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Analytical solution for estimating storage efficiency of geologic sequestration of CO2

https://doi.org/10.1016/j.ijggc.2009.11.002Get rights and content

Abstract

During injection of carbon dioxide (CO2) into deep saline aquifers, the available pore volume of the aquifer may be used inefficiently, thereby decreasing the effective capacity of the repository for CO2 storage. Storage efficiency is the fraction of the available pore space that is utilized for CO2 storage, or, in other words, it is the ratio between the volume of stored CO2 and the maximum available pore volume. In this note, we derive and present simple analytical expressions for estimating CO2 storage efficiency under the scenario of a constant-rate injection of CO2 into a confined, homogeneous, isotropic, saline aquifer. The expressions for storage efficiency are derived from models developed previously by other researchers describing the shape of the CO2-brine interface. The storage efficiency of CO2 is found to depend on three dimensionless groups, namely: (1) the residual saturation of brine after displacement by CO2; (2) the ratio of CO2 mobility to brine mobility; (3) a dimensionless group (which we call a “gravity factor”) that quantifies the importance of CO2 buoyancy relative to CO2 injection rate. In the particular case of negligible residual brine saturation and negligible buoyancy effects, the storage efficiency is approximately equal to the ratio of the CO2 viscosity to the brine viscosity. Storage efficiency decreases as the gravity factor increases, because the buoyancy of the CO2 causes it to occupy a thin layer at the top of the confined formation, while leaving the lower part of the aquifer under-utilized. Estimates of storage efficiency from our simple analytical expressions are in reasonable agreement with values calculated from simulations performed with more complicated multi-phase-flow simulation software. Therefore, we suggest that the analytical expressions presented herein could be used as a simple and rapid tool to screen the technical or economic feasibility of a proposed CO2 injection scenario.

Introduction

Storage of carbon dioxide (CO2) in deep saline aquifers has been proposed as a method of reducing atmospheric emissions of CO2 and thereby mitigating global climate change (Koide et al., 1992, Bachu, 2000, Holloway, 2001, Bruant et al., 2002, Pruess and Garcia, 2002, Bachu and Adams, 2003, White et al., 2003, IPCC, 2005). Among the challenges associated with this proposed technology is estimating the capacity of a candidate repository for CO2 storage (van der Meer, 1995, Kopp et al., 2009b). Bradshaw et al. (2007) and Bachu et al. (2007) have pointed out both the importance of reliable estimates of storage capacity, and the challenges associated with obtaining those estimates. For one thing, because supercritical CO2 is both less viscous and less dense than the brine found in saline aquifers, the injected CO2 does not displace resident brine in a “piston” or plug-flow fashion. Instead, the CO2 tends to ride over the brine as it is injected, forming a layer of CO2 at the top of the confined formation (Nordbotten et al., 2005). Thus, even if the overall pore volume of a confined aquifer can be estimated accurately, the fraction of that volume that is available for CO2 storage is not likely to be known a priori (Bachu et al., 2007).

Storage efficiency can be defined as the ratio between the amount of CO2 stored in an aquifer and the maximum amount of CO2 that could theoretically be stored in the same aquifer volume (van der Meer, 1995). Previous estimates of CO2 storage efficiency have often been based on numerical simulations (van der Meer, 1995, Obi and Blunt, 2006, Kopp et al., 2009b) that can be time-consuming or costly to perform. At present, a simple analytical method for estimating CO2 storage efficiency is lacking. Therefore, the main objective of this note is to develop a simple analytical equation for estimating storage efficiency during CO2 injection. The rationale for this study is that a fast and easy method of estimating CO2 storage efficiency may facilitate the prediction of the total amount of CO2 a given repository can sequester, and/or may indicate if more detailed numerical modeling or geologic investigation is warranted. The analysis given herein is limited to “early” injection times, i.e., when the primary trapping mechanisms for CO2 are stratigraphic and structural trapping, before the onset of significant CO2 dissolution into the resident brine (IPCC, 2005, Bachu et al., 2007).

Section snippets

Conceptual model

In this note, we consider the injection of CO2 at a constant injection rate into a confined, homogeneous, and isotropic saline aquifer via a single vertical well. Fig. 1 is a cartoon illustrating such an injection scheme.

In developing an expression for the storage efficiency, we also make the following assumptions or simplifications.

  • 1.

    The porous medium is inert, non-deformable, and initially saturated with brine.

  • 2.

    The radial extent of the confined aquifer is very large compared to its thickness.

  • 3.

Results

It can be seen from Eq. (8) that the storage efficiency depends on three dimensionless groups: Sr, the residual brine saturation following displacement of brine by CO2; λ, the ratio of CO2 mobility to brine mobility; and Γ, a dimensionless group that quantifies the importance of CO2 buoyancy relative to flow rate. In this section, we investigate further the dependence of storage efficiency on each of these variables. Previously, Kopp et al. (2009a) had predicted that storage capacity should

Discussion

A number of potentially important factors are not included in this analysis. For instance: (1) As CO2 is injected into the aquifer, the pressure in the formation will increase, which will in turn cause the densities and viscosities of the fluids to change. Both density and viscosity of supercritical CO2 are relatively strong functions of pressure near 45 °C and 120 bar. However, the analysis presented above assumes that fluid properties are constant, i.e., do not change during the injection. (2)

Summary and conclusions

The objective of this note is to develop a simple analytical equation for estimating storage efficiency during CO2 injection. Based on analytical models for CO2 plume shape developed previously by other researchers (Nordbotten et al., 2005, Nordbotten and Celia, 2006), a simple equation for the storage efficiency, ϵ, was derived. The derivation included some significant assumptions and simplifications, which will result in some uncertainty in estimates of ϵ; however, calculated values of ϵ are

Acknowledgements

This material is based on work supported by the Florida Energy Systems Consortium (FESC). Also, financial support has been awarded to Roland Okwen by the Alfred P Sloan Foundation via the National Action Council for Minorities in Engineering (NACME), and by a Diverse Student Success Fellowship at the University of South Florida (USF). Any opinions, findings, conclusions, or recommendations are those of the authors and do not necessarily reflect the views of FESC, NACME, USF, or the Alfred P

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