Experimental evidence for carbonate precipitation and CO2 degassing during sea ice formation

Chemical and stable carbon isotopic modifications during the freezing of artificial seawater were measured in four 4 m³ tank incubations. Three of the four incubations were inoculated with a nonaxenic Antarctic diatom culture. The 18 days of freezing resulted in 25 to 27 cm thick ice sheets overlying the residual seawater. The ice phase was characterized by a decrease in temperature from −1.9 to −2.2°C in the under-ice seawater down to −6.7°C in the upper 4 cm of the ice sheet, with a concurrent increase in the salinity of the under-ice seawater and brine inclusions of the ice sheet as a result of physical concentration of major dissolved salts by expulsion from the solid ice matrix. Measurements of pH, total dissolved inorganic carbon (C_T) and its stable isotopic composition (δ¹³C_T) all exhibited changes, which suggest minimal effect by biological activity during the experiment. A systematic drop in pH and salinity-normalized C_T by up to 0.37 pH_SWS units and 376 μmol C kg⁻¹ respectively at the lowest temperature and highest salinity part of the ice sheet were coupled with an equally systematic ¹³C enrichment of the C_T. Calculations based on the direct pH and C_T measurements indicated a steady increase in the in situ concentration of dissolved carbon dioxide (CO₂(aq)) with time and increasing salinity within the ice sheet, partly due to changes in the dissociation constants of carbonic acid in the low temperature-high salinity range within sea ice. The combined effects of temperature and salinity on the solubility of CO₂ over the range of conditions encountered during this study was a slight net decrease in the equilibrium CO₂(aq) concentration as a result of the salting-out overriding the increase in solubility with decreasing temperature. Hence, the increase in the in situ CO₂(aq) concentration lead to saturation or supersaturation of the brine inclusions in the ice sheet with respect to atmospheric pCO₂ (≈3.5 × 10⁻⁴ atm). When all physico-chemical processes are considered, we expect CO₂ degassing and carbonate mineral precipitation from the brine inclusions of the ice sheet, which were saturated or highly supersaturated with respect to both the anhydrous (calcite, aragonite, vaterite) and hydrated (ikaite) carbonate minerals.     

Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: Implications for detection of microbial CO2 signatures using δC-DIC

Laboratory experiments on reagent-grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric CO₂. There is preferential dissolution of Ca¹²CO₃ during hydrolysis, resulting in δ¹³C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as ε_carb. ε_carb is greater on average for glacial sediments (−17.4‰) than for calcium carbonate (−7.8‰) for the < 63 μm size fraction, a sediment concentration of 5 g L⁻¹ and closed system conditions at 5°C. This difference is most likely due to the preferential dissolution of highly reactive ultra-fine particles with damaged surfaces that are common in subglacial sediments. The kinetic isotopic fractionation has a greater impact on δ¹³C-DIC at higher CaCO₃:water ratios and is significant during at least the first 6 h of carbonate dissolution driven by atmospheric CO₂ at sediment concentrations of 5 g L⁻¹. Atmospheric CO₂ dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least ∼ 6 h after the start of the experiment at 5°C. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the δ¹³C-DIC signal in natural environments where rock:water contact times are short <6-24 h (e.g., glacial systems, headwaters in fluvial catchments) and there is an excess of carbonate in the sediments. It will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial CO₂ signatures in DIC from δ¹³C-DIC data.     

Mitigation of asphaltics deposition during CO2 flood by enhancing CO2 solvency with chemical modifiers

CO₂ injected in the reservoir of McElroy Field, TX, for a CO₂ flood was in the supercritical state. Supercritical CO₂ fluid is capable of extracting light and intermediate hydrocarbons from rocks but is unable to extract heavy hydrocarbons and asphaltics. Therefore, plugging of asphaltics in reservoir rocks and a consequent reduction in injectivity and recovery may result when CO₂ only is used in enhanced oil recovery. By adding common solvents as chemical modifiers, the flooding fluid shows marked improvement in solvency for heavy components of crudes due to its increased density and polarity. Numerous supercritical CO₂ fluid extractions of dolomite rock from the Grayburg Formation containing known amount of spiked McElroy crude oil have been carried out to evaluate extraction efficiencies of CO₂ and CO₂ with chemical modifiers at various temperatures and pressures. All experiments show that extraction efficiency increases with increasing CO₂ pressure but decreases with increasing temperature. Addition of chemical modifiers to CO₂ also shows improved extraction efficiency and reduced asphaltic deposits. Under the pressure and temperature similar to McElroy reservoir conditions; chemically modified CO₂ yielded almost 3 times as much oil extracts as those in extractions with CO₂ only. It also reduced the asphaltics content in extracted rocks to nearly one half; indicating its potential for mitigating asphaltics plugging of formation rocks     

