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This paper demonstrates the use of coupled fluid flow and geomechanical fault slip (fault reactivation) analysis to estimate the maximum sustainable injection pressure during geological sequestration of CO 2 . Two numerical modeling approaches for analyzing faultslip are applied, one using continuum stress-strain analysis and the other using discrete fault analysis. The results of these two approaches to numerical fault-slip analyses are compared to the results of a more conventional analytical fault-slip analysis that assumes simplified reservoir geometry. It is shown that the simplified analytical fault-slip analysis may lead to either overestimation or underestimation of the maximum sustainable injection pressure because it cannot resolve important geometrical factors associated with the injection induced spatial evolution of fluid pressure and stress. We conclude that a fully coupled numerical analysis can more accurately account for the spatial evolution of both in situ stresses and fluid pressure, and therefore results in a more accurate estimation of the maximum sustainable CO 2 injection pressure.2

Large volumes of CO 2 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 CO 2 plume itself. This study aimed at determining the three-dimensional region of influence during/after injection of CO 2 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 CO 2 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 CO 2 -brine flow around the single injection site and the single-phase water flow (with salinity changes) in the region away from the CO 2 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 CO 2 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 m 2 ). 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.

Approximately 300 kg/day of food-grade CO 2 was injected through a perforated pipe placed horizontally 2-2.3 m deep during July 9-August 7, 2008 at the MSU-ZERT field test to evaluate atmospheric and near-surface monitoring and detection techniques applicable to the subsurface storage and potential leakage of CO 2 . As part of this multidisciplinary research project, 80 samples of water were collected from 10 shallow monitoring wells (1.5 or 3.0 m deep) installed 1-6 m from the injection pipe, at the southwestern end of the slotted section (zone VI), and from two distant monitoring wells. The samples were collected before, during, and following CO 2 injection. The main objective of study was to investigate changes in the concentrations of major, minor, and trace inorganic and organic compounds during and following CO 2 injection. The ultimate goals were (1) to better understand the potential of groundwater quality impacts related to CO 2 leakage from deep storage operations, (2) to develop geochemical tools that could provide early detection of CO 2 intrusion into underground sources of drinking water (USDW), and (3) to test the predictive capabilities of geochemical codes against field data. Field determinations showed rapid and systematic changes in pH (7.0-5.6), alkalinity (400-1,330 mg/l as HCO 3 ), and electrical conductance (600-1,800 lS/cm) following CO 2 injection in samples collected from the 1.5 m-deep wells. Laboratory results show major increases in the concentrations of Ca (90-240 mg/l), Mg (25-70 mg/l), Fe (5-1,200 ppb), and Mn (5-1,400 ppb) following CO 2 injection. These chemical changes could provide early detection of CO 2 leakage into shallow groundwater from deep storage operations. Dissolution of observed carbonate minerals and desorptionion exchange resulting from lowered pH values following CO 2 injection are the likely geochemical processes responsible for the observed increases in the concentrations of solutes; concentrations generally decreased temporarily following four significant precipitation events. The DOC values obtained are 5 ± 2 mg/l, and the variations do not correlate with CO 2 injection. CO 2 injection, however, is responsible for detection of BTEX (e.g. benzene, 0-0.8 ppb), mobilization of metals, the lowered pH values, and increases in the concentrations of other solutes in groundwater. The trace metal and BTEX concentrations are all significantly below the maximum contaminant levels (MCLs). Sequential leaching of core samples is being carried out to investigate the source of metals and other solutes.

Capturing carbon dioxide (CO(2)) emissions from industrial sources and injecting the emissions deep underground in geologic formations is one method being considered to control CO(2) concentrations in the atmosphere. Sequestering CO(2) underground has its own set of environmental risks, including the potential migration of CO(2) out of the storage reservoir and resulting acidification and release of trace constituents in shallow groundwater. A field study involving the controlled release of groundwater containing dissolved CO(2) was initiated to investigate potential groundwater impacts. Dissolution of CO(2) in the groundwater resulted in a sustained and easily detected decrease of ~3 pH units. Several trace constituents, including As and Pb, remained below their respective detections limits and/or at background levels. Other constituents (Ba, Ca, Cr, Sr, Mg, Mn, and Fe) displayed a pulse response, consisting of an initial increase in concentration followed by either a return to background levels or slightly greater than background. This suggests a fast-release mechanism (desorption, exchange, and/or fast dissolution of small finite amounts of metals) concomitant in some cases with a slower release potentially involving different solid phases or mechanisms. Inorganic constituents regulated by the U.S. Environmental Protection Agency remained below their respective maximum contaminant levels throughout the experiment.

