Reservoir scale reactive-transport modeling of a buoyancy-controlled CO2 plume with impurities (SO2, NO2, O2)

A demonstration project for the geological storage of CO2 is currently being considered in the deep Precipice Sandstone formation of the Surat Basin, Queensland, Australia. Because of the presence of potential fresh water resources in this formation, a reservoir-scale two-dimensional reactive-transport model was developed to assess temporal and spatial changes in water quality imposed by co-injecting CO2 with SO2, NO2, and O2 at this location. The model shows that because the injection rate is relatively low (60,000 tons/year), flow is buoyancy-dominated and under these conditions the predicted CO2 flow pattern is quite sensitive to fine-scale heterogeneities and the resolution of the numerical mesh. The model also shows that SO2 and NO2 readily partition into the aqueous phase in close vicinity of their injection point, lowering pH somewhat beyond the acidification from CO2 dissolution. Only O2 under redox disequilibrium conditions is modeled to persist in the CO2 plume away from the injection point, however at sub-ppm levels. This modeling effort demonstrates acidification near the wellbore due to the preferential stripping of gas impurities, and accumulation of CO2 around a lithostratigraphic boundary above the target formation, where relatively rapid mineral dissolution (muscovite, chlorite and calcite) and precipitation (ankerite, kaolinite and chalcedony) occur

A Raman spectroscopic study of arsenite and thioarsenite species in aqueous solution at 25°C

The Raman spectra of thioarsenite and arsenite species in aqueous solution were obtained at room temperature. Solutions at constant ΣAs + ΣS of 0.1 and 0.5 mol kg(-1 )were prepared with various ΣS/ΣAs ratios (0.1–9.0) and pH values (~7–13.2). Our data suggest that the speciation of As under the conditions investigated is more complicated than previously thought. The Raman measurements offer evidence for at least six separate S-bearing As species whose principal bands are centered near 365, 385, 390, 400, 415 and 420 cm(-1). The data suggest that at least two different species may give rise to bands at 385 cm(-1), bringing the probable minimum number of species to seven. Several additional species are possible but could not be resolved definitively. In general, the relative proportions of these species are dependent on total As concentration, ΣS/ΣAs ratio and pH. At very low ΣS/ΣAs ratios we also observe Raman bands attributable to the dissociation products of H(3)AsO(3)(aq). Although we were unable to assign precise stoichiometries for the various thioarsenite species, we were able to map out general pH and ΣS/ΣAs conditions under which the various thioarsenite and arsenite species are predominant. This study provides a basis for more detailed Raman spectroscopic and other types of investigations of the nature of thioarsenite species

Calculation of the visible-UV absorption spectra of hydrogen sulfide, bisulfide, polysulfides, and As and Sb sulfides, in aqueous solution

Recently we showed that visible-UV spectra in aqueous solution can be accurately calculated for arsenic (III) bisulfides, such as As(SH)(3), As(SH)(2)S(- )and their oligomers. The calculated lowest energy transitions for these species were diagnostic of their protonation and oligomerization state. We here extend these studies to As and Sb oxidation state III and v sulfides and to polysulfides S(n)(2-), n = 2–6, the bisulfide anion, SH(-), hydrogen sulfide, H(2)S and the sulfanes, S(n)H(2), n = 2–5. Many of these calculations are more difficult than those performed for the As(iii) bisulfides, since the As and Sb(v) species are more acidic and therefore exist as highly charged anions in neutral and basic solutions. In general, small and/or highly charged anions are more difficult to describe computationally than larger, monovalent anions or neutral molecules. We have used both Hartree-Fock based (CI Singles and Time-Dependent HF) and density functional based (TD B3LYP) techniques for the calculations of absorption energy and intensity and have used both explicit water molecules and a polarizable continuum to describe the effects of hydration. We correctly reproduce the general trends observed experimentally, with absorption energies increasing from polysulfides to As, Sb sulfides to SH(- )to H(2)S. As and Sb(v) species, both monomers and dimers, also absorb at characteristically higher energies than do the analogous As and Sb(III)species. There is also a small reduction in absorption energy from monomeric to dimeric species, for both As and Sb III and v. The polysufides, on the other hand, show no simple systematic changes in UV spectra with chain length, n, or with protonation state. Our results indicate that for the As and Sb sulfides, the oxidation state, degree of protonation and degree of oligomerization can all be determined from the visible-UV absorption spectrum. We have also calculated the aqueous phase energetics for the reaction of S(8 )with SH(- )to produce the polysulfides, S(n)H(-), n = 2–6. Our results are in excellent agreement with available experimental data, and support the existence of a S(6 )species

