海底沉积层内CO_(2)水合物长期储存的地质风险评估

Geological Risk Assessment of Long⁃term Storage of Carbon Dioxide Hydrates in Seafloor Sedimentary Layers

新一代南海可燃冰开采、固碳和地质修复三联技术采用CO_(2)水合物置换开采法,将CO_(2)注入开采完后的天然气水合物储层,并在低温高压条件下以水合物形态进行长期地质封存。此方法既可实现能源开采,又能有效缓解温室效应,但目前对于海底沉积层内CO_(2)水合物长期储存的海底边坡稳定性问题研究较少。基于流固耦合方法和强度折减法,模拟得到海底边坡在不同位置、不同时期的应力、应变、位移变化情况,分析边坡在水合物全分解过程中的稳定性。依据南海可燃冰储藏区域的地形条件和土体参数,设计不同真实环境下的工况,探究各项参数的改变对边坡安全系数的影响。研究结果表明,将CO_(2)水合物分解度控制在安全范围内,即可施行长期海底碳封存。在水合物分解程度到一定程度后,斜坡的塑性应变值开始急剧增加,并逐渐形成了向上连通的塑性贯通区,此刻斜坡开始发生破坏。地形条件对斜坡稳定性的改变大于土体参数,影响程度排序为斜坡坡角>水合物层厚度>水合物层埋深,摩擦角>黏聚力>弹性模量,其中关键因素为坡角和摩擦角。

The latest generation technique of natural gas hydrate extraction,carbon sequestration,and geological remediation in the South China Sea involves the CO_(2) replacement mining method.This method injects CO_(2) into the extracted natural gas hydrate reservoir,enabling long-term geological storage in the form of hydrates under low temperature and high pressure conditions.This method can not only realize energy extraction but also effectively mitigate the greenhouse emissions.However,there has been limited research on the stability of seafloor slopes for long-term storage of CO2 hydrates in the sedimentary layers of the seafloor.Using the fluid-structure coupling method and strength reduc-tion method as the basis,the stress,strain and displacement changes of the seafloor slope are simulat-ed at various locations and during different periods.The analysis focuses on the stability of the slope during the complete decomposition of hydrates.Based on the topographic conditions and soil parame-ters,and considering the area for natural gas hydrate storage in the South China Sea,the operational conditions in various real environments were designed.The study aimed to explore the impact of changing different parameters on the safety factor.The results indicate that long-term seafloor carbon capture and storage can be achieved by managing the degree of CO_(2) hydrate decomposition within a safe range.Once the degree of hydrate decomposition reaches a certain level,plastic deformation be-gins to increase sharply,gradually forming a connected zone of plastic penetration extending upward,signaling the onset of slope failure.Changes in topographic conditions had a greater impact on slope stability compared to soil parameters,with the degree of influence ranked as follows:slope angle>hy-drate layer thickness>hydrate layer burial depth,friction angle>cohesion>elastic modulus,among which the key factors were slope angle and friction angle.The results can provide a technical reference for the long-term geological storage of seabed sediments of CO_(2) hydrates.