深部塩水層の二酸化炭素地中貯留量評価における閉鎖・開放境界条件の影響について

抄録

The volumetric method is the basis of carbon dioxide（CO₂）storage capacity estimation in many organizations implicitly assuming the storage formation is an open system. The compressibility method is another popular method among engineers assuming a closed system. The impacts of boundary conditions on CO₂ injection performance and storage capacity were investigated by literature survey and numerical simulations. The key conclusions of the investigation are summarized as follows: 1）a closed system has a small storage efficiency due to the constraints of pressure build-up: typically in the order of 0.5% or less of effective pore space, 2）a semi-closed system with finite seal permeability can be either effectively a closed system or an open system depending on seal permeability: 10^－5 md or less for a closed system and 10^－2 md or larger for an open system in the assumed case, 3）CO₂ flow from the storage formation to seal formations can be limited due to the difference in capillary pressure between the storage and seal formations even if the seal permeability is high enough to allow sufficient crossflow of formation water for pressure relaxation, 4）permeability cut-off criteria for the compressibility method can be smaller than that for the volumetric method because the rocks with an certain range of permeability do not allow CO₂ inclusion but can contribute pressure relaxation, 5）long-term fall-off data can include useful information to estimate the boundary conditions: e.g. the shut- in pressure recovering to the initial hydrostatic pressure together with a decline in pressure derivative in late time may indicate communication between the storage formation and overlaying and/or underlaying large aquifers.

The study finally highlights the importance of representing lateral and vertical boundary conditions appropriately and recommends modeling the whole reservoir complex consisting of not only the target storage formation, but also overlining/underling seal formations and aquifers for CO₂ storage simulations.