水对岩石孔隙毛细CO2的非平衡溶解过程

H2O and CO2 are two important geological fluids, they are active in many geological processes. The reaction between them in rock pores is of high geological and environmental importance. The dissolution process of CO2 trapped in rock capillary pores into ground water controls the migration of CO2 in geological formations. Thus, the process imposes influences on the subsequent geochemical processes, in which H2O and CO2 are involved. This dissolution process has often been assumed to be a “sufficiently fast reaction”. Under this assumption, the dissolution rate is determined by the solubility of CO2 in the saline; the equilibrium equation between the free supercritical phase and the dissolved CO2 is the key equation; and, the process is equilibrium dissolution. Nevertheless, the preliminary experiments of this study showed that the dissolution process was much lengthy than the calculated by the solubility of CO2 in the saline under the in situ pressure and temperature; the saline was often not saturated with CO2; the state of equilibrium between the free supercritical CO2 phase and the dissolved CO2 was rarely reached; and, the process was non-equilibrium dissolution. This study presents the concept of non-equilibrium dissolution, investigates the mechanisms and effecting factors in the dissolution process, and suggests mathematical equations to describe the rate of dissolution. This will provides a better understanding about CO2 migration in geological formations.

H2O和CO2是地层岩石中的两种重要流体，许多地质过程都是在二者共同作用下发生，研究二者在岩石中的相互反应具有重要的资源与环境意义。岩石孔隙毛细CO2转化成水中溶解CO2的过程，对CO2在地层岩石中的扩散迁移进而参与后续地球化学反应具有重要影响。目前对毛细CO2转化成水中溶解CO2的过程的处理一般过于简化，将其作为“充分快过程”进而使用局域平衡假定。假设CO2溶入水的速度主要由CO2在水中的溶解度决定,地层温度压力下水中的CO2与毛细孔隙中超临界CO2的平衡方程控制毛细CO2的溶解过程，为平衡溶解。但本项目的前期研究发现：此过程比按平衡溶解计算的慢的多，水中CO2经常达不到饱和状态，毛细CO2与水中CO2达不到平衡，发生的溶解为非平衡溶解。本项目模拟非平衡溶解过程，分析其发生机制和控制因素，建立描述此过程的普适性函数，将改善地层中CO2赋存和迁移的认识，具有重要的理论与实际意义。

H2O和CO2是地层岩石中的两种重要流体，许多地质过程都在二者共同作用下发生，研究二者在岩石中的相互作用为水-碳的地质循环及二氧化碳捕集和地质封存等碳中和、碳达峰课题提供理论基础。在深层咸水含水层CO2地质封存问题中，毛细作用下以超临界相态存在于毛细空隙中的CO2和溶解于水中的CO2是封存CO2在含水层中存在的两种主要形式，二者的相互转换过程对CO2在地层中的扩散迁移以及封存泄露风险具有控制作用。这一转换过程受控于岩石矿物成分、空隙大小分布、水的化学成分、地层压力温度等众多因素。本项目通过室内试验观测岩石微观结构，结合分离压理论计算了固液界面对流体作用力，确定了这种作用力对毛细力形成、吸附量及吸附速度、多项流体不同组分的存在形态、以及不同组分的扩散迁移的控制作用。确定了毛细束缚CO2与溶解CO2相互转换的控制机理，定量描述了二者之间的转化速度。本项目的试验及理论成果有利于改善对地层中水-二氧化碳等多项流体的赋存和迁移的认识，具有重要的理论与实际意义。本研究所发展的理论正逐渐完善，正着手应用推广工作，其成果正陆续总结发表。