煤层气与封存过程中的二元气固耦合作用。得出:将CO2注入煤层后,CO2不断驱替CH4;煤层的孔隙压力和渗透率受煤层吸附膨胀变形与孔隙压力变形耦合作用影响,在低压条件下吸附膨胀变形占主导控制地位,而在高压条件下吸附膨胀变形被气体压力变形抵消,后者控制煤层的渗透率;注气压力越大、煤层初始渗透率越大,以及煤弹性模量越小,二氧化碳的可注性越高。(6)基于COMSOL数值平台,对我国沁水盆地CO2驱采煤层气及地质处置中的煤层孔隙压力、二元气体组分以及煤层渗透率的演化特征进行了模拟研究,计算出在沁水盆地300×300m2场地中注气10年后CO2封存量为1.75×104t,注入CO2后可提高CH4产量达1.44倍。更多还原

【Abstract】 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.更多还原