Speciation of arsenic in sulfidic waters

Formation constants for thioarsenite species have been determined in dilute solutions at 25°C, ΣH(2)S from 10(-7.5 )to 10(-3.0 )M, ΣAs from 10(-5.6 )to 10(-4.8 )M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)(3)(0), and a mononuclear thioarsenite with an S/As ratio of 3:1. Thioarsenic species with S/As ratios of 1 : 1,2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10(-4.3 )M at neutral pH. Conversion from neutral As(OH)(3)(0 )to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions

Reactive transport modeling: A key performance assessment tool for the geologic disposal of nuclear waste

The disposal of spent nuclear fuel and high-level radioactive waste in the subsurface represents one of the greatest challenges for the geosciences. Most disposal strategies rely on a multiple barrier system, consisting of both natural and engineered materials, to prevent or delay the contact of groundwater with the waste and radionuclide release to the environment. Reactive transport models have been central to understanding and assessing how thermal, hydrological, and geochemical processes are coupled in these containment barriers, which are expected to experience a range of temperatures and geochemical conditions, yet, must maintain their integrity for millions of years

Reservoir scale reactive-transport modeling of a buoyancy-controlled CO2 plume with impurities (SO2, NO2, O2)

A demonstration project for the geological storage of CO2 is currently being considered in the deep Precipice Sandstone formation of the Surat Basin, Queensland, Australia. Because of the presence of potential fresh water resources in this formation, a reservoir-scale two-dimensional reactive-transport model was developed to assess temporal and spatial changes in water quality imposed by co-injecting CO2 with SO2, NO2, and O2 at this location. The model shows that because the injection rate is relatively low (60,000 tons/year), flow is buoyancy-dominated and under these conditions the predicted CO2 flow pattern is quite sensitive to fine-scale heterogeneities and the resolution of the numerical mesh. The model also shows that SO2 and NO2 readily partition into the aqueous phase in close vicinity of their injection point, lowering pH somewhat beyond the acidification from CO2 dissolution. Only O2 under redox disequilibrium conditions is modeled to persist in the CO2 plume away from the injection point, however at sub-ppm levels. This modeling effort demonstrates acidification near the wellbore due to the preferential stripping of gas impurities, and accumulation of CO2 around a lithostratigraphic boundary above the target formation, where relatively rapid mineral dissolution (muscovite, chlorite and calcite) and precipitation (ankerite, kaolinite and chalcedony) occur

Influence of Agricultural Managed Aquifer Recharge (AgMAR) and Stratigraphic Heterogeneities on Nitrate Reduction in the Deep Subsurface

Agricultural managed aquifer recharge (AgMAR) is a strategy whereby surface water is used to intentionally flood croplands to recharge underlying aquifers. However, nitrate (NO3−) contamination in agriculturally intensive regions poses a threat to groundwater resources under AgMAR. We use a reactive transport model to understand the effects of AgMAR management strategies (i.e., by varying the frequency, duration between flooding events, and amount of water) on NO3− leaching to groundwater under different stratigraphic configurations and antecedent moisture conditions. We examine the potential of denitrification and nitrogen retention in deep vadose zone sediments (∼15 m) using variable AgMAR application rates on two-dimensional representations of differently textured soils, soils with discontinuous bands/channels, and with preferential flow paths characteristic of agricultural fields. Simulations indicate finer textured sediments, alone or embedded within/adjacent to high flow regions, are important reducing zones providing conditions needed for denitrification. Simulation results suggest that applying water all-at-once rather than in increments transports higher concentrations of NO3− deeper into the profile, which may exacerbate groundwater quality. This transport into deeper depths can be aggravated by wetter antecedent soil moisture conditions. However, applying water all-at-once also increases denitrification within the vadose zone by promoting anoxic conditions. We conclude that AgMAR can be designed to enhance denitrification in the subsurface and reduce NO3− leaching to groundwater, while specifically accounting for lithologic heterogeneity, antecedent soil moisture conditions, and depth to the water table. Our findings are potentially relevant to other systems that experience flooding inundation such as floodplains and dedicated recharge basins

