CO2 Fate Comparison for Depleted Gas Field and Dipping Saline Aquifer

Abstract

An important metric for CO2 storage is the CO2 fate plot, which displays the partitioning of mobile/capillary trapped/dissolved/mineralised CO2 as a function of time. The archetype version of this plot (Fig. 1, from the IPCC 2005 report [1]) is idealised. While a simplification is a good way to illustrate physical mechanisms, the problem with this plot is that due to its popularity it is now easily misinterpreted to be a universal plot. However, for actual case studies the curves behave quantitatively and sometimes qualitatively differently from the archetype plot. The reason for this is that reservoir settings are different from case to case, having a profound impact on the characteristic times, shapes, and magnitudes of the CO2 fate curves. The objective of this work is to generate a realistic range of CO2 fate plots, derived from reservoir simulations on simple geometries but with realistic input parameters, expressing the impact of key control parameters. This new range of plots can be used for communication purposes to a wider audience, and also as a Quality Control tool for future modelling studies. Our results bring out major differences between a depleted gas field setting (more generally: structurally closed setting) and a dipping saline aquifer setting (more generally: structurally open setting). In a structurally closed setting the CO2 dissolution and/or mineralisation takes orders of magnitude longer than in a structurally open setting. This result is quite robust under variation of other parameters such as vertical to horizontal permeability, reservoir thickness, reservoir dip angle, relative permeabilities and geochemical variations. Moreover, only a small subset of structurally open cases, and none of the structurally closed cases, leads to CO2 fate plots that are similar to the one in the IPCC 2005 report. Therefore the CO2 fate plot from the IPCC 2005 report should not be used for quantitative statements regarding the relative size and timing of the various CO2 fate categories; for most potential CO2 storage sites such statements are likely to be invalid. Another implication is that for a structurally closed setting the CO2 tends to be relatively static: slow transitions between CO2 fate categories, and (due to structural closure) limited lateral migration of mobile gas. For a structurally open setting, however, the CO2 fate (spatial distribution and trapping category) is much more likely to vary significantly over time due to relatively quick CO2 plume migration laterally and relatively quick conversion of CO2 into the less mobile (capillary trapped/dissolved/mineralised) CO2 fate categories. For structurally open cases, the balance between, on the one hand, the lateral CO2 plume migration speed and, on the other hand, the capillary trapping/dissolution/mineralisation rate, determines the spatial distribution of the mobile CO2 in relation to potential leak paths. Therefore for CCS project screening/design in structurally open cases it is more critical to investigate this balance than for CCS in structurally closed reservoirs.