Task B. Large-scale Hydrological Evaluation and Modeling of the Impact on Groundwater Resources

 

Contact:	Jens Birkholzer and Chin-Fu Tsang
Key Personnel:	Quanlin Zhou Jonny Rutqvist

 

If carbon dioxide capture and storage (CCS) technologies are implemented on a large scale, the amounts of CO₂ injected and sequestered underground will be extremely large. 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 or the importance of other trapping mechanisms such as dissolution of CO₂ into formation water or mineral trapping as CO₂ reacts with the rock. Less emphasis has been placed on the understanding of the fate of the native brines or brackish waters that are being displaced by the injected volumes of CO₂. As discussed below, large-scale injection of CO₂ impacts subsurface volumes much larger than the CO₂ plume. Thus, even if the injected CO₂ itself is safely trapped in suitable geological structures, large-scale injection and related brine displacement may affect shallow groundwater resources. The issue of brine displacement, and its possible environmental impact on groundwater hydrology, is addressed in the research effort reported here.

The figure below shows schematically the large-scale subsurface impacts that will 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 anticlinal structure, the footprint area of the plume is smaller than the footprint area of the displaced brine, which in turn is much smaller than the footprint area of elevated pressure. The footprint area of displaced brine illustrates the approximate location of a displaced fluid volume that was originally located within the CO₂ plume footprint. Of course, brine displacement occurs, to some degree, wherever a pressure gradient develops in response to injection, suggesting the possibility of water quality changes as brines or brackish water may migrate into freshwater regions. The footprint area of elevated pressure indicates the extremely large subsurface volumes where such pressure impacts might be expected.

The environmental impact of large-scale brine displacement depends to a large extent on the hydraulic connectivity between deep storage formations and the freshwater aquifers overlying them. One primary concern would be a storage formation that extends updip to form a freshwater resource used for domestic or commercial water supply. Via this direct hydraulic communication, CO₂ storage at depth will impact the shallow portions of the aquifer, which could experience pressure increase and water table rise, changes in discharge and recharge zones, and changes in water quality. Even if separated from deep storage formations by sequences of 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. This can be, for example, via local high-permeability flow paths such as faults and abandoned boreholes. In addition, seals may pinch out or have higher permeabilities locally, allowing for vertical interlayer migration. Finally, land-surface deformation or uplift is expected in response to CO₂ injection, which may change surface and near-subsurface flow patterns even without a hydraulic direct impact of brine displacement. The reverse effect, land subsidence in response to groundwater withdrawal (e.g., for water supply, agriculture, or related to oil production), is a common problem throughout the United States.

Issues related to large-scale pressure buildup and brine displacement may also cause operational and capacity problems. For example, if more than one large point source was to store CO₂ into the same formation, the operational scheme and the location of the injection zone would have to be carefully planned to avoid unwanted feedback between neighboring sites. Storage capacity may be a concern in compartmentalized formations, from which the displaced CO₂ can not easily escape laterally to make room for the injected CO₂ (closed systems). When large volumes of CO2 are injected into a closed system, a significant pressure buildup will be produced, which can severely limit CO₂ storage capacity, because overpressure and geomechanical damage need to be avoided.

Schematic showing different regions of influence related to CO₂

To date, the impact of large-scale CO₂ injection and related brine displacement on regional multilayered groundwater systems has not been systematically assessed. To build confidence in carbon capture and storage technologies, there needs to be an adequate understanding of the magnitude and extent of water pressure increase in the storage formation and in the shallower aquifers, which may be separated by aquitards of much smaller hydraulic conductivities. In particular, the change in groundwater table level, the effect on discharge and recharge zones in the groundwater system, and the impact of these changes on the properties and characteristics of underground sources of drinking water (USDWs) should be investigated. Estimates of CO₂ storage capacity should consider constraints imposed by brine displacement, either to avoid shallow-water impacts in open systems or to account for pressure constraints in closed systems.

To address these research needs, the following three-year research project was initiated in 2006. The tasks included in this research project have a successive degree of complexity, starting with systematic studies of idealized formations leading up to detailed modeling studies of one or two groundwater basins in the U.S. that are likely candidates for future storage of CO₂.

Sub-Task B1:   Storage Capacity and Pressure Buildup in Idealized Pressure-Constrained Storage Formations (Closed Systems)
Analytical or numerical solutions are employed or developed in this ongoing task that allow a fast evaluation of brine displacement by injected CO₂ and the related pressure buildup in simplified geological systems. Only the storage formation is considered in this task (no interaction with overlying or underlying units). The objectives are to:

• Develop a basic understanding of potential pressure buildup in closed systems
• Determine sensitivity to injected volume, formation size, hydraulic properties, compressibility, depth, and other key parameters
• Develop a quick-assessment method for estimating storage capacity in pressure-constrained formations

Sub-Task B2:    Pressure Buildup and Brine Displacement in Idealized Multilayered Groundwater Systems (Open Systems)
A semi-analytical or numerical simulation study is conducted in this ongoing task to evaluate the brine displacement by injected CO₂ and the related pressure buildup in multilayered geological systems. In this case, the storage formation interacts with overlying or underlying units. The objectives are to:

• Develop a basic understanding of potential pressure buildup (in lateral and vertical direction) in a laterally open storage formation.
• Explore the effects of the interlayer communication types through porous medium low-permeability seals, major faults, fracture zones, or wells.
• Evaluate the impact on pressure rises and discharge-recharge zones in the shallower groundwater layers, including USDWs.

Sub-Task B3:    Hydromechanical Aspects of Injection in Idealized Multilayered Groundwater Systems
This future task is an extension of Task B2 to account for hydromechanical effects that could potentially have a major effect on the expected pressure buildup and multilayer interaction. The objectives are to:

• Evaluate the role of mechanical deformation, with associated permeability changes, on pressure buildup, water displacement, and land surface uplift, etc.
• Identify and characterize data needs.
• Evaluate hydrogeological and mechanical effects in shallower units and USDWs.

Sub-Task B4:    Analysis and Modeling of One or Two Representative Cases of Regional Groundwater Systems
This future task involves modeling evaluation of one or two regional groundwater systems in response to CO₂ injection. Two typical deep saline aquifer systems will be selected, based on an extensive literature survey and in interaction with regional carbon sequestration partnerships. The objectives are to:

• Conduct a literature survey of existing information on the hydrogeology of deep saline aquifers.
• Collect data on volume, gradients, interaction with overlying or underlying layers.
• Select two realistic examples.
• Conduct large-scale hydrologic modeling studies of CO₂ storage for these examples.