ARTICLE pubs.acs.org/est
Variation in Energy Available to Populations of Subsurface Anaerobes in Response to Geological Carbon Storage Matthew F. Kirk* Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-0754, United States
bS Supporting Information ABSTRACT: Microorganisms can strongly influence the chemical and physical properties of the subsurface. Changes in microbial activity caused by geological CO2 storage, therefore, have the potential to influence the capacity, injectivity, and integrity of CO2 storage reservoirs and ultimately the environmental impact of CO2 injection. This analysis uses free energy calculations to examine variation in energy available to Fe(III) and SO42 reducers and methanogens because of changes in the bulk composition of brine and shallow groundwater following subsurface CO2 injection. Calculations were performed using data from two field experiments, the Frio Formation experiment and an experiment at the Zero Emission Research and Technology test site. Energy available for Fe(III) reduction increased significantly during CO2 injection in both experiments, largely because of a decrease in pH from near-neutral levels to just below 6. Energy available to SO42 reducers and methanogens varied little. These changes can lead to a greater rate of microbial Fe(III) reduction following subsurface CO2 injection in reservoirs where Fe(III) oxides or oxyhydroxides are available and the rate of Fe(III) reduction is limited by energy available prior to injection.
’ INTRODUCTION Carbon capture and geological storage has been identified as a potential solution to limit the increase of CO2 in the atmosphere.1 In most cases, CO2 would be injected at depths >800 m where it would exist in a supercritical state, which is less dense than oil and saline water.1 A thick low-permeability formation overlying a storage reservoir is necessary to prevent upward migration of the supercritical CO2. CO2 would also be retained in the reservoir by capillary trapping in pores as water migrates into a CO2 plume, dissolution into pore water, and mineral reactions.2 The long-term performance of a storage reservoir will depend on physical and chemical controls within the storage reservoir.3 Biological controls within the reservoir are also important to consider, however, because microorganisms strongly influence the physical and chemical properties of subsurface environments. Microorganisms, for example, can degrade organic pollutants,4 attenuate acid mine water,5 control the distribution of hazardous trace elements,6 and cause the permeability of porous medium to decrease by orders of magnitude.7 The effect that CO2 injection has on biological processes, therefore, could have a direct bearing on the capacity, injectivity, and integrity of a storage reservoir and ultimately the environmental impact of CO2 sequestration. Where supercritical CO2 is present in a storage reservoir, much of the pre-existing microbial community may perish. Highpressure CO2 can kill microorganisms by penetrating cell membranes and extracting intracellular materials.8,9 It may also disrupt r 2011 American Chemical Society
microbial activity by disabling enzymes 10 and interfering with protein synthesis.11 Mitchell et al.,12 for example, exposed planktonic and biofilm cultures in porous medium to supercritical CO2 and observed a thousand and 10-fold reduction in viable cells, respectively, in only 19 min. Microorganisms tolerant to supercritical CO2 may persist, although the degree to which they would remain metabolically active is unclear. Sudden exposure to high levels of dissolved CO2 can also stress and kill cells.13 In contrast to supercritical CO2, however, numerous studies have demonstrated that microorganisms are capable of colonizing aqueous environments with CO2 concentrations that are significantly higher than most natural waters.1417 Although microbial activity would likely persist in water with high dissolved CO2 concentration, geochemical changes triggered by CO2 injection could potentially cause microbial community composition to change. CO2 injection can decrease groundwater pH, drive mineral dissolution and precipitation, increase salinity, and enhance the mobility of organic compounds and inorganic contaminants.1826 These changes could affect microbial communities in many ways. Species that are poorly suited to withstand stresses imposed by increased levels of aqueous CO2 and lower pH will likely decrease in population.27 Received: April 14, 2011 Accepted: July 8, 2011 Revised: July 1, 2011 Published: July 08, 2011 6676
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Figure 1. Variation in alkalinity, Fe(II) concentration, and pH during the Frio and ZERT CO2 injection experiments. Note changes in scale and units of Fe(II) concentration between the graphs. Multiple samples were collected each day during the Frio experiments. Data points for that experiment show average concentrations in samples collected before (n = 3), during (n = 3), and after (n = 2) CO2 injection. The shaded background in each graph corresponds to the CO2 injection interval. The dashed vertical lines in the ZERT graph show the dates of the three largest precipitation events during the experiment. Data were originally published in Kharaka et al. 23,30,32 and are reproduced here with permission.
