Large-scale impact of CO2 storage in deep saline aquifers: A sensitivity study on pressure response in stratified systems
Introduction
Geologic carbon sequestration in deep formations (e.g., saline aquifers, oil and gas reservoirs, and coalbeds) has drawn increasing consideration as a promising method to mitigate the adverse impacts of climate change (Holloway, 1996, Gale, 2004, IPCC, 2005, Hepple and Benson, 2005). Deep saline aquifers offer the largest storage potential of all the geological CO2 storage options and are widely distributed throughout the globe in all sedimentary basins. For CO2 storage to have a significant impact on atmospheric levels of greenhouse gases, the amounts of CO2 injected and sequestered underground need to be extremely large (Holloway, 2005). Various research studies have been conducted to date evaluating under which hydrogeological conditions the injected volumes of CO2 can be safely stored over hundreds or even thousands of years. For example, many of these studies address issues such as the long-term efficiency of structural trapping of CO2 under sealing layers. Less emphasis has been placed on evaluating the large-scale pressure changes caused by industrial-scale injection of CO2 into deep saline formations or understanding the fate of the native brines that are being displaced by the injected fluids (Van der Meer, 1992, Holloway, 1996, Gunter et al., 1996). Large-scale injection of CO2 will impact subsurface volumes much larger than the CO2 plume. Thus, even if the injected CO2 itself is safely trapped in suitable geological structures, pressure changes and brine displacement may affect shallow groundwater resources, for example, by increasing the rate of discharge into a lake or stream, or by mixing of brine into drinking water aquifers (Bergman and Winter, 1995).
Fig. 1 shows schematically the large-scale subsurface impacts that may be experienced during and after industrial-scale injection of CO2. While the CO2 plume at depth may be safely trapped under a low-permeability caprock with an anticlinal structure, the footprint area of the plume is much smaller than the footprint area of elevated pressure expected in the storage formation. The environmental impact of large-scale pressure buildup and related brine displacement depends mainly on the hydraulic connectivity between deep saline formations and the freshwater aquifers overlying them. One concern would be a storage formation that extends updip to form a freshwater resource used for domestic or commercial water supply (Bergman and Winter, 1995, Nicot, 2008). Via this direct hydraulic communication, CO2 storage at depth could impact the shallow portions of the aquifer, which may experience water table rise, changes in discharge and recharge zones, and changes in water quality. Even if separated from deep storage formations by low-permeability seals, freshwater resources may be hydraulically communicating with deeper layers, and the pressure buildup at depth would then provide a driving force for upward brine migration. Interlayer pressure propagation and brine leakage may occur, for example, if high-permeability conduits such as faults and abandoned boreholes are present. Pressure may also propagate in a slow, diffuse process if the sealing layers have a relatively high permeability.
A recent study of CO2 storage capacity in compartmentalized saline formations suggests that the hydraulic characteristics of seal layers may strongly affect the lateral and vertical volumes affected by pressure buildup (Zhou et al., 2008). Suitable sites for CO2 sequestration would typically have thick, laterally continuous shale, mudstone, or siltstone seals that act as permeability and capillary barriers to impede or prevent upward migration of buoyant CO2. These sealing units also play a role in reducing the interlayer pressure perturbation and limiting flow of native brine out of the storage formation into overlying and underlying strata. In contrast to supercritical CO2, however, this process is limited only by the small seal permeability; capillary sealing is not a factor. Interlayer pressure propagation and brine leakage may occur anywhere in the storage formation where pressure increases in response to CO2 injection. Thus, these processes can occur over a large area.
How far the pressure buildup induced by CO2 injection will extend into the lateral versus the vertical direction depends on the characteristics and properties of the stratigraphic units. If brine leakage out of the storage formation were important, the lateral displacement of brine within the formation would become less extensive, and vice versa. For very small seal permeabilities, the native brine displaced by injected CO2 is expected to migrate mostly within the storage formation, which could potentially affect freshwater resources located further updip (Fig. 1) (Nicot, 2008). On the other hand, if the sealing layers have a relatively higher permeability, the pressure front (and the native brine) may slowly propagate into and through the seals into neighboring formations, and may reach shallow levels in extreme cases. At the same time, such considerable vertical leakage would attenuate pressure buildup within the storage formation.
To our knowledge, no research has been conducted to date to systematically estimate the area of influence in response to CO2 storage within multilayer systems where lateral and vertical brine flow may compete. This article describes an attempt to address these issues quantitatively, providing a basis for further studies directly addressing the potential environmental risks to groundwater resources. Numerical simulations are conducted to estimate the pressure perturbation and brine migration in response to industrial-scale CO2 injection into a large, laterally open saline aquifer. The model domain includes an idealized multilayer groundwater system, with a sequence of aquifers and aquitards (sealing layers) extending from the deep saline storage formation to the top of the uppermost freshwater aquifer. Thereby, the region of influence is evaluated in both lateral and vertical directions. Recognizing the possible importance of vertical interlayer communication, we conduct sensitivity studies, varying the hydrologic properties of the aquitards. Our research aims at: (1) developing a basic understanding of flow and pressure conditions in a CO2 storage formation embedded in a sequence of aquifers and aquitards, (2) exploring the effects of interlayer communication through low-permeability seals and the impact on lateral/vertical displacement, and (3) determining the region of influence during/after injection of CO2 and evaluating possible implications for shallow groundwater resources.
Section snippets
Model setup and parameters
A numerical model is developed to investigate the multiphase flow and multicomponent transport of CO2 and brine in response to CO2 injection into an idealized multilayer formation. The transient pressure buildup, spatial CO2 plume evolution, and brine flow and transport are simulated for various sensitivity cases, using the TOUGH2/ECO2N simulator (Pruess et al., 1999, Pruess, 2005).
Spatial distribution of CO2 plume
Before elaborating on the large-scale impacts of CO2 injection, we may briefly focus on the characteristics of the CO2 plume at the end of the injection period, shown in Fig. 4, together with pressure buildup contours and brine flow vectors. The case depicted in the figure has a seal permeability of 10−18 m2; all other properties are given in Table 1. Only a small part of the entire model domain is shown, concentrating on the storage formation near the injection point.
The CO2 plume size is
Discussion
With respect to pressure changes within the storage formation, the region of influence in response to CO2 injection can be extremely large. For the radial-symmetric domain evaluated in this study, considerable pressure buildup was observed at large distances of more than 100 km from the injection zone. Such pressure changes may cause problems if experienced in near-surface groundwater systems, a possible concern in a storage formation that extends updip to a shallow freshwater resource zone (
Summary and conclusions
Through numerical modeling of idealized subsurface formations with a single injection site, we have evaluated the possible impact of industrial-scale CO2 injection on regional multilayered groundwater systems. For the conditions evaluated in this study, considerable pressure buildup in the storage formation is predicted more than 100 km away from the injection zone, while the lateral brine transport velocity and migration distance are less significant. Large-scale pressure changes appear to be
Acknowledgments
The authors wish to thank Larry Myer at Lawrence Berkeley National Laboratory (LBNL) for his careful internal review of the manuscript. Thanks are also due to two anonymous reviewers for their constructive suggestions for improving the quality of the manuscript. This work was funded by the Assistant Secretary for Fossil Energy, Office of Sequestration, Hydrogen, and Clean Coal Fuels, National Energy Technology Laboratory, of the U.S. Department of Energy, and by Lawrence Berkeley National
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