Large-scale impact of CO₂ storage in deep saline aquifers: A sensitivity study on pressure response in stratified systems

Abstract

Large volumes of CO₂ captured from carbon emitters (such as coal-fired power plants) may be stored in deep saline aquifers as a means of mitigating climate change. Storing these additional fluids may cause pressure changes and displacement of native brines, affecting subsurface volumes that can be significantly larger than the CO₂ plume itself. This study aimed at determining the three-dimensional region of influence during/after injection of CO₂ and evaluating the possible implications for shallow groundwater resources, with particular focus on the effects of interlayer communication through low-permeability seals. To address these issues quantitatively, we conducted numerical simulations that provide a basic understanding of the large-scale flow and pressure conditions in response to industrial-scale CO₂ injection into a laterally open saline aquifer. The model domain included an idealized multilayered groundwater system, with a sequence of aquifers and aquitards (sealing units) extending from the deep saline storage formation to the uppermost freshwater aquifer. Both the local CO₂-brine flow around the single injection site and the single-phase water flow (with salinity changes) in the region away from the CO₂ plume were simulated. Our simulation results indicate considerable pressure buildup in the storage formation more than 100 km away from the injection zone, whereas the lateral distance migration of brine is rather small. In the vertical direction, the pressure perturbation from CO₂ storage may reach shallow groundwater resources only if the deep storage formation communicates with the shallow aquifers through sealing units of relatively high permeabilities (higher than 10⁻¹⁸ m²). Vertical brine migration through a sequence of layers into shallow groundwater bodies is extremely unlikely. Overall, large-scale pressure changes appear to be of more concern to groundwater resources than changes in water quality caused by the migration of displaced saline water.

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.