FY12 ARRA-NRAP Report ? Studies to Support Risk Assessment of Geologic Carbon Sequestration

Abstract

SummaryThis report summarizes results of research conducted during FY 2012 to support the assessment of environmental risks associated with geologic carbon dioxide (CO 2 ) sequestration and storage. Several research focus areas are ongoing as part of this project. This includes the quantification of the leachability of metals and organic compounds from representative CO 2 storage reservoir and caprock materials, the fate of metals and organic compounds after release, and the development of a method to measure pH in situ under supercritical CO 2 (scCO 2 ) conditions.Metal leachability experiments were completed on six different rock samples in brine in equilibrium with scCO 2 at representative geologic reservoir conditions. In general, the leaching of Resource Conservation and Recovery Act of 1976 metals and other metals of concern was found to be limited and not likely to be a significant issue (at least, for the rocks tested). Metals leaching experiments were also completed on one rock sample with scCO 2 containing oxygen at concentrations of 0%, 1%, 4%, and 8% to simulate injection of CO 2 originating from the oxy-fuel combustion process. Significant differences in the leaching behavior of certain metals were observed when oxygen is present in the CO 2 . These differences resulted from oxidation of sulfides, release of sulfate, ferric iron and other metals, and subsequent precipitation of iron oxides and some sulfates such as barite.Experiments to evaluate the potential for mobilization of organic compounds from representative reservoir materials and cap rock and their fate in porous media (quartz sand) have been conducted. Results with Fruitland coal and Gothic shale indicate that lighter organic compounds were more susceptible to mobilization by scCO 2 compared to heavier compounds. Alkanes demonstrated very low extractability by scCO 2 . No significant differences were observed between the extractability of organic compounds by dry or water saturated scCO 2 . Reaction equilibrium appears to have been reached by 96 hours.When the scCO 2 was released from the reactor, less than 60% of the injected lighter compounds (benzene, toluene) were transported through the dry sand column by the CO 2 , while more than 90% of the heavier organics were trapped in the sand column. For wet sand columns, most (80% to 100%) of the organic compounds injected into the sand column passed through, except for naphthalene which was substantially removed from the CO 2 within the column.A spectrophotometric method was developed to measure pH in brines in contact with scCO 2 . This method provides an alternative to fragile glass pH electrodes and thermodynamic modeling approaches for estimating pH. The method was tested in simulated reservoir fluids (CO 2 -NaCl-H 2 O) at different temperatures, pressures, and ionic strengths, and the results were compared with other experimental studies and geochemical models. Measured pH values were generally in agreement with the models, but inconsistencies were present between some of the models.v

Experimental Study of Organic Ligand Transport in Supercritical CO2 Fluids and Impacts to Silicate Reactivity

“…In the subsurface, the injected scCO 2 produces regions in which the full spectrum of mutual CO 2 -H 2 O solubility can occur [6][7][8][9]. In basalt reservoirs, the CO 2 -rich (water-bearing scCO 2 ) fluids are highly reactive with regard to the minerals present [10], and the scCO 2 may mobilize the organics, as scCO 2 is an effective solvent of organic matter [11], which has been observed in the field and in experiments with geomaterials [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. In our recent investigation, we demonstrated that simple, naturally abundant organic ligands (the protonated forms of acetate, malonate, oxalate, and citrate) were transported through water-saturated scCO 2 and their presence impacted the carbonation of forsterite [25].…”