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Abstract

The ideas of open and closed-boundary systems are explored to investigate the technical limits of storage capacity of subsurface porous media, as required for the CO₂-mitigation technology of carbon capture and storage (CCS). In particular, the effects of reservoir characteristics as well as the nature and mix of fluids that originally saturate the porous media are elucidated. An improved analytic method, premised on the concept of in-situ fluid displacement and replacement, of estimating the storage capacity of open systems is proposed and evaluated.

For closed systems, our analyses show that the general order of attractiveness (technical storage capacity) of potential sites is aquifer < water-depleted oil < undepleted oil < gas-depleted oil < depleted gas < undepleted gas formations. However, owing to the complexity of microscopic and macroscopic events, similar simple conclusion can not be drawn on the relative attractiveness of various open-boundary sinks.

As a case study, we examine the applicability of CCS in Nigeria. From first-order estimates of the pore volumes and other reservoir-fluid properties in Nigeria, we quantify the limits and time-scale of “available” geologic sinks to accommodate current and future loads of anthropogenic CO₂ emissions from the Nigerian fossil-fired power sources.

Due to the relatively poor storage capacity of the potential sites compared to the anticipated CO₂ load from power plants, which is just one of the CO₂ sources, sequestration into underground formations may not, on its own, be a sustainable technical solution for Nigeria. Additionally, for the open systems, which define the upper limits of storage capacity, the potential challenges of managing displaced formation brine (and other less valuable fluids) may be overwhelming. As a complement, we offer some potential outlets for CO₂ generated from these captive and other sources. Finally, long-term carbon-mitigation strategies hinged on mixed solutions are enumerated.