The form and availability of electron donors and acceptors, as well as the energy available to various metabolic groups, could also be altered. Both changes could potentially influence subsurface microbial reaction rates.28 The objective of this study is to use a bioenergetics analysis to consider the question; how does variation in aqueous chemistry following CO2 injection affect the balance between subsurface anaerobic microbial reactions? Variation in energy available for Fe(III) and SO42 reducers and methanogens during CO2 injection was assessed in two field experiments, the Frio Formation experiment and Zero Emission Research and Technology (ZERT) experiment. Both acetate-oxidizing (acetotrophic) and hydrogen-oxidizing (hydrogentrophic) microbial reactions were considered. The Frio experiment provides an example of CO2 storage in a deep saline reservoir. The ZERT experiment is representative of CO2 leakage from deep storage into a shallow freshwater aquifer. Neither data set from these experiments includes concentrations for all of the chemical species that are involved in the microbial metabolisms considered here. This preliminary analysis, therefore, is limited to examining how changes in the bulk chemical composition of water affect energy available relative to conditions prior to CO2 injection. Lastly, this analysis does not explicitly consider individual microbial species. Instead, functional groups of microorganisms, groups of species that catalyze a specific oxidationreduction reaction in their catabolism, are considered. This analysis focuses on Fe(III)- and SO42-reducing microorganisms and methanogens because these groups commonly dominate subsurface microbial activity.29
’ MATERIALS AND METHODS Frio CO2 Injection Experiment. During October 2004, approximately 1600 t of CO2 were injected into a 24 m thick sandstone horizon in the Oligocene Frio Formation. The injection site is located on the flank of a salt dome in the South Liberty oil field near Dayton, Texas. Downhole and wellhead samples of formation water and gas were collected before, during, and after injection from the injection well and at an observation well 30 m up-dip from the injection well using methods described previously.26 The observation well and injection well are completed from 1528 to 1534 m and 15411546 m depth, respectively, where the formation
has a temperature of about 59 °C. Data for this study came from analysis of samples collected from the monitoring well. Geochemical results from the injection experiment are reported in Kharaka et al.23,30 The formation contains Na CaCl-type brine with 93 g 3 L1 of total dissolved solids (TDS) content. CO2 injection created dramatic changes in pH and Fe and alkalinity concentration (Figure 1). Before and after CO2 injection, pH decreased from values as high as 7.2 to 5.7, Fe increased from 0.5 to 8 mM, and alkalinity increased from 2 to 20 mM as HCO3. pH measurements were performed on samples at the surface after they had lost some CO2. Geochemical modeling calculations indicate that the pH of the brine after CO2 injection was as low as 4.7 prior to degassing.31 Acetate content also varied during the experiment, although much less than pH, Fe, and alkalinity. Acetate levels measured in the monitoring well averaged 6 μM before the experiment and 8 μM after injection was completed. ZERT CO2 Injection Experiment. From July 9 to August 7, 2008, food-grade CO2 was injected at a rate of about 300 kg/day into poorly consolidated Cenozoic sandy gravel capped by about 1 m of silt, clay, and top soil. The injection site is located in the Gallatin Valley and is maintained by Montana State University. The injection well is a perforated horizontal pipe 22.3 m below the surface. Several observation wells were sampled during the experiment. Data used in this study came from analysis of samples collected from a single observation well 1.5 m deep located about 1 m away from the injection well (well 2B; 32). The temperature of groundwater from the observation well averaged 10.5 °C during the experiment. Geochemical results from the ZERT injection experiment are presented in Kharaka et al.32 Shallow groundwater at the site is CaMgNaHCO3 type water with 600 mg 3 L1 TDS. Similar to the shifts in groundwater composition observed during the Frio experiment, CO2 injection affected pH and Fe and alkalinity concentration during the experiment (Figure 1). CO2 injection caused pH to decrease from about 7 to as low as 5.6, Fe increased from 0.09 to 21 μM, and alkalinity increased from 7 to 22 mM as HCO3. Noticeable changes to Ca, Mg, and Mn concentrations also occurred following CO2 injection. Free Energy Calculations. The energy available (ΔGA) for a microbial reaction is the negative of the free energy change of that 6677
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Table 1. Net Reactions for Acetotrophic and Hydrogentrophic Fe(III) and SO42 Reduction and Methanogenesisa Fe(III) reduction
