Yongman Kim

Dewetting of Silica Surfaces upon Reactions with Supercritical CO2 and Brine: Pore-Scale Studies in Micromodels

Wettability of reservoir minerals and rocks is a critical factor controlling CO(2) mobility, residual trapping, and safe-storage in geologic carbon sequestration, and currently is the factor imparting the greatest uncertainty in predicting capillary behavior in porous media. Very little information on wettability in supercritical CO(2) (scCO(2))-mineral-brine systems is available. We studied pore-scale wettability and wettability alteration in scCO(2)-silica-brine systems using engineered micromodels (transparent pore networks), at 8.5 MPa and 45 °C, over a wide range of NaCl concentrations up to 5.0 M. Dewetting of silica surfaces upon reactions with scCO(2) was observed through water film thinning, water droplet formation, and contact angle increases within single pores. The brine contact angles increased from initial values near 0° up to 80° with larger increases under higher ionic strength conditions. Given the abundance of silica surfaces in reservoirs and caprocks, these results indicate that CO(2) induced dewetting may have important consequences on CO(2) sequestration including reducing capillary entry pressure, and altering quantities of CO(2) residual trapping, relative permeability, and caprock integrity.

Transport and deposition of functionalized CdTe nanoparticles in saturated porous media

Comprehensive understanding of the transport and deposition of engineered nanoparticles (NPs) in subsurface is required to assess their potential negative impact on the environment. We studied the deposition behavior of functionalized quantum dot (QD) NPs (CdTe) in different types of sands (Accusand, ultrapure quartz, and iron-coated sand) at various solution ionic strengths (IS). The observed transport behavior in ultrapure quartz and iron-coated sand was consistent with conventional colloid deposition theories. However, our results from the Accusand column showed that deposition was minimal at the lowest IS (1mM) and increased significantly as the IS increased. The effluent breakthrough occurred with a delay, followed by a rapid rise to the maximum normalized concentration of unity. Negligible deposition in the column packed with ultrapure quartz sand (100mM) and Accusand (1mM) rules out the effect of straining and suggests the importance of surface charge heterogeneity in QD deposition in Accusand at higher IS. Data analyses further show that only a small fraction of sand surface area contributed in QD deposition even at the highest IS (100mM) tested. The observed delay in breakthrough curves of QDs was attributed to the fast diffusive mass transfer rate of QDs from bulk solution to the sand surface and QD mass transfer on the solid phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to examine the morphology and elemental composition of sand grains. It was observed that there were regions on the sand covered with layers of clay particles. EDX spectra collected from these regions revealed that Si and Al were the major elements suggesting that the clay particles were kaolinite. Additional batch experiments using gold NPs and SEM analysis were performed and it was observed that the gold NPs were only deposited on clay particles originally on the Accusand surface. After removing the clays from the sand surface, we observed negligible QD deposition even at 100mM IS. We proposed that nanoscale charge heterogeneities on clay particles on Accusand surface played a key role in QD deposition. It was shown that the value of solution IS determined the extent to which the local heterogeneities participated in particle deposition.

Aqueous Uranium(VI) Concentrations Controlled by Calcium Uranyl Vanadate Precipitates

Elevated concentrations of U in contaminated environments necessitate understanding controls on its solubility in groundwaters. Here, calculations were performed to compare U(VI) concentrations expected in typical oxidizing groundwaters in equilibrium with different U(VI) minerals. Among common U(VI) minerals, only tyuyamunite (Ca(UO(2))(2)V(2)O(8)·8H(2)O), uranophane (Ca(UO(2))(2)(SiO(3)OH)(2)·5H(2)O), and a putative well-crystallized becquerelite (Ca(UO(2))(6)O(4)(OH)(6)·8H(2)O) were predicted to control U concentrations around its maximum contaminant level (MCL = 0.13 μM), albeit over narrow ranges of pH. Given the limited information available on uranyl vanadates, room temperature Ca-U-V precipitation experiments were conducted in order to compare aqueous U concentrations with tyuyamunite equilibrium predictions. Measured U concentrations were in approximate agreement with predictions based on Langmuirs estimated ΔG(f)°, although the precipitated solids were amorphous and had wide ranges of Ca/U/V molar ratios. Nevertheless, high initial U concentrations were decreased to below the MCL over the pH range 5.5-6.5 in the presence of newly formed CaUV solids, indicating that such solids can be important in controlling U in some environments.

Capillary pressure‐saturation relations in quartz and carbonate sands: Limitations for correlating capillary and wettability influences on air, oil, and supercritical CO2 trapping

Author(s): Wang, S; Tokunaga, TK; Wan, J; Dong, W; Kim, Y | Abstract:

Water Saturation Relations and Their Diffusion‐Limited Equilibration in Gas Shale: Implications for Gas Flow in Unconventional Reservoirs

Author(s): Tokunaga, TK; Shen, W; Wan, J; Kim, Y; Cihan, A; Zhang, Y; Finsterle, S | Abstract:

Ion Diffusion Within Water Films in Unsaturated Porous Media

Diffusion is important in controlling local solute transport and reactions in unsaturated soils and geologic formations. Although it is commonly assumed that thinning of water films controls solute diffusion at low water contents, transport under these conditions is not well understood. We conducted experiments in quartz sands at low volumetric water contents (θ) to quantify ion diffusion within adsorbed films. At the lowest water contents, we employed fixed relative humidities to control water films at nm thicknesses. Diffusion profiles for Rb+ and Br- in unsaturated sand packs were measured with a synchrotron X-ray microprobe, and inverse modeling was used to determine effective diffusion coefficients, De, as low as ∼9 × 10-15 m2 s-1 at θ = 1.0 × 10-4 m3 m-3, where the film thickness = 0.9 nm. Given that the diffusion coefficients (Do) of Rb+ and Br- in bulk water (30 °C) are both ∼2.4 × 10-9 m2 s-1, we found the impedance factor f = De/(θDo) is equal to 0.03 ± 0.02 at this very low saturation, in agreement with the predicted influence of interface tortuosity (τa) for diffusion along grain surfaces. Thus, reduced cross-sectional area (θ) and tortuosity largely accounted for the more than 5 orders of magnitude decrease in De relative to Do as desaturation progressed down to nanoscale films.

Experimental and Modeling Study of Methane Adsorption onto Partially Saturated Shales

Author(s): Wang, L; Wan, J; Tokunaga, TK; Kim, Y; Yu, Q | Abstract: ©2018. American Geophysical Union. All Rights Reserved. Shale gas equilibrates through gas-liquid-solid interactions in reservoirs, but the role of moisture is rarely investigated. To determine how adsorbed water influences methane behavior, three carboniferous shale samples from the Qaidam Basin, China, were humidified at five levels up to a relative humidity of 89%, and their methane capacities at pressures up to 12 MPa were studied. The experimental results indicate that two water-related mechanisms, “water blocking for methane transport” and “surface competition for gas-solid interaction,” are primarily responsible for the methane capacity variations. A compositional comparison suggests that a high abundance of clay minerals plays a favorable role in methane migration by retaining water in interlayer pores. Based on the experimental data, an optimized method for calculating the adsorption amount based on an approximation of density distribution is proposed. The model predicts the average thickness of the adsorption layer and the adsorbed methane density distribution on the surface at a given pressure. The methane adsorption layer “thins” in a stepped pattern by up to 45% in the presence of water, with little further change observed at relative humidities greater than 75% in the studied samples.

Supercritical CO2 uptake by nonswelling phyllosilicates

Significance Reliable estimates of geologic carbon storage capacities (needed for policymaking) in both saline aquifers and unconventional gas/oil shales rely on understanding trapping mechanisms. We found that CO2 uptake by muscovite (a common mineral and a conservative proxy for illite) far exceeds the maximum adsorption capacity of its external surface area. Our measurements using different methods collectively suggest that CO2 enters muscovite interlayers without bulk interlayer expansion, contrary to the conventional wisdom that only swelling clays take up CO2 into interlayers. Because the nonswelling illitic clay is the major clay mineral in deep subsurface tight rocks, their excess uptake of CO2 may significantly contribute to CO2 storage capacity and warrants further in-depth studies. Interactions between supercritical (sc) CO2 and minerals are important when CO2 is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO2 alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO2 on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO2), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO2 uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO2 partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO2 to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO2, can increase CO2 storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO2 uptake constitutes a previously unrecognized potential trapping mechanism.

Mesoscale Biotransformations of Uranium: Identifying Sites and Strategies where Reductive Immobilization is Practical

Bioreduction of U in contaminated sediments is an attractive strategy because of its low cost, and because of short-term studies supporting its feasibility. However, any in-situ immobilization approach for U will require assurance of either permanent fixation, or of very low release rates into the biosphere. Our previous long-term (2 years) laboratory experiments have shown that organic carbon (OC) based U(VI) bioreduction to UO2 can be transient even under sustained reducing (methanogenic) conditions. The biogeochemical processes underlying this finding urgently need to be understood. The current research is designed to identify mechanisms responsible for anaerobic U oxidation, and identify conditions that will support long-term stability of bioreduced U. We are investigating: (1) effects of OC concentration and supply rate on remobilization of bioreduced U, (2) the roles of Fe- and Mn-oxides as potential U oxidants in sediments, and (3) the role of microorganisms in U reoxidation, and (4) influences of pH on U(IV)/U(VI) redox equilibrium.