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

Molecular dynamics simulations have been carried out to study decomposition of methane hydrate at different cage occupancies. The decomposition rate is found to depend sensitively on the hydration number. The rate of the destruction of the cages displays Arrhenius behavior, consistent with an activated mechanism. During the simulations, reversible formation of partial water cages around methane molecules in the liquid was observed at the interface at temperatures above the computed hydrate decomposition temperature.

A single-sided transient plane source technique has been used to determine the thermal conductivity and thermal diffusivity of a compacted methane hydrate sample over the temperature range of 261.5-277.4 K and at gas-phase pressures ranging from 3.8 to 14.2 MPa. The average thermal conductivity, 0.68 +/- 0.01 W/(m K), and thermal diffusivity, 2.04 x 10(-7) +/- 0.04 x 10(-7) m2/s, values are, respectively, higher and lower than previously reported values. Equilibrium molecular dynamics (MD) simulations of methane hydrate have also been performed in the NPT ensemble to estimate the thermal conductivity for methane compositions ranging from 80 to 100% of the maximum theoretical occupation, at 276 K and at pressures ranging from 0.1 to 100 MPa. Calculations were performed with three rigid potential models for water, namely, SPC/E, TIP4P-Ew, and TIP4P-FQ, the last of which includes the effects of polarizability. The thermal conductivities predicted from MD simulations were in reasonable agreement with experimental results, ranging from about 0.52 to 0.77 W/(m K) for the different potential models with the polarizable water model giving the best agreement with experiments. The MD simulation method was validated by comparing calculated and experimental thermal conductivity values for ice and liquid water. The simulations were in reasonable agreement with experimental data. The simulations predict a slight increase in the thermal conductivity with decreasing methane occupation of the hydrate cages. The thermal conductivity was found to be essentially independent of pressure in both simulations and experiments. Our experimental and simulation thermal conductivity results provide data to help predict gas hydrate stability in sediments for the purposes of production or estimating methane release into the environment due to gradual warming.

Nonequilibrium molecular dynamics simulations with the nonpolarizable SPC/E (Berendsen et al., J. Phys. Chem. 1987, 91, 6269) and the polarizable COS/G2 (Yu and van Gunsteren, J. Chem. Phys. 2004, 121, 9549) force fields have been employed to calculate the thermal conductivity and other associated properties of methane hydrate over a temperature range from 30 to 260 K. The calculated results are compared to experimental data over this same range. The values of the thermal conductivity calculated with the COS/G2 model are closer to the experimental values than are those calculated with the nonpolarizable SPC/E model. The calculations match the temperature trend in the experimental data at temperatures below 50 K; however, they exhibit a slight decrease in thermal conductivity at higher temperatures in comparison to an opposite trend in the experimental data. The calculated thermal conductivity values are found to be relatively insensitive to the occupancy of the cages except at low (T

Predicting the fate of subsea hydrocarbon gases escaping into seawater is complicated by potential formation of hydrate on rising bubbles that can enhance their survival in the water column, allowing gas to reach shallower depths and the atmosphere. The precise nature and influence of hydrate coatings on bubble hydrodynamics and dissolution is largely unknown. Here we present high-definition, experimental observations of complex surficial mechanisms governing methane bubble hydrate formation and dissociation during transit of a simulated oceanic water column that reveal a temporal progression of deep-sea controlling mechanisms. Synergistic feedbacks between bubble hydrodynamics, hydrate morphology, and coverage characteristics were discovered. Morphological changes on the bubble surface appear analogous to macroscale, sea ice processes, presenting new mechanistic insights. An inverse linear relationship between hydrate coverage and bubble dissolution rate is indicated. Understanding and incorporating these phenomena into bubble and bubble plume models will be necessary to accurately predict global greenhouse gas budgets for warming ocean scenarios and hydrocarbon transport from anthropogenic or natural deep-sea eruptions.

Two-phase equilibrium between CO 2 hydrate (H) and a water-rich liquid (L) are experimentally measured and theoretically described between 273 and 281 K, at pressures below 30 MPa, and at aqueous CO 2 concentrations between 0.0163 and 0.0242 mole fraction. These data represent the conditions where hydrates form from a single-phase aqueous solution of fixed composition. Both theoretical and experimental results indicate that the equilibrium pressure is very sensitive to concentration at all temperatures. The concentrations reported represent the solubility of CO 2 in a water phase in equilibrium with hydrate at the given temperature and pressure. When a constant aqueous composition LH curve is extrapolated to the three-phase VLH curve, the composition characterizing the LH curve also represents the solubility of CO 2 in water at the VLH conditions. Since the solubility of CO 2 in water at hydrate-forming conditions is difficult to obtain, this method provides an excellent way of indirectly measuring this three-phase solubility. The effect of salinity on hydrate formation from water-rich-liquid systems was also studied. A modified model was introduced to describe the experimental results and produced good agreement between calculated and experimental pressures. A simplified version of the model can provide quick and reasonable estimations of the equilibrium conditions of hydrates at low concentrations and medium to low pressures. Interestingly, the increase of salt increases the maximum temperature at which hydrates are stable for a constant pressure and constant composition system. This is because the salt increases the chemical potential of the dissolved gases, which more than offsets the reduction in the chemical potential of the liquid water. The model can also be used for prediction of LH equilibrium for other gas hydrates. An example is given for methane hydrate at three different concentrations of methane in water.

Deep saline aquifers are reported to have the largest estimated capacity for CO 2 sequestration. Knowledge of possible geochemically-induced changes to the porosity and permeability of host CO 2 storage sandstone and seal rock will enhance our capability to predict CO 2 storage capacity and long-term reservoir behavior.An experimental study of the potential interaction of CO 2 /brine/rock on saline formations in a static system under CO 2 sequestration conditions was conducted. Chemical interactions in the Mount Simon sandstone environment upon exposure to CO 2 mixed with brine under sequestration conditions were studied. Samples were exposed to the estimated in-situ reaction conditions for six months. The experimental parameters used were two core samples of Mount Simon sandstone; Illinois Basin model brine; temperature of 85°C, pressure of 23.8 MPa (3,500 psig), and CO 2 . Micro-CT, CT, XRD, SEM, petrography, and brine, porosity, and permeability analyses were performed before and after the exposure. Preliminary permeability measurements obtained from the sandstone sample showed a significant change after it was exposed to CO 2 -saturated brine for six months. This observation suggests that mineral dissolution and mineral precipitation could occur in the host deposit altering its characteristics for CO 2 storage over time.

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