动态超临界CO2流体对水平衡煤体CO2封存性能的作用规律及机理

CO2 sequestration in deep coal seams can not only store the main anthropogenic greenhouse gas (CO2) but also enhance coal-bed methane (CH4) recovery. Due to their special characteristics of both CO2 and moisture-equilibrated coals under the optimum reservoir conditions for CO2 sequestration, although the physisorption effect between CO2 and moisture-equilibrated coals is widely acknowledged, other potential interactions mainly including extraction effect between supercritical CO2 fluid and small organic compounds dissolved in coal matrix, chemical dissolution effect between CO2 fluid and inorganic compounds of moisture-equilibrated coals, and chemisorption effect between CO2 fluid and organic structure of coals are still ambiguous. The aforementioned complex interactions can change physico-chemical characteristics of coals mainly including pore morphology, aromaticity, alkyl carbons and oxygen-containing functional groups, and further show influences on CO2 sequestration capacity of target coal seams. Thus, investigations are performed to study the effect and mechanism of dynamic supercritical CO2 fluid on CO2 sequestration performance of moisture-equilibrated coals. This research work consists of three parts: the effect of dynamic supercritical CO2 fluid on physico-chemical characteristics of moisture-equilibrated coals, the mechanism of CO2 fluid dependence of physico-chemical characteristics of moisture-equilibrated coal, and the influence of physico-chemical characteristics alterations of moisture-equilibrated coals on CO2 sequestration capacity. The implementation of the above work will contribute to improve the research framework of CO2 sequestration in deep coal seams.

深部煤层封存CO2技术既能存储主要人为温室气体（CO2），又能提高煤层气（CH4）采收率。基于适宜CO2封存的储层条件下的CO2特性和水平衡煤体特征，CO2与水平衡煤体之间除了已明确的物理吸附作用以外，还涉及尚未明确的超临界CO2流体对煤基质内部小分子有机物的萃取作用、CO2流体对水平衡煤体中无机矿物的化学溶解作用和CO2流体与煤有机结构间的化学吸附作用。上述CO2流体和煤体间的复杂作用会改变煤体理化性质（主要指孔隙结构、芳香性、脂肪性和含氧官能团），进而影响煤体CO2封存性能。因此，本项目重点研究封存过程中动态超临界CO2流体对水平衡煤体CO2封存性能的作用规律及机理。研究内容包括：①动态超临界CO2流体对水平衡煤体理化性质的作用；②动态超临界CO2流体改变水平衡煤体理化性质的原因；③水平衡煤体理化性质变化对后续CO2封存性能的影响。实施本项目有助于完善深部煤层封存CO2技术的研究体系。

深部煤层封存CO2技术既能减排主要人为温室气体（CO2），又能提高煤层气（CH4）采收率。基于储层中CO2流体特性和煤体特征，二者间涉及尚未探明的超临界CO2流体（scCO2）对煤基质小分子有机物的萃取作用、对水平衡煤体中无机矿物溶解作用、与煤有机结构间的化学吸附作用和诱导煤基质溶胀作用。上述复杂作用关系会改变煤体关键理化性质（孔隙结构、芳香性、脂肪性和含氧官能团），进而影响其CO2封存性能。因此，本项目重点研究封存过程中动态scCO2对水平衡煤体CO2封存性能的作用规律及机理。研究内容包括：（1）scCO2对水平衡煤体理化性质的作用规律；（2）scCO2对水平衡煤体理化性质的作用机理；（3）scCO2对水平衡煤体CO2封存性能的影响机制。研究结果包括：（1）scCO2-H2O作用能够改变煤岩孔隙结构及表面化学性质。其减少了低变质煤岩微孔数量，增多了中变质煤岩微孔数量，但不会显著影响高变质煤岩微孔结构；同时，scCO2-H2O作用减少了所有煤岩介孔数量。此外，scCO2-H2O作用减少了煤岩表面主要官能团（C-C/C-H、C-O、C=O和COOH）数量。（2）煤岩理化性质的改变主要归因于：① scCO2-H2O对煤基质中正构烷烃（C10-C31）、环烷烃和芳烃（芳环数介于1-3）的萃取作用；② scCO2-H2O对煤岩中高岭石、方解石、勃姆石和石英的溶解作用；③ scCO2-H2O与不同变质煤岩模型替代物（氧化石墨）表面含氧基团的化学吸附作用；④ scCO2-H2O与氧化石墨结构间插层效应诱导的片层溶胀作用。（3）scCO2-H2O作用能够改变煤岩的CO2吸附静力学性能，即：低变质煤岩CO2吸附容量升高，高变质煤岩CO2吸附容量降低。此外，scCO2-H2O作用后所有煤岩CO2吸附/解吸滞后效应增强，有利于提高煤层长期封存CO2的稳定性。综上，本项目研究成果既有助于丰富煤层封存CO2技术的理论体系，又有助于为评价煤层封存CO2有效性、长期性和安全性提供研究依据。