8 goethite + CH3COO + 17 H+ f 2 CO2(aq) + 14 H2O + 8 Fe2+ 8 goethite + 4 H2(aq) + 16 H+ f 16 H2O + 8 Fe2+ 4 hematite + CH3COO + 17 H+ f 2 CO2(aq) + 10 H2O + 8 Fe2+ 4 hematite + 4 H2(aq) + 16 H+ f 12 H2O + 8 Fe2+ 4 magnetite + CH3COO + 25 H+ f 2 CO2(aq) + 14 H2O + 12 Fe2+ 4 magnetite + 4 H2(aq) + 24 H+ f 16 H2O + 12 Fe2+ SO42 reduction SO42 + CH3COO + 3 H+ f 2 CO2(aq) + 2 H2O + H2S(aq) SO42 + 4 H2(aq) + 2 H+ f 4 H2O + H2S(aq) methanogenesis CH3COO + H+ f CO2(aq) + CH4(aq) CO2(aq) + 4 H2(aq) f 2 H2O + CH4(aq) a
Reactions are balanced on the basis of an eight electron transfer (1 CH3COO or 4 H2).
reaction (ΔGr). Energy available in kJ 3 mol1 for acetotrophic and hydrogentrophic Fe(III) and SO42 reduction and methanogenesis (Table 1) before, during, and after each experiment was calculated as follows: Y ðγi mi Þvi ð1Þ ΔGA ¼ ΔGr ¼ ½ΔG°T þ RT ln i
where ΔGT° is the standard Gibbs free-energy change for reaction r at temperature T (K), R represents the gas constant (kJ 3 mol1 3 K1), γi and mi are the activity coefficient (molal1) and molality of the ith chemical species in the reaction, and vi is the stoichiometric coefficient of that species, which is positive for products and negative for reactants. Standard Gibbs free energy values at in situ temperature were calculated using the Geochemists Workbench software package,33 version 8.0.10, and the LLNL (Lawrence Livermore National Laboratory) thermodynamic database.34 Activities were calculated from chemical data with Geochemists Workbench software using an extended form of the DebyeH€uckel equation, the B-dot equation.35 Although virial methods are better suited for modeling brine speciation, the B-dot equation was used because data for extending virial methods beyond 25 °C are limited to relatively simple chemical systems 36 that do not include all of the chemical species required for this study. Successful modeling of Frio Formation chemistry using the B-dot equation in a previous study 31 supports using this approach. Nonetheless, to minimize this source of error, the results of each calculation are presented normalized to the amount of energy available prior to CO2 injection. To normalize the results, the quantity of energy available for each functional group before CO2 injection was subtracted from each value calculated for conditions after CO2 injection began 2 ΔGCO ΔGinitial ¼ ΔGnA A A
ð2Þ
where the superscript “CO2” designates each value calculated during or after CO2 injection began, “initial” designates the value calculated prior to injection, and “n” represents the normalized value. Calculations results provided in the Supporting Information
demonstrate that this normalization procedure minimized error due to activity modeling. As a result of this normalization, the results of the calculations show how energy available changed during the experiment and should not be interpreted in an absolute sense. In addition to minimizing error associated with activity modeling, normalization also eliminates the need to account for pressure, which has a relatively small impact on free energy values for aqueous-phase reactions.37 Calculations were performed at standard pressure, which is appropriate for the ZERT experiment but much lower than the total pressure measured during the Frio experiment (about 150 bar). Normalization cancels out corrections for in situ pressures because total pressure was largely invariant in each experiment.31,32 This analysis focuses on oxidation of acetate and H2, two of the most common electron donors used by microorganisms in the subsurface.38 In doing so, both organotrophic and lithotrophic reactions, respectively, are considered. Measured acetate concentrations were used in the analysis of the Frio experiment. Acetate levels were not measured during the ZERT experiment, however, and H2(aq) content was not reported for either injection experiment. Instead, arbitrarily small values for those electron donors were assumed. Energy was calculated at 1 nM H2(aq) for analysis of both experiments and at 1 μM acetate for analysis of the ZERT experiment. In addition to electron donor concentrations, CH4(aq) levels are unavailable. For the Frio experiment, CH4(aq) content was assumed to have remained at 45 mM, the CH4(aq) concentration in the reservoir prior to the experiment.23 For the ZERT experiment, an arbitrary small value of 1 μM CH4(aq) was used. Lastly, detection limit concentrations were assumed for H2S and Fe(II) when the concentration of those chemical species was below the detection limit of the methods used in each experiment. Uncertainty regarding these chemical species is unavoidable in this study. However, because the results are normalized, they are insensitive to the arbitrary electron donor concentrations used in the calculations. Increasing