CO2 production in tar sand reservoirs under in situ steam temperatures: Reactive calcite dissolution

Recovery of highly viscous oil from some of the deeper oil sand deposits of northern Alberta, Canada, is made possible through injection of heat by steam or hot water flooding of the reservoirs. The rise in temperature lowers the viscosity of the bitumen allowing it to be produced. The increase in temperature accelerates the reactions between the matrix and pore minerals of the formation and can produce reaction products which can significantly alter the permeability of the reservoir. If carbonate minerals are present in the reservoir, inorganic CO₂ may also be a reaction product.

The Grand Rapids reservoir consists of relatively clean quartz sand containing 7 wt.% kaolinite, 1 wt.% calcite and a trace of smectite. Core floods of this sand by a neutral NaCl brine at 265°C, 8.2-MPa overburden pressure, 6.0-MPa fluid pressure and a flow velocity of 0.4 pore volumes per hour were used to determine the potential for hydrothermal reactions between clays and carbonate minerals in a natural reservoir sand. Reaction progress was followed by continuous sampling of the production fluids. The produced water was analyzed and the phase chemistry was calculated back to the run conditions using the computer code SOLMNEQF.

Mass-balance considerations on produced total inorganic carbon (TIC) show that calcite broke down very quickly, the maximum in CO₂ production occurring after only one pore volume of fluid had passed through the core. The Ca released from the breakdown of calcite was incorporated in the formation of smectite as was shown by post-run clay mineral analysis by the following unbalanced chemical reaction:

calcite+kaolinite+H₄Si04Ca-smectite+H₂0+CO₂

Silica was supplied by the dissolution of quartz. Silica concentrations analyzed in the production fluid were depressed from those predicted by previously published quartz rate equations because of the rapid rate of smectite synthesis.

These observations were used to formulate the following model for the passage of the first pore volume of NaCl brine through the core. Initially calcite is present throughout the core. As the brine enters the inlet of the core, it equilibrates with calcite. The brine remains in equilibrium with calcite throughout the core as quartz and kaolinite react to form smectite. This model was tested with the computer code PATH.UBC using CO₂ production as a measure of the progress variable ξ. A best fit was achieved to the produced fluid chemistry by varying relative dissolution rates of kaolinite and quatz and varying the suppression of precipitation of certain minerals.     

Laboratory study of self-potential signals during releasing of CO2 and N2 plumes

Carbon Capture Sequestration (CCS) projects require, for safety reasons, monitoring programmes focused on surveying gas leakage on the surface. Generally, these programmes include detection of chemical tracers that, once on the surface, could be associated with CO₂ degassing. We take a different approach by analysing feasibility of applying electrical surface techniques, specifically Self-Potential. A laboratory-scale model, using water-sand, was built for simulating a leakage scenario being monitored with non-polarisable electrodes. Electrical potentials were measured before, during and after gas injection (CO₂ and N₂) to determine if gas leakage is detectable. Variations of settings were done for assessing how the electrical potentials changed according to size of electrodes, distance from electrodes to the gas source, and type of gas. Results indicated that a degassing event is indeed detectable on electrodes located above injection source. Although the amount of gas could not be quantified from signals, injection timespan and increasing of injection rate were identified. Even though conditions of experiments were highly controlled contrasting to those usually found at field scale, we project that Self-Potential is a promising tool for detecting CO₂ leakage if electrodes are properly placed.     

