基于有限元数值模拟的地质封存CO2运移行为预测方法

A Prediction Method for CO2 Migration Behavior in Geological Storage based on Finite Element Numerical Simulation

摘要:

针对注入咸水层CO2运移特征预测的工程需求，基于COMSOL Multiphysics平台建立了矿藏尺度的咸水层CO2地质封存有限元数值模型，探讨了地层孔隙度、温度及CO2注入流量等因素对咸水层中CO2运移行为的影响规律，开发了可移植使用的仿真软件，最终形成了一种基于有限元数值模拟的地质封存CO2运移行为预测方法。以中国神华集团咸水层二氧化碳捕集与地质封存项目中的石千峰地层为例，开展了有限元数值建模、CO2运移行为影响因素和影响规律研究。研究结果表明:(1) 在CO2注入阶段的前期，CO2沿着近似水平方向运移并在注入井周围形成高压区域；随着CO2的持续注入，CO2在咸水层中的分布区域逐渐形成上宽下窄的“舌状”，咸水层压力有所降低；在CO2停注后，咸水层的压力逐渐降低至原地层压力水平，出现咸水反向驱替CO2的现象；(2) 随着地层孔隙度的减小，CO2注入咸水层后地层压力呈指数增长趋势，孔隙度的改变影响CO2的运移形态；在CO2停注后，随着咸水层孔隙度的增大，咸水的反向驱替现象愈加明显，咸水层顶部的CO2堆积量增加；(3) 地层温度的升高增强了咸水的反向驱替作用，CO2运移时“舌状”区域的边界倾角随之增大，咸水层顶部的CO2侧向分布范围逐渐扩大；(4) CO2的扩散范围随注入流量的增加而扩大；注入流量相同的条件下，CO2的扩散速率随时间的推移而降低；在CO2停注后，注入流量越低反向驱替现象越明显；(5) 所开发的仿真软件具有数值模型建立、仿真数据分析等功能模块，测试结果表明了软件的可用性。研究成果可为CO2地质封存点的选址、注入条件优化、泄漏监测提供理论和模型基础。

Abstract:

To meet the engineering requirements for predicting the migration characteristics of CO2 injected into saline layers, a finite element numerical model for geological storage of CO2 in saline layers was established based on the COMSOL Multiphysics platform. The influences of factors such as formation porosity, temperature, and CO2 injection flow rate on CO2 migration behavior in saline layers were explored, and a portable simulation software was developed. Finally, a finite element numerical simulation based method for predicting CO2 migration behavior in geological storage was developed. Taking the Shiqianfeng Formation of China Shenhua Group"s Saline Layer Carbon Dioxide Capture and Geological Storage Project as an example, the finite element numerical modeling, influencing factors and laws of CO2 migration behavior were studied. It has been shown that: (1) in the early stage of CO2 injection, CO2 migrates approximately horizontally and forms a high-pressure area around the injection well; with the continuous injection of CO2, the distribution area of CO2 in the saline layer gradually forms a tongue shape with a wide top and a narrow bottom, and the pressure in the saline layer decreases to some extent; after the injection of CO2 is stopped, the pressure of the saline water layer gradually decreases to the original formation pressure level, leading to the phenomenon of reverse displacement of CO2 by saline water; (2) as the porosity of the formation decreases, the formation pressure increases exponentially after CO2 is injected into the saline water layer, and the change in porosity affects the migration pattern of CO2; after the injection of CO2 is stopped, as the porosity of the saline layer increases, the reverse displacement phenomenon of saline water becomes more obvious, and the accumulation of CO2 at the top of the saline layer increases; (3) the increase in formation temperature enhances the reverse displacement effect of saline water, and the boundary angle of the tongue-shaped area increases during CO2 migration; the lateral distribution range of CO2 at the top of the saline layer gradually expands; (4) the diffusion range of CO2 expands with the increase of the injection flow rate; under the same injection flow rate, the diffusion rate of CO2 decreases over time; after the injection of CO2 is stopped, the lower the injection flow rate, the more obvious the reverse displacement phenomenon; (5) the simulation software developed in this work has functional modules such as numerical model establishment and simulation data analysis, and the test results indicate the usability of the software. The research results can provide a theoretical and model basis for the selection of CO2 geological storage sites, optimization of injection conditions, and leakage monitoring.