H2(aq) concentration by a factor of 100, for example, does not change the results. These results are useful, therefore, to examine how energy changed relative to the amount of energy available prior to CO2 injection for each functional group as a result of changes in the bulk composition of the water during each experiment. The analysis of Fe(III) reduction is generalized by considering three potential sources of Fe(III) in the calculations: goethite (R-FeOOH), hematite (R-Fe2O3), and magnetite (Fe3O4). Fe oxides and oxyhydroxides are widespread in natural environments 39 and can persist in anoxic environments.40 The Frio Formation contains minor amounts of Fe(III) oxyhydroxides 26 and hematite is common in U.S. Gulf Coast sandstone formations.31 Sand at the ZERT test site contains about 40% magnetite and oxyhydroxides are also present.32 Poorly crystalline Fe(III) oxides and oxyhydroxides, such as ferrihydrite, may also be present in the shallow sediment at the ZERT site. However, oxides and oxyhydroxides with greater stability, such as those considered here, are more representative of Fe(III) minerals in the deep subsurface, where reaction associated with CO2 storage will largely occur.
’ RESULTS AND DISCUSSION Frio Experiment. The results of the calculations demonstrate that CO2 injection created conditions that were more favorable for Fe(III) reduction in the Frio Formation. For both 6678
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Figure 2. Variation in energy available (ΔGA) for acetotrophic and hydrogentrophic Fe(III) and SO42 reduction and methanogenesis during the Frio experiment relative to conditions before CO2 injection. Multiple samples were collected each day. Data points show the average of values calculated from chemical analyses of samples collected before (n = 3), during (n = 3), and after (n = 2) CO2 injection. The shaded background in each graph corresponds to the CO2 injection interval.
acetotrophic and hydrogentrophic reactions, the energy available for Fe(III) reduction in the Frio Formation significantly increased (i.e., ΔGr decreased) for all three Fe(III) minerals considered (Figure 2). Energy available for reduction of Fe(III) in magnetite increased the most while energy available for reduction of Fe(III) in goethite and hematite changed an identical amount. This result primarily reflects the decrease in pH caused by dissolution of CO2 into the brine (Figure 1). Fe(III) reduction consumes a large number of H+ (Table 1). As a result, energy available for Fe(III) reduction increases sharply as pH decreases, a relationship recently noted by Bethke et al. 40 In contrast to pH, increases in CO2(aq) and Fe(II) levels during the experiment worked to lower the energy yield of hydrogentrophic and acetotrophic Fe(III) reduction, which produce Fe(II) and both Fe(II) and CO2, respectively. Fe(II) and CO2 content increased by about a factor of 10 during the experiment, approximately the same order of magnitude as the increase in the H+ concentration. These changes in brine composition had less impact on energy yield than pH, however, because fewer CO2 molecules and Fe(II) ions are produced than H+ consumed in the net reactions considered for Fe(III) reducers. In contrast to Fe(III) reduction, energy available for SO42 reduction and methanogenesis in the Frio Formation changed little (Figure 2). Reactions for both groups contain H+ and CO2 molecules. Energy available for the reactions does not vary sharply with pH and CO2 content, however, because few H+ and CO2 molecules are involved. Regardless of electron acceptor, energy available for hydrogentrophic reactions increased slightly over that available for acetotrophic reactions on the basis of an eight electron transfer (i.e., 1 acetate or 4 H2) (Figure 2). Unlike acetotrophic reactions, hydrogentrophic reactions do not produce CO2. Hydrogentrophic reactions became more favorable than acetotrophic reactions, therefore, because an increase in CO2 concentration in response to CO2 injection did not lower the energy available for those reactions. Furthermore, because hydrogentrophic methanogens consume CO2, an increase in CO2
concentration works to increase the energy available to that functional group. ZERT Experiment. Energy available for the functional groups considered varied considerably more during the ZERT experiment than the Frio Experiment, reflecting the occurrence of precipitation events, as discussed below. Nonetheless, variation in the bulk chemical composition of groundwater during the ZERT experiment also benefitted microbial Fe(III) reduction more than SO42 reduction or methanogenesis. Energy available for Fe(III) reduction increased for all three minerals considered during CO2 injection and was largely invariant for SO42 reduction and methanogenesis (Figure 3). Furthermore, hydrogentrophic reactions were again favored slightly over