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POTENTIAL RISKS
Since supercritical CO2 is buoyant at the relevant crustal pressures and temperatures, it will seek the Earth’s surface in most settings. A large CO2 accumulation would exert forces on the reservoir, cap rock, faults, and wells. CO2 must also be injected at pressures above reservoir pressures, creating a pressure transient during and after injection. In addition, dissolved CO2 forms carbonic acid, which can alter rock and well-bore properties and composition. Therefore, despite confidence in the storage mechanisms discussed above, the possibility of leakage from storage sites remains.
These risks were recently highlighted by geochemical analysis and laboratory experiments carried out at a pilot injection in South Liberty, Texas (Hovorka et al. 2006). Kharaka et al. (2006) observed rapid dissolution of some minerals, chiefly carbonate, oxide, and hydroxide minerals. Although this population represented a small fraction of the rock volume (<2%),>.
These studies suggest that while Kharaka et al. (2006) may have discovered a new element of risk, that element does not appear to represent a major concern to CO2 storage. The geomechanical response to CO2 injection may still cause concerns. In a parallel set of studies, Johnson et al. (2005) simulated large pressure excursions from CO2 injection. They concluded that under certain conditions, such excursions lead to fracture dilation, with some seepage of CO2 into overlying units. In the case of most cap rocks, which have both fractures and reactive minerals (e.g. chlorite), this creates a competing rates problem between dilation of fracture and precipitation of reactive minerals in fracture voids. In this system, fracture closure or dilation is sensitive to CO2 diffusion distance and reaction rate. In addition, the pressure transient from injection could lead to fault-slip-induced fluid migration (e.g. Wiprut and Zoback 2002). While it is generally possible to predict the conditions under which this might occur (e.g. Chiaramonte et al. 2006), effective storage will require proper system calibration and injection management.
As mentioned previously, achievement of substantial CO2 emissions reductions through GCS will require hundreds to thousands of large-volume injection facilities distributed around the world. Each existing large project and some small projects (e.g. the Frio Brine Pilot; Hovorka et al. 2006) have provided some demonstration of effectiveness, monitoring technologies, and operational economics. Importantly, each existing large project also has revealed an important aspect of the geology that was not previously known or in some cases incorrectly characterized. For example, at Sleipner, the importance of small flow heterogeneities was not anticipated but was clearly seen (Arts et al. 2004). At Weyburn, CO2 migrated in unexpected ways along secondary fractures (Wilson and Monea 2004). Such features would not have been revealed through a small-scale (<100,000 style="font-weight: bold;">CO2 Dissolution and Precipitation Kinetics
The rate at which CO2 dissolves in brines of varying composition, temperature, pressure, and mixing degree greatly affects the long-term trapping mechanisms (i.e. the formation of carbonic acid, bicarbonate, and new minerals). These issues in turn affect other important concerns, such as the configuration and infrastructure of storage reservoir engineering (e.g. Keith et al. 2005) or the long-term fate of CO2 (Ennis-King and Paterson 2003). Although there has been some work on the controls of dissolution rate (e.g. interfacial effects; Yang et al. 2005) more could be done.
Similarly, knowledge of CO2-brine-mineral dissolution and precipitation kinetics is limited. Recent years have seen many experimental studies on individual minerals or classes of minerals (e.g. Carroll and Knauss 2005 and references therein). Still, much remains to be learned about rock systems, including true multiphase chemistry, mineral–CO2 equations of state, and minerals that may represent only a small volume of the rock but have rapid dissolution kinetics
(e.g. metal oxides or hydroxides).
As a subtopic, most GCS work on mineral reaction kinetics has focused on pure CO2–rock–brine systems. Very little work has been done on gas streams with small concentrations of other gases, in particular SOx, NOx, and H2S. These co-contaminant gases have the potential to dramatically alter the chemical response of a gas–brine–rock system, even if very small amounts of these gases are present (Knauss et al. 2005). Because the capture and separation of trace gases with CO2 may save capital and operating expenses, investigation of reactions in such systems may prove useful in the near future to determine if mixed-gas systems present additional concerns or risks.
Groundwater
The majority of sequestered CO2 will be stored in saline formations at depth. CO2 stored in these formations should have little or no effect on groundwater. However, if CO2 were to reach the surface along some fast pathway, then CO2 might enter fresh groundwater systems. It is highly unlikely that the rate or volume of CO2 would present a problem. However, the results of Kharaka et al. (2006) have raised the possibility that rapid local reactions could release unwanted elements and compounds into groundwater. Again, for most reservoirs this may have no effect. However, some reservoirs have elevated levels of natural arsenic. In such a system, even a small release of CO2 might result in an increase in local arsenic concentrations that could bring a municipal water supply out of compliance with U.S. Environmental Protection Agency regulations. Similarly, widespread deployment of GCS could potentially displace enough water to create saline groundwater intrusions in contiguous formations.
Wells
Wells almost certainly present the greatest risk to leakage because they are drilled to bring large volumes of fluid quickly to the Earth’s surface. In addition, they remove the aspects of the rock volume that prevent buoyant migration. Well casings and cements are susceptible to corrosion from carbonic acid. When wells are adequately plugged and completed, they are likely to trap CO2 at depth effectively. However, large numbers of orphaned or abandoned wells may not be adequately plugged, completed, or cemented (Ide et al. 2006), and such wells represent potential leak points for CO2. One analog site is particularly well suited for study. Crystal Geyser, Utah (Shipton et al. 2005), is a well that penetrated a natural CO2 accumulation in 1936, was poorly completed, and has erupted CO2 ever since (FIG. 3). Eruptions are episodic and vary in size. Measurements of individual and sequential eruptions suggest that large events bring tens of tonnes of CO2 to the surface, with an average daily flux of 40–50 tonnes (Gouveia et al. 2005). During the eruptions, atmospheric concentrations of CO2 were not recorded at or above dangerous levels. While this style of eruption does not appear to present a substantial risk (Bogen et al. 2006), more study is needed to understand how representative of well leakage this site may be.
Little is known about the probability of escape from a given well, the likelihood of such a well existing within a potential site, or the risk such a well presents in terms of potential leakage volume or consequence. Current approaches involve statistical characterization of many wells and semiquantitative analysis (Celia et al. 2006), or modeling and simulation of features and processes in well-bore environments (Gerard et al. 2006). Work in understanding the key features of wells (e.g. fracture geometry and character), the chemical response of well components to CO2 systems, and the evolution of natural and engineered interfaces could provide both better estimates of well-bore integrity risk and potential mitigation and remediation strategies (IPCC 2005).