Influence of Agricultural Managed Aquifer Recharge (AgMAR) and Stratigraphic Heterogeneities on Nitrate Reduction in the Deep Subsurface

Agricultural managed aquifer recharge (AgMAR) is a strategy whereby surface water is used to intentionally flood croplands to recharge underlying aquifers. However, nitrate (NO3−) contamination in agriculturally intensive regions poses a threat to groundwater resources under AgMAR. We use a reactive transport model to understand the effects of AgMAR management strategies (i.e., by varying the frequency, duration between flooding events, and amount of water) on NO3− leaching to groundwater under different stratigraphic configurations and antecedent moisture conditions. We examine the potential of denitrification and nitrogen retention in deep vadose zone sediments (∼15 m) using variable AgMAR application rates on two-dimensional representations of differently textured soils, soils with discontinuous bands/channels, and with preferential flow paths characteristic of agricultural fields. Simulations indicate finer textured sediments, alone or embedded within/adjacent to high flow regions, are important reducing zones providing conditions needed for denitrification. Simulation results suggest that applying water all-at-once rather than in increments transports higher concentrations of NO3− deeper into the profile, which may exacerbate groundwater quality. This transport into deeper depths can be aggravated by wetter antecedent soil moisture conditions. However, applying water all-at-once also increases denitrification within the vadose zone by promoting anoxic conditions. We conclude that AgMAR can be designed to enhance denitrification in the subsurface and reduce NO3− leaching to groundwater, while specifically accounting for lithologic heterogeneity, antecedent soil moisture conditions, and depth to the water table. Our findings are potentially relevant to other systems that experience flooding inundation such as floodplains and dedicated recharge basins

Play-fairway analysis for geothermal resources and exploration risk in the Modoc Plateau region

The region surrounding the Modoc Plateau, encompassing parts of northeastern California, southern Oregon, and northwestern Nevada, lies at an intersection between two tectonic provinces; the Basin and Range province and the Cascade volcanic arc. Both of these provinces have substantial geothermal resource base and resource potential. Geothermal systems with evidence of magmatic heat, associated with Cascade arc magmatism, typify the western side of the region. Systems on the eastern side of the region appear to be fault controlled with heat derived from high crustal heat flow, both of which are typical of the Basin and Range. As it has the potential to host Cascade arc-type geothermal resources, Basin and Range-type geothermal resources, and/or resources with characteristics of both provinces, and because there is relatively little current development, the Modoc Plateau region represents an intriguing potential for undiscovered geothermal resources. It remains unclear however, what specific set(s) of characteristics are diagnostic of Modoc-type geothermal systems and how or if those characteristics are distinct from Basin and Range-type or Cascade arc-type geothermal systems. In order to evaluate the potential for undiscovered geothermal resources in the Modoc area, we integrate a wide variety of existing data in order to evaluate geothermal resource potential and exploration risk utilizing ‘play-fairway’ analysis. We consider that the requisite parameters for hydrothermal circulation are: 1) heat that is sufficient to drive circulation, and 2) permeability that is sufficient to allow for fluid circulation in the subsurface. We synthesize data that indicate the extent and distribution of these parameters throughout the Modoc region. ‘Fuzzy logic’ is used to incorporate expert opinion into the utility of each dataset as an indicator of either heat or permeability, and thus geothermal favorability. The results identify several geothermal prospects, areas that are highly favorable for the occurrence of both heat and permeability. These are also areas where there is sufficient data coverage, quality, and consistency that the exploration risk is relatively low. These unknown, undeveloped, and under-developed prospects are well-suited for continued exploration efforts. The results also indicate to what degree the two ‘play-types,’ i.e. Cascade arc-type or Basin and Range-type, apply to each of the geothermal prospects, a useful guide in exploration efforts