CO2 in haplo-phonolite melt: solubility, speciation and carbonate complexation

CO₂ solubility was measured in a synthetic iron-free phonolite (haplo-phonolite) by equilibrating melt with excess CO₂ fluid in a piston cylinder apparatus for a range of pressures (1.0- 2.5 GPa) and temperatures (1300 to 1550°C). The quenched glasses were then analysed using a bulk carbon analytical method (LECO). The measured solubilities are between 0.65 and 2.77 wt.% for the range of conditions studied and show a negative correlation with temperature as reported for most other silicate melt compositions.A range of carbonate species are present within the glass, as well as minor amounts of molecular CO₂. FTIR and NMR analyses suggest that carbonate is present as both ‘network’ and ‘depolymerised’ units as shown for relatively highly polymerised compositions in the model of Brooker et al. (2001b). The bulk CO₂ analyses were used to calibrate the IR extinction coefficient for the carbonate groups. However, the results show that the values obtained for the glasses vary with the melt equilibration conditions, presumably because the ratio of the different carbonate species changes as a complex function of run pressure, temperature and quench rate. Thus the use of IR may not be a reliable method for the quantification of dissolved CO₂ concentrations in natural glasses of ‘intermediate’ composition.     

Oxygen isotope exchange between H2O and CO2 at elevated CO2 pressures: Implications for monitoring of geological CO2 storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.     

The effect of crystallinity on dissolution rates and CO2 consumption capacity of silicates

Comparison of measured far-from-equilibrium dissolution rates of natural glasses and silicate minerals at 25 °C and pH 4 reveals the systematic effects of crystallinity and elemental composition on these rates. Rates for both minerals and glasses decrease with increasing Si:O ratio, but glass dissolution rates are faster than corresponding mineral rates. The difference between glass and mineral dissolution rates increases with increasing Si:O ratio; ultra-mafic glasses (Si:O ? 0.28) dissolve at similar rates as correspondingly compositioned minerals, but Si-rich glasses such as rhyolite (Si:O ∼ 0.40) dissolve ?1.6 orders of magnitude faster than corresponding minerals. This behaviour is interpreted to stem from the effect of Si-O polymerisation on silicate dissolution rates. The rate controlling step of dissolution for silicate minerals and glasses for which Si:O > 0.28 is the breaking of Si-O bonds. Owing to rapid quenching, natural glasses will exhibit less polymerisation and less ordering of Si-O bonds than minerals, making them less resistant to dissolution. Dissolution rates summarized in this study are used to determine the Ca release rates of natural rocks at far-from-equilibrium conditions, which in turn are used to estimate their CO₂ consumption capacity. Results indicate that Ca release rates for glasses are faster than those of corresponding rocks. This difference is, however, significantly less than the corresponding difference between glass and mineral bulk dissolution rates. This is due to the presence of Ca in relatively reactive minerals. In both cases, Ca release rates increase by ∼two orders of magnitude from high to low Si:O ratios (e.g., from granite to gabbro or from rhyolitic to basaltic glass), illustrating the important role of Si-poor silicates in the long-term global CO₂ cycle.     

Melting relationships in the systems CaOCO2 and MgOCO2 to 33 kilobars

The melting temperatures of calcite and magnesite in the presence of excess CO₂ have been measured using Ag₂C₂O₄ in sealed capsules m a piston-cylinder apparatus. At 27 kbar, 11.5 wt % CO₂ dissolves in molten CaCO₂, depressing the freezing temperature from 1610 to 1505°C; and 6.5 wt % CO₂ dissolves in molten MgCO₃, depressing the freezing temperature from 1590 to 1510°C. The eutectic between calcite and lime was located at 1385°C at 27 kbar. These and other new results, combined with previously published data, permit completion of PT diagrams for the systems CaO-CO₂ and MgO-CO₂ from 1 bar to 35 kbar. The dissociation curve for each carbonate terminates at an invariant point where melting begins, at 40 bars and 1230°C for CaO-CO₂ and 23 kbar and 1550°C for MgO-CO₂ The differences between the two systems are explained by the different solubilities of CO₂ in the invariant liquids consequent upon the large pressure difference between the locations of these two invariant points. The results show that the temperatures for the beginning of melting of carbonates in the asthenosphere are lowered by about 100°C in the presence of CO₂.     

