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A flow‐through experiment was performed to investigate evolution of a fractured carbonate caprock during flow of CO2‐acidified brine. A core was taken from the Amherstburg limestone, a caprock formation overlying the Bois Blanc and Bass Islands formations, which have been used to demonstrate CO2 storage in the Michigan basin. The inlet brine was representative of deep saline brines saturated with CO2, resulting in a starting pH of 4.4. Experimental conditions were 27 °C and 10 MPa. X‐ray computed tomography and scanning electron microscopy were used to observe evolution of fracture geometry and to investigate mineralogical changes along the fracture surface. The initial brine flow corresponded to an average fluid velocity of 110 cm hr−1. After one week, substantial mineral dissolution caused the average cross‐sectional area of the fracture to increase from 0.09 cm2 to 0.24 cm2. This demonstrates that carbonate caprocks, if fractured, can erode quickly and may jeopardize sealing integrity when hydrodynamic conditions promote flow of CO2‐acidified brine. However, changes to fracture permeability due to mineral dissolution may be offset by unaltered constrictions along the flow path and by increases in surface roughness. In this experiment, preferential dissolution of calcite over dolomite led to uneven erosion of the fracture surface and an increase in roughness. In areas with clay minerals, calcite dissolution left behind a silicate mineral‐rich microporous coating along the fracture wall. Thus, the evolution of fracture permeability will depend in a complex way on the carbonate content, as well as the heterogeneity of the minerals and their spatial patterning. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd

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Coal tar, a dense nonaqueous phase liquid (NAPL), is a common subsurface contaminant at sites of former manufactured gas plants. A proposed remediation technology is water-miscible solvent extraction, which requires understanding of the effect of water-miscible solvents on the solubility of coal tar. This study investigated this effect and the extent to which multicomponent coal tar could be represented as a pseudocomponent in thermodynamic modeling. The coal tar used in this study showed a predominance of polycyclic aromatic hydrocarbons with no single compound accounting for more than 4% (wt). The bulk solubility of coal tar in water was estimated to be 16 mg/L using composition data and Raoult's law assumption for aqueous solubility. For three solvents, n-butylamine, acetone, and 2-propanol, equilibrium phase compositions of two-phase coal tar/solvent/water mixtures were experimentally determined using radiolabeled materials and are presented as ternary phase diagrams. Results showed n-butylamine to be a good water-miscible solvent for coal tar dissolution. The validity of thermodynamic modeling of coal tar as a pseudocomponent was explored by examining the liquid-liquid solute partitioning of naphthalene, phenanthrene and pyrene and by assessing the effect of solvent extraction on coal tar phase composition. It was found that coal tar partitions as a pseudocomponent in systems with appreciable solvent, but not in systems with only coal tar and water.

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Geochemical reactions may alter the permeability of leakage pathways in caprocks, which serve a critical role in confining CO 2 in geologic carbon sequestration. A caprock specimen from a carbonate formation in the Michigan sedimentary Basin was fractured and studied in a high-pressure core flow experiment. Inflowing brine was saturated with CO 2 at 40°C and 10 MPa, resulting in an initial pH of 4.6, and had a calcite saturation index of -0.8. Fracture permeability decreased during the experiment, but subsequent analyses did not reveal calcite precipitation. Instead, experimental observations indicate that calcite dissolution along the fracture pathway led to mobilization of less soluble mineral particles that clogged the flow path. Analyses of core sections via electron microscopy, synchrotron-based X-ray diffraction imaging, and the first application of microbeam Ca K-edge X-ray absorption near edge structure, provided evidence that these occlusions were fragments from the host rock rather than secondary precipitates. X-ray computed tomography showed a significant loss of rock mass within preferential flow paths, suggesting that dissolution also removed critical asperities and caused mechanical closure of the fracture. The decrease in fracture permeability despite a net removal of material along the fracture pathway demonstrates a nonintuitive, inverse relationship between dissolution and permeability evolution in a fractured carbonate caprock.