Summary

Carbon dioxide capture and sequestration (CCS) has emerged as an important part of the portfolio of technologies for lowering emissions of greenhouse gases. While much of the science and technology for CCS can be borrowed from the oil and gas industry, there are outstanding issues that should be addressed to support implementation at the enormous scale that will be needed if this technology is to make a major contribution to reducing emissions. One of these important areas involves developing a better scientific understanding of fate and transport of CO2 in saline aquifers. Saline aquifers have the largest sequestration capacity and are the mostly widely distributed geographically. Studies are now going on at a variety of spatial and temporal scales, from the pore-scale to the field scale to assess the fate and transport of injected CO2. Laboratory experiments and high resolution simulations of multi-phase flow of CO2 and brine elucidate the influence of small scale heterogeneity on brine displacement efficiency and CO2 saturation distributions. Field scale pilot tests and full-scale demonstration projects elucidate the importance of buoyancy and stratigraphy on CO2 migration. Geophysical measurements-from seismic imaging to X-rays-are crucial for improving our understanding of the fate and transport of CO2. This paper focuses on an often overlooked spatial scale, that is, the influence of sub-core scale heterogeneity on CO2 transport and brine displacement efficiency. Core-scale experiments indicate that brine displacement efficiency and relative permeability can depend on the flow rate under conditions investigated in the experiments. Flow rate dependence is not typically considered in simulations of largescal sequestration projects. Implications for large scale CO2 sequestration are discussed, including storage capacity and spatial extent of the plume of injected CO2.

Introduction

Carbon dioxide capture and sequestration (CCS) in deep geological formations has emerged over the past ten years as an important component of the portfolio of options for reducing greenhouse emissions. Three commercial projects now operating provide valuable experience for assessing the efficacy of CCS. To date, these projects are performing as expected, with no evidence for leakage or other unexpected difficulties. If CCS is implemented on the scale needed for large reductions in CO2 emissions, a billion of tonnes or more of CO2 will be sequestered annually-a 250 fold increase over the amount sequestered annually today. Effectively sequestering these large volumes will require building a strong scientific foundation of the coupled hydrological-geochemical-geomechanical processes that govern the long term fate of CO2 in the subsurface. In addition, we will need methods to characterize and select sequestration sites, subsurface engineering to optimize performance and cost, safe operations, monitoring technology, remediation methods, regulatory oversight, and an institutional approach for managing long term liability.

This paper focuses on the fundamental science underpinning sequestration in saline aquifers. Saline aquifers have the largest sequestration capacity, as compared to oil and gas reservoirs or deep unminable coal beds. Saline aquifers are also more broadly distributed and thus, closer to more emission sources.

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