acetotrophic reactions. Like the thermodynamic findings from the Frio experiment, these results primarily reflect variation in pH during the experiment. Within one week after the injection began, pH had dropped from about 7 to below 6, where it remained until after CO2 injection ceased (Figure 1). Energy available to Fe(III) reducers also changed as a result of precipitation events, however. During the experiment, there were multiple precipitation events that increased the elevation of the water table at the site and altered aqueous chemistry.32 The largest events occurred on July 18, July 22, and August 8. Recharge during these events transported O2 into the shallow aquifer, which reacted with Fe(II) and caused Fe(II) levels to fall sharply. This decrease in Fe(II) content, coupled with an acidic pH during the injection, allowed energy available for Fe(III) reduction to increase even further than if Fe(II) concentration remained elevated during the experiment. Such events are less likely at greater depth but represent an important consideration for the impact of CO2 sequestration on microbial communities in near surface aquifers. Rate of Microbial Reaction. Factors that control the rate of microbial respiration include the kinetics of electron donation and acceptance, as well as the amount of energy available for reaction in the environment.41 The rate of respiration for microorganisms that utilize reactions having a strong thermodynamic driving force is dominated by kinetic controls.42 Energy available for O2 respiration and denitrification, for example, is 6679
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Figure 3. Variation in energy available (ΔGA) for acetotrophic and hydrogentrophic Fe(III) and SO42 reduction and methanogenesis during the ZERT experiment relative to conditions before CO2 injection. Data points show values calculated for individual samples. The shaded background in each graph corresponds to the CO2 injection interval. The dashed vertical lines show the dates of the three largest precipitation events during the experiment.
large even when O2 and NO3 concentrations are very small. However, for microorganisms that catalyze reactions with less effective thermodynamic driving forces, such as those considered in this study, the amount of energy available can be the dominant control on reaction rate.42 This relationship is particularly relevant in energy-limited geochemical environments, where microbial reactions are often close to equilibrium.28 The results presented here show that energy available for microbial Fe(III) reduction can increase considerably as a result of CO2 injection into both brine and shallow groundwater. The rate of microbial Fe(III) reduction could increase, therefore, where this occurs. During the Frio and ZERT experiments, energy available for Fe(III) reducers increased because pH decreased. Increases in the rate of Fe(III) reduction, therefore, may be limited to zones in deep storage reservoirs and overlying aquifers, if CO2 leaks from storage, where buffering is unable to prevent a decrease in pH. In either setting, furthermore, Fe(III) oxides or oxyhydroxides would need to be available and the rate of microbial Fe(III) reduction would need to be limited by energy available prior to CO2 injection for this shift in energy available to trigger an increase in the rate of Fe(III) reduction. Where CO2 injection does cause an increase in the rate of Fe(III) reduction, the change would be temporary. Unless Fe(III) is resupplied, Fe(III) would eventually become depleted, forcing a decline in Fe(III) reducer population size. This limitation may explain why Oppermann et al.16 observed a lower abundance of Fe(III) reducers in the soil of a natural CO2 vent compared to a nearby control soil. The high CO2 flux through the vent (200 kg 3 day1) may have initially enhanced the rate of microbial Fe(III) reduction. However, the relatively low mass of Fe(III) oxide remaining in the vent soil after >75 years of exposure 43 limits the amount of Fe(III) reduction the system can presently support.16 Abiological dissolution triggered by high CO2 levels could also be responsible for Fe(III) depletion from the soil, particularly if poorly crystalline Fe(III) oxides/oxyhydroxides were present. In contrast to Fe(III) reducers, SO42 reducers and methanogens appear to gain little from changes in the bulk chemistry of water following CO2 injection. Although energy available to these functional groups changed little, their activity could eventually decrease where Fe(III) reducers consume electron
