Brian R. Ellis

Dissolution potential of SO2 Co-injected with CO2 in geologic sequestration.

Sulfur dioxide is a possible co-injectant with carbon dioxide in the context of geologic sequestration. Because of the potential of SO2 to acidify formation brines, the extent of SO2 dissolution from the CO2 phase will determine the viability of co-injection. Pressure-, temperature-, and salinity-adjusted values of the SO2 Henrys Law constant and fugacity coefficient were determined. They are predicted to decrease with depth, such that the solubility of SO2 is a factor of 0.04 smaller than would be predicted without these adjustments. To explore the potential effects of transport limitations, a nonsteady-state model of SO2 diffusion through a stationary cone-shaped plume of supercritical CO2 was developed. This model represents an end-member scenario of diffusion-controlled dissolution of SO2, to contrast with models of complete phase equilibrium. Simulations for conditions corresponding to storage depths of 0.8-2.4 km revealed that after 1000 years, 65-75% of the SO2 remains in the CO2 phase. This slow release of SO2 would largely mitigate its impact on brine pH. Furthermore, small amounts of SO2 are predicted to have a negligible effect on the critical point of CO2 but will increase phase density by as much as 12% for mixtures containing 5% SO2.

Suggested Reporting Parameters for Investigations of Wastewater from Unconventional Shale Gas Extraction

from Unconventional Shale Gas Extraction Kyle J. Bibby,†,‡ Susan L. Brantley, Danny D. Reible, Karl G. Linden, Paula J. Mouser, Kelvin B. Gregory, Brian R. Ellis, and Radisav D. Vidic*,† †Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ‡Department of Computational and Systems Biology, University of Pittsburgh Medical School, Pittsburgh, Pennsylvania 15261, United States Earth and Environmental Systems Institute and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, United States Department of Civil and Environmental Engineering, Texas Tech University, Lubbock, Texas 79409, United States Department of Civil, Environmental and Architectural Engineering, University of Colorado, UCB 428, Boulder, Colorado 80309, United States Department of Civil, Environmental and Geodetic Engineering, The Ohio State University, Columbus, Ohio 43210, United States Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States

Comparative Human Toxicity Impact of Electricity Produced from Shale Gas and Coal

The human toxicity impact (HTI) of electricity produced from shale gas is lower than the HTI of electricity produced from coal, with 90% confidence using a Monte Carlo Analysis. Two different impact assessment methods estimate the HTI of shale gas electricity to be 1-2 orders of magnitude less than the HTI of coal electricity (0.016-0.024 DALY/GWh versus 0.69-1.7 DALY/GWh). Further, an implausible shale gas scenario where all fracturing fluid and untreated produced water is discharged directly to surface water throughout the lifetime of a well also has a lower HTI than coal electricity. Particulate matter dominates the HTI for both systems, representing a much larger contribution to the overall toxicity burden than VOCs or any aquatic emission. Aquatic emissions can become larger contributors to the HTI when waste products are inadequately disposed or there are significant infrastructure or equipment failures. Large uncertainty and lack of exposure data prevent a full risk assessment; however, the results of this analysis provide a comparison of relative toxicity, which can be used to identify target areas for improvement and assess potential trade-offs with other environmental impacts.

Roles of transport limitations and mineral heterogeneity in carbonation of fractured basalts

Basalt formations could enable secure long-term carbon storage by trapping injected CO2 as stable carbonates. Here, a predictive modeling framework was designed to evaluate the roles of transport limitations and mineral spatial distributions on mineral dissolution and carbonation reactions in fractured basalts exposed to CO2-acidified fluids. Reactive transport models were developed in CrunchTope based on data from high-temperature, high-pressure flow-through experiments. Models isolating the effect of transport compared nine flow conditions under the same mineralogy. Heterogeneities were incorporated by segmenting an actual reacted basalt sample, and these results were compared to equivalent flow conditions through randomly generated mineral distributions with the same bulk composition. While pure advective flow with shorter retention times promotes rapid initial carbonation, pure diffusion sustains mineral reactions for longer time frames and generates greater net carbonate volumes. For the same transport conditions and bulk composition, exact mineral spatial distributions do not impact the amount of carbonation but could determine the location by controlling local solution saturation with respect to secondary carbonates. In combination, the results indicate that bulk mineralogy will be more significant than small-scale heterogeneities in controlling the rate and extent of CO2 mineralization, which will likely occur in diffusive zones adjacent to flow paths or in dead-end fractures.

