Short communicationAnalytical solution for estimating storage efficiency of geologic sequestration of CO2
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: , the residual brine saturation following displacement of brine by CO2; , the ratio of CO2 mobility to brine mobility; and , 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, , was derived. The derivation included some significant assumptions and simplifications, which will result in some uncertainty in estimates of ; however, calculated values of 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
References (32)
Sequestration of CO2 in geological media: criteria and approach for site selection in response to climate change
Energy Conversion and Management
(2000)- et al.
Sequestration of CO2 in geological media in response to climate change: capacity of deep saline aquifers to sequester CO2 in solution
Energy Conversion and Management
(2003) - et al.
CO2 storage capacity estimation: methodology and gaps
International Journal of Greenhouse Gas Control
(2007) - et al.
CO2 storage capacity estimation: issues and developments of standards
International Journal of Greenhouse Gas Control
(2007) - et al.
Subterranean containment and long-term storage of carbon dioxide in unused aquifers and in depleted natural-gas reservoirs
Energy Conversion and Management
(1992) - et al.
Investigations on CO2 storage capacity in saline aquifers. Part 1. Dimensional analysis of flow processes and reservoir characteristics
International Journal of Greenhouse Gas Control
(2009) - et al.
Investigations on CO2 storage capacity in saline aquifers. Part 2. Estimation of storage capacity coefficients
International Journal of Greenhouse Gas Control
(2009) - et al.
ECO2N – A fluid property module for the TOUGH2 code for studies of CO2 storage in saline aquifers
Energy Conversion and Management
(2007) - et al.
Code intercomparison builds confidence in numerical simulation models for geologic disposal of CO2
Energy
(2004) - et al.
Behavior of supercritical CO2 injected into porous media containing water
Energy
(2005)
CO2 storage efficiency of aquifers
Energy Conversion and Management
TOUGHREACT—a simulation program for non-isothermal multiphase reactive geochemical transport in variably saturated geologic media: Applications to geothermal injectivity and CO2 geological sequestration
Computers and Geosciences
Equations of state for basin geofluids: algorithm review and intercomparison for brines
Geofluids
Screening and ranking of sedimentary basins for sequestration of CO2 in geological media in response to climate change
Environmental Geology
Effects of in-situ conditions on relative permeability characteristics of CO2-brine systems
Environmental Geology
Safe storage of CO2 in deep saline aquifers
Environmental Science & Technology
Cited by (73)
CO<inf>2</inf>-brine interface evolution and corresponding overpressure in high-gravity CO<inf>2</inf> storage complex
2024, Geoenergy Science and EngineeringCoupled wellbore-reservoir modelling to evaluate CO<inf>2</inf> injection rate distribution over thick multilayer storage zones
2023, International Journal of Greenhouse Gas ControlReservoir simulation of the CO<inf>2</inf> storage potential for the depositional environments of West Siberia
2023, Gas Science and EngineeringThe effect of temperature on CO<inf>2</inf> injectivity in sandstone reservoirs
2022, Scientific AfricanCitation Excerpt :Injected CO2 may be stored in depleted oil and gas reservoirs, deep saline reservoirs, unmineable coal seams or injected into active oil and gas reservoirs for Enhanced Oil Recovery (EOR) [4,5]. In terms of storage space, deep saline aquifers offer the most promising storage capacity [6–8]. A large volumetric storage potential, high well injectivity and reliable containment is needed to inject the amount of CO2 required to reduce global emissions by appreciable margins [9].
CO<inf>2</inf> geological storage: Critical insights on plume dynamics and storage efficiency during long-term injection and post-injection periods
2020, Journal of Natural Gas Science and Engineering