donors at an increased rate. Recent findings by Morozova et al. 44 showed that SO42 reducers were able to adapt to conditions within five months after CO2 was injected into a deep saline reservoir but found no evidence of a significant decline in SO42 reducer abundance. Before injection, SO42- reducers accounted for as much as 24% of the active cells observed in brine samples using fluorescence in situ hybridization (FISH). Following a N2 lift and CO2 injection, SO42 reducer abundance fell below detection before eventually rebounding to account for 19% of the active cells. These results may have been affected by the presence of drilling mud before the N2 lift, which appears to have stimulated SO42 reducing activity and precipitation of Fesulfide solids.45 Whether Fe(III) reducer abundance varied during the CO2 injection experiment is unclear because Fe(III)-reducing species were not quantified. In addition to creating an environment that is more favorable for Fe(III) reduction, this study shows that the shifts in geochemistry following CO2 injection favor lithotrophic reactions over organotrophic reactions. This energy advantage is very small. Whether or not any significant shift in microbial reaction rates would occur as a result, therefore, is unclear. Variation in electron donor concentration triggered by CO2 injection, such as that observed during CO2 injection for enhanced oil recovery46 and storage in coal beds,47 may have more potential to affect the balance between organotrophic and lithotrophic reactions in CO2 reservoirs. Implications. Over long periods of time following injection, solubility trapping is expected to become the primary mechanism of CO2 retention in the subsurface.1 How changes to the geochemical environment due to solubility trapping affect what microbial communities are doing, therefore, may have long-term implications for the physical and chemical properties of storage reservoirs and overlying freshwater aquifers, where leakage occurs. This thermodynamic analysis provides only a first approximation of the impact that CO2 injection will have on the balance of microbial metabolisms in the subsurface. Many other factors can also influence microbial reaction rates. Additional studies are warranted to examine the interplay between geological CO2 storage and microbiology further. Nonetheless, the results underscore the need to characterize microbial communities in CO2 storage reservoirs prior to CO2 injection and to provide 6680
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Environmental Science & Technology detailed analysis of reservoir geochemistry including analysis of formation mineralogy and the concentrations of chemical species that are involved in microbial reactions. The results of this study imply additional pathways for enhanced trace element mobility following CO2 injection. Previous studies showed that abiological reactions following CO2 injection can enhance the mobility of hazardous trace elements.19,21,24 This potential consequence of CO2 injection becomes particularly concerning if CO2 is able to leak from storage reservoirs into overlying freshwater aquifers. An enhanced rate of microbial Fe(III) reduction would provide a biological pathway for trace element mobilization by enhancing dissolution Fe(III) oxide and oxyhydroxide minerals, which can adsorb large amounts of trace elements. Where the rate of Fe(III) reduction increases at the expense of SO42- reduction, furthermore, formation of sulfide minerals would decrease. Diminished sulfide mineralization would also favor enhanced trace element mobility because sulfides such as pyrite can sequester a wide variety of trace elements.48,49 Lastly, the findings of this study indicate that Fe(III) reducers or other metabolic groups that are strongly acid consuming may provide a logical choice for biological strategies to enhance CO2 storage. Stimulating microbial biomass production could provide a strategy for enhancing CO2 storage by limiting permeability in caprocks and providing surfaces that facilitate calcite precipitation.5052 Stimulated microbial activity can also work to increase pH, which increases CO2 solubility and the saturation state of calcite.53 The success of these strategies may be improved if the geochemical environment becomes more favorable for the microorganisms that are used in the strategy. As an added advantage, furthermore, greater Fe(II) production by Fe(III) reducers could ultimately enhance CO2 mineral trapping by siderite (FeCO3) precipitation where pH is favorable.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information about the normalization procedure used in this study is available as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; phone: 505-844-2094 (office); fax: 505-844-7354.
’ ACKNOWLEDGMENT I thank Susan Altman, Randall Cygan, Qusheng Jin, Hongkyu Yoon, Jay Santillan, and two anonymous reviewers for providing valuable suggestions that improved this paper and Yousif Kharaka and Susan Hovorka for assisting with this work. This material is based upon work supported as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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