If carbon dioxide stored in deep saline aquifers were to leak into an overlying aquifer containing potable groundwater, the intruding CO 2 would change the geochemical conditions and cause secondary effects mainly induced by changes in pH In particular, hazardous trace elements such as lead and arsenic, which are present in the aquifer host rock, could be mobilized. In an effort to evaluate the potential risks to potable water quality, reactive transport simulations were conducted to evaluate to what extent and mechanisms through which lead and arsenic might be mobilized by intrusion of CO 2 . An earlier geochemical evaluation of more than 38,000 groundwater quality analyses from aquifers throughout the United States and an associated literature review provided the basis for setting up a reactive transport model and examining its sensitivity to model variation. The evaluation included identification of potential mineral hosts containing hazardous trace elements, characterization of the modal bulk mineralogy for an arenaceous aquifer, and augmentation of the required thermodynamic data. The reactive transport simulations suggest that CO 2 ingress into a shallow aquifer can mobilize significant lead and arsenic, contaminating the groundwater near the location of intrusion and further downstream. Although substantial increases in aqueous concentrations are 2 predicted compared to the background values, the maximum permitted concentration for arsenic in drinking water was exceeded in only a few cases, whereas that for lead was never exceeded.

Integrated modeling of basin-and plume-scale processes induced by full-scale deployment of CO 2 storage was applied to the Mt. Simon aquifer in the Illinois Basin. A 3D mesh was generated with local refinement around 20 injection sites, with ~30 km spacing. A total annual injection rate of 100 Mt CO 2 over 50 years was employed. The CO 2 -brine flow at the plume scale and the single-phase flow at the basin scale were simulated. Simulation results show the overall shape of a CO 2 plume consisting of a typical gravity-override subplume in the bottom injection zone of high injectivity and a pyramid-shaped subplume in the overlying multilayered Mt. Simon, indicating the important role of a secondary seal with relatively low permeability and high entry capillary pressure. The secondary seal effect is manifested by retarded upward CO 2 migration as a result of multiple secondary seals, coupled with lateral preferential CO 2 viscous fingering through high-permeability layers. The plume width varies from 9.0 to 13.5 km at 200 years, indicating the slow CO 2 migration and no plume interference between storage sites. On the basin scale, pressure perturbations propagate quickly away from injection centers, interfere after less than a year, and eventually reach basin margins.The simulated pressure buildup of 35 bar in the injection area is not expected to affect caprock geomechanical integrity. Moderate pressure buildup is observed in Mt. Simon in northern Illinois. However, its impact on groundwater resources is less than the hydraulic drawdown induced by long-term extensive pumping from overlying freshwater aquifers.

Coupled reservoir-geomechanical simulations were conducted to study the potential for tensile and shear failure-e.g., tensile fracturing and shear slip along pre-existing fractures-associated with underground CO 2 injection in a multilayered geological system. This failure analysis aimed to study factors affecting the potential for breaching a geological CO 2 storage system and to study methods for estimating the maximum CO 2 injection pressure that could be sustained without causing such a breach. We pay special attention to geomechanical stress changes resulting from upward migration of the CO 2 and how the initial stress regime affects the potential for inducing failure. We conclude that it is essential to have an accurate estimate of the three-dimensional in situ stress field to support the design and performance assessment of a geological CO 2 injection operation. Moreover, we also conclude that it is important to consider mechanical stress changes that might occur outside the region of increased reservoir fluid pressure (e.g., in the overburden rock) between the CO 2 -injection reservoir and the ground surface.2