Optimization method of water-quality observation positions in offshore CCS sites using the adjoint marginal sensitivity method

Carbon dioxide (CO2) Capture and Storge (CCS) is recognized as a promising method to mitigate the global warming. In Japan, sub-seabed CO2 storage is considered to be commercialized in a decade. However, offshore CO2 storage has a risk of leakage. Since CO2 seepage into seawater may affect the marine environment, CO2 concentration must be monitored in CO2 storage sea sites under a Japanese law. However, there is no scientifically logical base to determine observation positions. In this study, we therefore developed a method of determining optimal observation positions in CO2 storage sea sites. First, we defined optimal observation positions as those that can estimate seepage positions with high accuracy with the minimal number using the adjoint marginal sensitivity method, even if the seepage positions locate anywhere in a target sea site. Second, non-dimensional parameters were proposed to determine whether a datum at a candidate observation position can be successfully used to estimate a possible seepage position using the adjoint method. Then, we defined a coverage index that indicates how much of a target sea site is “covered” in total by observation points and proposed an algorithm to optimally place observation positions. Finally, we validated the developed method numerically and indicated that, to perform highly accurate estimation, it is desirable that a seepage position be covered by at least three observation points.