QICS Paper: Numerical study of the fate of CO2 purposefully injected into the sediment and seeping from seafloor in Ardmucknish Bay

To quantify the impact of leaked CO2 purposefully stored in subsea geological formations on the marine ecosystem, CO2 gas was injected into sandy sediments in a small bay in Scotland in 2012. Alongside the experiment, a numerical study was conducted to predict CO2 fate in the bay. CO2 may take the form both of the gas and dissolved phases when it seeps out from the seafloor. The bubble CO2 rises in the water column forming bubble plumes and dissolves into the seawater during its ascent. Measurements indicated that approximately 8–15% of the injected CO2 escaped the sediments in the gas phase and no empirical evidence was seen for fluxes in the dissolved phase. Therefore, it is thought that 85–92% of the CO2 remained within the sediments. However, the results of our numerical study suggest that 10–40% of the injected CO2 stayed in the sediment. Apart from unexpected errors in the present numerical simulation, a possible explanation for this discrepancy may be the heterogeneous nature of the sediment and observations limited in time and space. It is also recognised that the CO2 concentration away from the injection site is undetectably small and that the readily detectable signal is confined to a small area in the vicinity of the injection point. This is a publication in QICS Special Issue - International Journal of Greenhouse Gas Control, Chiaki Mori et. al. Doi:10.1016/j.ijggc.2014.11.023.为量化有意封存于海底地质构造中的二氧化碳发生泄漏后对海洋生态系统的影响，研究团队于2012年在苏格兰一处小型海湾的砂质沉积物内注入了二氧化碳气体。同步开展配套数值模拟研究，以预测该海湾内二氧化碳的运移与归宿。二氧化碳从海底渗出时，可呈现气相与溶解相两种形态。气相二氧化碳在水柱中上升形成气泡羽流，并在上升过程中溶解于海水。监测数据显示，约8%至15%的注入二氧化碳以气相形式从沉积物中逸出，未观测到溶解相通量的实证依据。因此推测，85%至92%的二氧化碳留存于沉积物内部。但本次数值模拟的结果却表明，10%至40%的注入二氧化碳滞留在沉积物中。除当前数值模拟存在未预见的误差外，造成这一差异的潜在原因还包括沉积物的非均质性，以及观测在时间与空间尺度上的局限性。同时研究亦指出，注入点以外区域的二氧化碳浓度极低，无法被检测到，仅在注入点附近的小范围区域内存在可被轻易检出的信号。本研究成果发表于《国际温室气体控制期刊》（International Journal of Greenhouse Gas Control）QICS特刊，作者为Chiaki Mori等人，DOI:10.1016/j.ijggc.2014.11.023。