Estimating Radium Activity in Shale Gas Produced Brine

Shale gas reservoir-produced brines may contain elevated levels of naturally occurring radioactive material, including Ra-226 and Ra-228, which come from the decay of U-238 and Th-232 in shale. While the total Ra activity in shale gas wastewaters can vary by over 3 orders of magnitude, the parent radionuclides tend to only vary by 1 order of magnitude. The extent of Ra mobilization from the shale into produced brines is thought to be largely controlled by adsorption/desorption from the shale, which is influenced by shale cation exchange capacity (CEC) and reservoir brine salinity, often reported as the total dissolved solids (TDS). To determine how these factors lead to such large variation in Ra activity of produced brines, the U content and CEC of shale samples from the Antrim and Utica-Collingwood shales in Michigan and the Marcellus shale in Pennsylvania were evaluated. Analysis of produced brine from 17 Antrim shale gas wells was then used to develop an empirical relationship between Ra-226 activity and produced water TDS for a given U content of the shale. This correlation will provide an a priori estimate of the expected Ra activity of a produced brine from a given shale gas play when the brine salinity and U content of the shale are known. Such information can serve as a guide for optimal wastewater treatment and disposal strategies prior to any drilling activity, thereby reducing risks associated with elevated Ra activity in shale gas wastewaters.

Permanent CO2 Trapping through Localized and Chemical Gradient-Driven Basalt Carbonation

Recent laboratory and field studies have demonstrated that basalt formations may present one of the most secure repositories for anthropogenic CO2 emissions through carbon mineralization. In this work, a series of high-temperature, high-pressure core flooding experiments was conducted to investigate how transport limitations, reservoir temperature, and brine chemistry impact carbonation reactions following injection of CO2-rich aqueous fluids into fractured basalts. At 100 °C and 6.3 mM [NaHCO3], representative of typical reservoir conditions, carbonate precipitates were highly localized on reactive mineral grains contributing key divalent cations. Geochemical gradients promoted localized reaction fronts of secondary precipitates that were consistent with 2D reactive transport model predictions. Increasing [NaHCO3] to 640 mM dramatically enhanced carbonation in diffusion-limited zones, but an associated increase in clays filling advection-controlled flow paths could ultimately obstruct flow and limit sequestration capacity under such conditions. Carbonate and clay precipitation were further enhanced at 150 °C, reducing the pre-reaction fracture volume by 48% compared to 35% at 100 °C. Higher temperature also produced more carbonate-driven fracture bridging, which generally increased with diffusion distance into dead-end fractures. In combination, the results are consistent with field tests indicating that mineralization will predominate in buffered diffusion-limited zones adjacent to bulk flow paths and that alkaline reservoirs with strong geothermal gradients will enhance the extent of carbon trapping.

Emerging investigators series: using an analytical solution approach to permit high volume groundwater withdrawals

Sustainable water management is paramount to ensuring continued access to fresh water resources. Some states have chosen to use analytical solutions to predict pumping-induced drawdowns and the reduction in groundwater baseflow to streams in an effort to predict negative impacts associated with high volume groundwater withdrawals (HVGWs). In line with this approach, the State of Michigan has developed the Water Withdrawal Assessment Tool (WWAT), which estimates streamflow depletion to evaluate whether a proposed HVGW activity will have an adverse impact on stream ecology. To assess the tools performance, this study compared calculations for streamflow depletion estimated using the Hunt (1999) solution, as implemented in the WWAT, with those of a numerical groundwater flow model developed in MODFLOW for two different locations in Michigan where HVGW wells have been permitted. In addition, sensitivity and uncertainty analyses were conducted. The results showed that the WWAT, in general, provides a conservative estimate of stream depletion. However, to obtain a more accurate estimate, the type of aquifer (unconfined versus semi-confined) needs to be taken into account. The most critical parameters are the storativity, S, and the streambed conductance, λ. Since S has a fixed value of 0.01 in the WWAT, the role of streambed conductance becomes paramount. Given the paucity of information regarding λ, its estimation merits additional scrutiny.