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Research for Deployment: Incorporating Risk, Regulation,and Liability for Carbon Capture and Sequestration

Carbon capture and sequestration (CCS) has the potential to enable deep reductions in global carbon dioxide (CO₂) emissions, however this promise can only be fulfilled with large-scale deployment. For this to happen, CCS must be successfully embedded into a larger legal and regulatory context, and any potential risks must be effectively managed. We developed a list of outstanding research and technical questions driven by the demands of the regulatory and legal systems for the geologic sequestration (GS) component of CCS. We then looked at case studies that bound uncertainty within two of the research themes that emerge. These case studies, on surface leakage from abandoned wells and groundwater quality impacts from metals mobilization, illustrate how research can inform decision makers on issues of policy, regulatory need, and legal considerations. A central challenge is to ensure that the research program supports development of general regulatory and legal frameworks, and also the development of geological, geophysical, geochemical, and modeling methods necessary for effective GS site monitoring and verification (M&V) protocols, as well as mitigation and remediation plans. If large-scale deployment of GS is to occur in a manner that adequately protects human and ecological health and does not discourage private investment, strengthening the scientific underpinnings of regulatory and legal decision-making is crucial.

Potential Groundwater Quality Impacts from Metals Mobilization.

Groundwater merits special attention because it is a precious resource, and it is subject to current regulation. While there are several ways in which large-scale injection or leakage might affect water supplies, most attention has focused on CO₂−brine−rock interactions. While processes in this system could affect both organic and inorganic geochemistry of aquifers, only the mobilization of inorganic compounds, chiefly metals, through dissolution and transport will be considered here.

South Liberty (Frio) Pilot, TX.

In 2004, a DOE pilot field experiment in South Liberty, TX injected 1800 t of CO₂ into the Frio saline formation (29). This injection was designed to validate simulations of CO₂ transport and fate in one of the largest saline formations in the United States. A monitoring well located 100 ft. from the injection well collected direct fluid samples using a U-tube apparatus (30). This tool and others detected arrival of a CO₂ plume in the monitoring well 7 days after injection.

Of note, a substantial amount of dissolved metal was recovered in the U-tube (31). Initially, workers thought that the well casing was reacting to carbonic acid in the reservoir. However, laboratory studies and geochemical analyses confirmed that a substantial fraction of the metals were the product of mineral dissolution, specifically the oxide and hydroxide coatings of mineral grains that represent <2%>31). The rapidity of mobilization and the high concentrations suggested strongly that carbonic acid formed from dissolved CO₂ in formation brines might quickly and dramatically alter groundwater chemistry.

The Frio was the first saline formation analyzed in this way. Ultimately, this result is not unexpected, yet it is not clear whether this effect of metal mobilization is common or significant at depth. It is also not clear what fraction of metals would be transported with CO₂ should it leak to other formations. However, it raised the concern that should CO₂ leak into a shallow freshwater aquifer, there could be consequences that could negatively affect groundwater quality, potentially impacting public health and acceptance of CCS deployment.

Carbonate and Siliclastic Systems.

To a first order, both injection targets and shallow aquifers can be divided into siliciclastic or carbonate systems. This division reflects the primary composition of the reservoir rocks. Carbonate systems chiefly comprise calcite, aragonite, dolomite, and other carbonate minerals that form limestones and dolostones, whereas siliciclastic systems chiefly comprise frag ments of quartz, feldspar, and other siliceous minerals that form sandstones, siltstones, and shales.

This compositional difference greatly affects the response of carbon acid. Silicate minerals react slowly with CO₂, which means that there is little change in porosity and permeability over the duration of injection; however, the brines with dissolved CO₂ will remain acidic. In contrast, carbonate rocks react quickly with CO₂ and could change permeability and porosity quickly; however, the rapid kinetics will result in rapid increase of brine pH and buffering of the brine−CO₂ system, reducing reactivity over time. Because of these competing effects, it is not clear which fundamental rock composition is more prone to leakage or to mobilizations of metals, and little work has focused on direct comparison of these two primary aquifer compositions.

Key U.S. Freshwater Aquifer Settings.

Given the distribution of CO₂ point sources and potential GS reservoirs, it is likely that CO₂ storage will be concentrated in a small number of basins. To understand and appraise the potential for metal mobilization in shallow aquifers, it would be helpful to understand the composition and acid-reaction response near the surface of these basins. Table 2 provides a subset of key shallow aquifers in these basins and some issues around their geology.

Potential Leakage and Toxicity Problems with CO2 Sequestration

(From Green Car Congress web site April 28, 2009)

31 July 2006

Cross-well seismic difference tomogram of the Frio Brine project shows the CO2 plume.

Results from a field test on CO₂ sequestration in an old brine-filled oil reservoir suggest that the mixture of CO₂ and brine dissolves minerals in the rock walls, including carbonate, that could lead to pathways in the rock through which the gas could escape.

In a paper published in the July edition of Geology, the researchers in the Frio Brine Pilot also note the potential for the mobilization of toxic trace metals and toxic organic compounds.

The Frio Brine Pilot was the first test of closely monitored CO₂ injection in a brine formation in the United States, and was funded by the Department of Energy (DOE) National Energy Technology Laboratory (NETL) under the leadership of the Bureau of Economic Geology (BEG) at the Jackson School of Geosciences, The University of Texas at Austin, with major collaboration from GEO-SEQ, a national lab consortium led by Lawrence Berkeley National Laboratory (LBNL).

The researchers injected 1,600 metric tons of CO₂ 1,500 meters down into a sandstone site representative of a target for large-volume storage. The sandstones of the Oligocene Frio Formation are part of a thick, regionally extensive sandstone trend that underlies a concentration of industrial sources and power plants along the Gulf Coast of the United States.

Monitoring strategy at Frio.

The team then measured and monitored the CO₂ plume using a diverse suite of technologies in three intervals: the injection zone, the area above the injection zone, and the shallow near-surface environment.

Each monitoring strategy used a preinjection and one or more postinjection measurements. Wireline logging, pressure and temperature measurement, and geochemical sampling were also conducted during injection, and at follow-up intervals subsequent to the injection.

While the sequestration to-date has been successful—there have been no detected CO₂ leakages—the researchers conclude in their latest published assessment of on-going findings and analysis that the chemistry of the process might prove problematic.

Fluid samples obtained from the injection and observation wells before CO₂ injection showed a Na-Ca-Cl–type brine with 93,000 mg/L total dissolved solids (TDS) at near saturation with CH₄ at reservoir conditions.

Following CO₂ breakthrough, samples showed sharp drops in pH (6.5–5.7), pronounced increases in alkalinity (100–3,000 mg/L as HCO₃) and Fe (30–1,100 mg/L), and significant shifts in the isotopic compositions of H₂O, dissolved inorganic carbon (DIC), and CH₄.

Geochemical modeling indicates that brine pH would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides.

This rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for CO₂ and brine leakage. Dissolution of minerals, especially iron oxyhydroxides, could mobilize toxic trace metals and, where residual oil or suitable organics are present, the injected CO₂ could also mobilize toxic organic compounds.

Environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater.