Hongbo Shao

Dissolution and precipitation of clay minerals under geologic CO2 sequestration conditions: CO2-brine-phlogopite interactions.

To ensure efficiency and sustainability of geologic CO2 sequestration (GCS), a better understanding of the geochemical reactions at CO2-water-rock interfaces is needed. In this work, both fluid/solid chemistry analysis and interfacial topographic studies were conducted to investigate the dissolution/precipitation on phlogopite (KMg3Si3AlO10(F,OH)2) surfaces under GCS conditions (368 K, 102 atm) in 1 M NaCl. Phlogopite served as a model for clay minerals in potential GCS sites. During the reaction, dissolution of phlogopite was the predominant process. Although the bulk solution was not supersaturated with respect to potential secondary mineral phases, interestingly, nanoscale precipitates formed. Atomic force microcopy (AFM) was utilized to record the evolution of the size, shape, and location of the nanoparticles. Nanoparticles first appeared on the edges of dissolution pits and then relocated to other areas as particles aggregated. Amorphous silica and kaolinite were identified as the secondary mineral phases, and qualitative and quantitative analysis of morphological changes due to phlogopite dissolution and secondary mineral precipitation are presented. The results provide new information on the evolution of morphological changes at CO2-water-clay mineral interfaces and offer implications for understanding alterations in porosity, permeability, and wettability of pre-existing rocks in GCS sites.

Effects of salinity and the extent of water on supercritical CO2-induced phlogopite dissolution and secondary mineral formation.

To ensure the viability of geologic CO2 sequestration (GCS), we need a holistic understanding of reactions at supercritical CO2 (scCO2)-saline water-rock interfaces and the environmental factors affecting these interactions. This research investigated the effects of salinity and the extent of water on the dissolution and surface morphological changes of phlogopite [KMg2.87Si3.07Al1.23O10(F,OH)2], a model clay mineral in potential GCS sites. Salinity enhanced the dissolution of phlogopite and affected the location, shape, size, and phase of secondary minerals. In low salinity solutions, nanoscale particles of secondary minerals formed much faster, and there were more nanoparticles than in high salinity solutions. The effect of water extent was investigated by comparing scCO2-H2O(g)-phlogopite and scCO2-H2O(l)-phlogopite interactions. Experimental results suggested that the presence of a thin water film adsorbed on the phlogopite surface caused the formation of dissolution pits and a surface coating of secondary mineral phases that could change the physical properties of rocks. These results provide new information for understanding reactions at scCO2-saline water-rock interfaces in deep saline aquifers and will help design secure and environmentally sustainable CO2 sequestration projects.

Trace Metal Source Terms in Carbon Sequestration Environments

Carbon dioxide sequestration in deep saline and depleted oil geologic formations is feasible and promising; however, possible CO(2) or CO(2)-saturated brine leakage to overlying aquifers may pose environmental and health impacts. The purpose of this study was to experimentally define a range of concentrations that can be used as the trace element source term for reservoirs and leakage pathways in risk simulations. Storage source terms for trace metals are needed to evaluate the impact of brines leaking into overlying drinking water aquifers. The trace metal release was measured from cements and sandstones, shales, carbonates, evaporites, and basalts from the Frio, In Salah, Illinois Basin, Decatur, Lower Tuscaloosa, Weyburn-Midale, Bass Islands, and Grand Ronde carbon sequestration geologic formations. Trace metal dissolution was tracked by measuring solution concentrations over time under conditions (e.g., pressures, temperatures, and initial brine compositions) specific to the sequestration projects. Existing metrics for maximum contaminant levels (MCLs) for drinking water as defined by the U.S. Environmental Protection Agency (U.S. EPA) were used to categorize the relative significance of metal concentration changes in storage environments because of the presence of CO(2). Results indicate that Cr and Pb released from sandstone reservoir and shale cap rocks exceed the MCLs by an order of magnitude, while Cd and Cu were at or below drinking water thresholds. In carbonate reservoirs As exceeds the MCLs by an order of magnitude, while Cd, Cu, and Pb were at or below drinking water standards. Results from this study can be used as a reasonable estimate of the trace element source term for reservoirs and leakage pathways in risk simulations to further evaluate the impact of leakage on groundwater quality.

Viability and Metal Reduction of Shewanella oneidensis MR-1 under CO2 Stress: Implications for Ecological Effects of CO2 Leakage from Geologic CO2 Sequestration

To study potential ecological impacts of CO(2) leakage to shallow groundwater and soil/sediments from geologic CO(2) sequestration (GCS) sites, this work investigated the viability and metal reduction of Shewanella oneidensis MR-1 under CO(2) stress. While MR-1 could grow under high-pressure nitrogen gas (500 psi), the mix of 1% CO(2) with N(2) at total pressures of 15 or 150 psi significantly suppressed the growth of MR-1, compared to the N(2) control. When CO(2) partial pressures were over 15 psi, the growth of MR-1 stopped. The reduced bacterial viability was consistent with the pH decrease and cellular membrane damage under high pressure CO(2). After exposure to 150 psi CO(2) for 5 h, no viable cells survived, the cellular contents were released, and microscopy images confirmed significant cell structure deformation. However, after a relatively short exposure (25 min) to 150 psi CO(2), MR-1 could fully recover their growth within 24 h after the stress was removed, and the reduction of MnO(2) by MR-1 was observed right after the stress was removed. Furthermore, MR-1 survived better if the cells were aggregated rather than suspended, or if pH buffering minerals, such as calcite, were present. To predict the cell viability under different CO(2) pressures and exposure times, a two-parameter mathematical model was developed.

Supercritical CO2–brine induced dissolution, swelling, and secondary mineral formation on phlogopite surfaces at 75–95 °C and 75 atm

To safely implement geologic carbon sequestration (GCS), a better understanding of geochemical reactions at supercritical CO2 (scCO2)–brine–clay mineral interfaces is necessary. This work investigated phlogopite dissolution and secondary mineral formation after freshly cleaved (001) surfaces were exposed to scCO2–brine systems. Phlogopite was used as a model clay mineral, and scCO2–1 M NaCl–phlogopite systems at 75 °C and 75 atm were chosen to mimic CO2 storage conditions in deep saline aquifers. Additional experiments were also performed at 95 °C to explore the effect of temperature on phlogopite dissolution. The dissolution activation energies for each element were calculated to be 64.2 kJ mol−1 for Si, 53.6 kJ mol−1 for Mg, and 78.4 kJ mol−1 for Al. Over 43 h of reaction time, the activation energy for K dissolution was calculated to be 35.9 kJ mol−1. A whole-mineral activation energy for phlogopite, 62.5 kJ mol−1, was estimated from the weighted mean values of the activation energies of the framework elements (Al, Si, and Mg). Swelling of the phlogopite outer layers, dissolution pit formation, and precipitation of both illite and amorphous silica were dominant at both temperatures. At 75 °C, normalized volumetric surface coverage (μm3/μm2) was 0.34 ± 0.74 for illite and 0.05 ± 0.90 for amorphous silica nanoparticles.

The Relative Importance of Abiotic and Biotic Transformation of Carbon Tetrachloride in Anaerobic Soils and Sediments

The relative contributions of abiotic and microbial processes and the role of dissolved species in the reductive dechlorination of carbon tetrachloride (CT) by natural soils and sediments were investigated. Microcosms were constructed using soils or sediments and site water from three locations, and then amended with electron acceptors and/or donors to stimulate the growth of iron- and sulfate-reducing bacteria and to promote the formation of minerals that can react with CT. Before spiking with CT, half the replicate microcosms were sterilized in order to measure the rates of abiotic CT transformation without any direct contribution from microbial dechlorination. Abiotic reaction rates were significantly greater than microbial rates for a range of initial CT concentrations, and for both iron- and sulfate-reducing conditions. In most cases, abiotic reaction rates were indistinguishable from total reaction rates (abiotic plus microbial), indicating a negligible microbial contribution to CT transformation. While in most microcosms the soil/sediment acted as the abiotic reductant, under certain conditions the supernatant was more reactive with CT than was the solid phase. For these conditions, we propose that the reactive species in the supernatant consisted of aqueous natural organic matter that underwent reduction or other transformation by S(-II) generated by sulfate-reducing bacteria.

In Situ Spectrophotometric Determination of pH under Geologic CO2 Sequestration Conditions: Method Development and Application

CO(2) injection into deep geologic formations for long-term storage will cause a decrease in aqueous pH due to CO(2) dissolution into reservoir water/brine. Current studies seeking to assess chemical changes under geological CO(2) sequestration (GCS) conditions rely largely on thermodynamic modeling due to the lack of reliable experimental methods. In this work, a spectrophotometric method utilizing bromophenol blue to measure pH in laboratory experiments under GCS-relevant conditions was developed. 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 those from other experimental studies and geochemical models. Measured pH values were generally in agreement with the models, but inconsistencies were present between the models. In situ pH measurements for a basalt rock-CO(2)-brine system were conducted under GCS conditions. The pH increased to 3.52 during a 10-day period due to rock dissolution, compared to pH 2.95 for the CO(2)-brine system without rock. The calculated pH values from geochemical models were 0.22-0.25 units higher than the measured values (assuming all iron in the system was in the form of Fe(2+)). This work demonstrates the use of in situ spectrophotometry for pH measurement under GCS-relevant conditions.

Determination of Organic Partitioning Coefficients in Water-Supercritical CO2 Systems by Simultaneous in Situ UV and Near-Infrared Spectroscopies

CO2 injected into depleted oil or gas reservoirs for long-term storage has the potential to mobilize organic compounds and distribute them between sediments and reservoir brines. Understanding this process is important when considering health and environmental risks, but little quantitative data currently exists on the partitioning of organics between supercritical CO2 and water. In this work, a high-pressure, in situ measurement capability was developed to assess the distribution of organics between CO2 and water at conditions relevant to deep underground storage of CO2. The apparatus consists of a titanium reactor with quartz windows, near-infrared and UV spectroscopic detectors, and switching valves that facilitate quantitative injection of organic reagents into the pressurized reactor. To demonstrate the utility of the system, partitioning coefficients were determined for benzene in water/supercritical CO2 over the range 35-65 °C and approximately 25-150 bar. Density changes in the CO2 phase with increasing pressure were shown to have dramatic impacts on benzenes partitioning behavior. Our partitioning coefficients were approximately 5-15 times lower than values previously determined by ex situ techniques that are prone to sampling losses. The in situ methodology reported here could be applied to quantify the distribution behavior of a wide range of organic compounds that may be present in geologic CO2 storage scenarios.