Selection of coals of different maturities for CO2 Storage by modelling of CH4 and CO2 adsorption isotherms

CO₂ injection in unmineable coal seams could be one interesting option for both storage and methane recovery processes. The objective of this study is to compare and model pure gas sorption isotherms (CO₂ and CH₄) for well-characterised coals of different maturities to determine the most suitable coal for CO₂ storage. Carbon dioxide and methane adsorption on several coals have been investigated using a gravimetric adsorption method. The experiments were carried out using both CO₂ and CH₄ pure gases at 25 °C from 0.1 to 5 MPa (1 to 50 bar). The experimental results were fitted using Temkin's approach but also with the corrected Langmuir's and the corrected Tóth's equations. The two last approaches are more accurate from a thermodynamical point of view, and have the advantage of taking into account the fact that experimental data (isotherms) correspond to excess adsorption capacities. These approaches allow better quantification of the adsorbed gas. Determined CO₂ adsorption capacities are from 0.5 to 2 mmol/g of dry coal. Modelling provides also the affinity parameters of the two gases for the different coals. We have shown these parameters determined with adsorption models could be used for classification and first selection of coals for CO₂ storage. The affinity ratio ranges from a value close to 1 for immature coals to 41 for high rank coals like anthracites. This ratio allows selecting coals having high CO₂ adsorption capacities. In our case, the modelling study of a significant number of coals from various ranks shows that anthracites seem to have the highest CO₂ storage capacities. Our study provides high quality affinity parameters and values of CO₂ and CH₄ adsorption capacities on various coals for the future modelling of CO₂ injection in coal seams.     

Impact of nitrogenous fertilizers on carbonate dissolution in small agricultural catchments: Implications for weathering CO2 uptake at regional and global scales

The goal of this study was to highlight the occurrence of an additional proton-promoted weathering pathway of carbonate rocks in agricultural areas where N-fertilizers are extensively spread, and to estimate its consequences on riverine alkalinity and uptake of CO₂ by weathering. We surveyed 25 small streams in the calcareous molassic Gascogne area located in the Garonne river basin (south-western France) that drain cultivated or forested catchments for their major element compositions during different hydrologic periods. Among these catchments, the Hay and the Montoussé, two experimental catchments, were monitored on a weekly basis. Studies in the literature from other small carbonate catchments in Europe were dissected in the same way. In areas of intensive agriculture, the molar ratio (Ca + Mg)/HCO₃ in surface waters is significantly higher (0.7 on average) than in areas of low anthropogenic pressure (0.5). This corresponds to a decrease in riverine alkalinity, which can reach 80% during storm events. This relative loss of alkalinity correlates well with the content in surface waters. In cultivated areas, the contribution of atmospheric/soil CO₂ to the total riverine alkalinity (CO_{2 ATM-SOIL}/HCO₃) is less than 50% (expected value for carbonate basins), and it decreases when the nitrate concentration increases. This loss of alkalinity can be attributed to the substitution of carbonic acid (natural weathering pathway) by protons produced by nitrification of N-fertilizers (anthropogenic weathering pathway) occurring in soils during carbonate dissolution. As a consequence of these processes, the alkalinity over the last 30 years shows a decreasing trend in the Save river (one of the main Garonne river tributaries, draining an agricultural catchment), while the nitrate and calcium plus magnesium contents are increasing.We estimated that the contribution of atmospheric/soil CO₂ to riverine alkalinity decreased by about 7-17% on average for all the studied catchments. Using these values, the deficit of CO₂ uptake can be estimated as up to 0.22-0.53 and 12-29 Tg¹ yr⁻¹ CO₂ on a country scale (France) and a global scale, respectively. These losses represent up to 5.7-13.4% and only 1.6-3.8% of the total CO₂ flux naturally consumed by carbonate dissolution, for France and on a global scale, respectively. Nevertheless, this loss of alkalinity relative to the Ca + Mg content relates to carbonate weathering by protons from N-fertilizers nitrification, which is a net source of CO₂ for the atmosphere. This anthropogenic CO₂ source is not negligible since it could reach 6-15% of CO₂ uptake by natural silicate weathering and could consequently partly counterbalance this natural CO₂ sink.     

CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic melts

The solubility of CO₂ in dacitic melts equilibrated with H₂O-CO₂ fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO₂ is dissolved in dacitic glasses as molecular CO₂ and carbonate. The quantification of total CO₂ in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm⁻¹ can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO_2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO₂ species concentrations from the MIR spectra we have measured total CO₂ contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO₂ solubility in dacitic melts on x^fluid_CO2,total shows a strong positive deviation from linearity with almost constant CO₂ solubility at x_CO2fluid > 0.8 (maximum CO₂ solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO₂. A similar nonlinear variation of CO₂ solubility with x_CO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO₂ (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO₂ molecules.A thermodynamic model describing the dependence of the CO₂ solubility in hydrous rhyolitic and dacitic melts on T, P, f_CO2 and the mol fraction of water in the melt (x_water) has been developed. An exponential variation of the equilibrium constant K₁ with x_water is proposed to account for the nonlinear dependence of x_CO2,totalmelt on x_CO2fluid. The model reproduces the CO₂ solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low x_CO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO₂ at low pressures (a Henrian behavior of the CO₂ solubility is observed at low pressure and low H₂O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the x_CO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO₂ is much lower in rhyolitic melts (31.7 cm³/mol) than in dacitic melts (46.6 cm³/mol). The dissolution enthalpy for CO₂ in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO₂ solubility in silicic melts.     

Feldspar dissolution rates measured using phase-shift interferometry: Implications to CO2 underground sequestration

To assess CO₂ underground sequestration from a geochemical viewpoint, the anorthite dissolution rate, which is an important parameter of risk analysis, was measured in a CO₂–water system. The authors sought to obtain precise dissolution rate data in a short time observing a crystal surface on a nanoscale. For this purpose, phase-shift interferometry was applied. Using this method, uncertainty of the reactive surface area that is imparted on calculation of the dissolution rate constant can also be avoided. The time-course profile of vertical retreat of the surface revealed that the anorthite dissolution process changes from the initial transient state to a later steady state, which is consistent with results of numerous precedent studies. The transient dissolution rate depends strongly on local features (e.g., density of defects, variation of chemical compositions) of the crystal surface, rather than on temperature. Therefore, it is very important to determine the original properties of the anorthite surface for the examination of subsequent dissolution process. Contrary to general expectations, the anorthite dissolution can alter the physical properties of reservoir rock immediately after CO₂ injection. The simple estimation using the anorthite dissolution rate obtained in this study, which was done as a test case for the CO₂ underground sequestration project conducted by RITE, revealed that porosity of reservoir rock increased about 2% (23–23.4%) of initial values during 60 a. That change in physical property in such a short time might enhance the diffusion of injected CO₂ and formation water, and therefore accelerate further geochemical reactions. Results of this study demonstrate that the geochemical water–rock interaction, which is generally regarded as a longer-term phenomenon than various physical processes, can also affect the reservoir system from the initial stage.     

火山岩吸附CO2气的成藏潜力及实例分析

火山岩的脱气实验和对昌德东CO2气藏气源的分析结果表明加热火山岩到250℃时,脱出挥发分总量为0.0299～0.0790mL/g,其中CO2脱出量为0.0218～0.0706mL/g(0.429～1.387wt%);挥发组分以CO2为主,还含有H2、CO、CH4等还原性气体,以及少量低碳烷烃,CO2含量和总烃呈现反比关系;基性岩的CO2脱出量、脱出率高于中、酸性岩;CO2脱出量与岩石碱质含量正相关.松辽盆地北部昌德东CO2气藏成藏模式为"自生自储",成藏CO2气主要来自深部被火山岩吸附的气.随岩浆上升,在岩浆冷凝成火山岩的过程中被吸附于火山岩的节理、劈理和晶体位错之中的CO2气,连同火山岩包体中的残留气,成为高纯CO2气藏的主要补给源,并非地幔气体沿大断裂上来直接充注成藏.     

Evolution of the chemistry of Fe bearing waters during CO2 degassing

The rates of Fe(II) oxidation and precipitation from groundwater are highly pH dependent. Elevated levels of dissolved CO₂ can depress pH and cause difficulty in removing dissolved Fe and associated metals during treatment of ferruginous water. This paper demonstrates interdependent changes in pH, dissolved inorganic C species, and Fe(II) oxidation rates that occur as a result of the removal (degassing) of CO₂ during aeration of waters discharged from abandoned coal mines. The results of field monitoring of aeration cascades at a treatment facility as well as batchwise aeration experiments conducted using net alkaline and net acidic waters in the UK are combined with geochemical modelling to demonstrate the spatial and temporal evolution of the discharge water chemistry. The aeration cascades removed approximately 67% of the dissolved CO₂ initially present but varying the design did not affect the concentration of Fe(II) leaving the treatment ponds. Continued removal of the residual CO₂ by mechanical aeration increased pH by as much as 2 units and resulted in large increases in the rates of Fe(II) oxidation and precipitation. Effective exsolution of CO₂ led to a reduction in the required lime dose for removal of remaining Fe(II), a very important factor with regard to increasing the sustainability of treatment practices. An important ancillary finding for passive treatment is that varying the design of the cascades had little impact on the rate of CO₂ removal at the flow rates measured.     

Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine

Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO₂ (CO₂–brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO₂ storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO₂ (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N₂, in order to mimic O₂-depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO₂-free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks.Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO₂ increased in all experiments. Release rates of Fe and SiO₂ were more pronounced in solutions reacted with CO₂–brine as compared to those reacted with CO₂-free brine. For samples reacted with the CO₂–brine, lower temperature reactions (80 °C) released much more Fe and SiO₂ than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO₂–brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO₂-free brine; while no trace of analcime was observed in either of the samples reacted with CO₂–brine.     

CO2-water-basalt interaction. Numerical simulation of low temperature CO2 sequestration into basalts

The interaction between CO₂-rich waters and basaltic glass was studied using reaction path modeling in order to get insight into the water-rock reaction process including secondary mineral composition, water chemistry and mass transfer as a function of CO₂ concentration and reaction progress (ξ). The calculations were carried out at 25-90 °C and pCO₂ to 30 bars and the results were compared to recent experimental observations and natural systems. A thermodynamic dataset was compiled from 25 to 300 °C in order to simulate mineral saturations relevant to basalt alteration in CO₂-rich environment including revised key aqueous species for mineral dissolution reactions and apparent Gibbs energies for clay and carbonate solid solutions observed to form in nature. The dissolution of basaltic glass in CO₂-rich waters was found to be incongruent with the overall water composition and secondary mineral formation depending on reaction progress and pH. Under mildly acid conditions in CO₂ enriched waters (pH <6.5), SiO₂ and simple Al-Si minerals, Ca-Mg-Fe smectites and Ca-Mg-Fe carbonates predominated. Iron, Al and Si were immobile whereas the Mg and Ca mobility depended on the mass of carbonate formed and water pH. Upon quantitative CO₂ mineralization, the pH increased to >8 resulting in Ca-Mg-Fe smectite, zeolites and calcite formation, reducing the mobility of most dissolved elements. The dominant factor determining the reaction path of basalt alteration and the associated element mobility was the pH of the water. In turn, the pH value was determined by the concentration of CO₂ and extent of reaction. The composition of the carbonates depended on the mobility of Ca, Mg and Fe. At pH <6.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg. At pH >8, the mobility of Fe and Mg was limited due to the formation of clays whereas Ca was incorporated into calcite, zeolites and clays. Competing reactions between clays (Ca-Fe smectites) and carbonates at low pH, and zeolites and clays (Mg-Fe smectites) and carbonates at high pH, controlled the availability of Ca, Mg and Fe, playing a key role for low temperature CO₂ mineralization and sequestration into basalts. Several problems of the present model point to the need of improvement in future work. The determinant factors linking time to low temperature reaction path modeling may not only be controlled by the primary dissolving phase, which presents challenges concerning non-stoichiometric dissolution, the leached layer model and reactive surface area, but may include secondary mineral precipitation kinetics as rate limiting step for specific reactions such as retrieved from the present reaction path study.     

GEOCARBSULF: A combined model for Phanerozoic atmospheric O2 and CO2

A model for the combined long-term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the GEOCARB III model [Berner R.A., Kothavala Z., 2001. GEOCARB III: a revised model of atmospheric CO₂ over Phanerozoic time. Am. J. Sci. 301, 182-204] for CO₂ with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for O₂ [Berner R.A., 2001. Modeling atmospheric O₂ over Phanerozoic time. Geochim. Cosmochim. Acta65, 685-694]. New isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models. Sensitivity analysis is done by determining (1) the effect on weathering rates of using rapid recycling (rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks); (2) the effect on O₂ of using different initial starting conditions; (3) the effect on O₂ of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic δ¹³C for the past 200 million years; (4) the effect on sulfur isotope fractionation and on O₂ of varying the size of O₂ feedback during sedimentary pyrite formation; (5) the effect on O₂ of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion, and the degree of exposure of coastal lands by sea level change; (6) the effect on CO₂ of adding the variability of volcanic rock weathering over time [Berner, R.A., 2006. Inclusion of the weathering of volcanic rocks in the GEOCARBSULF model. Am. J. Sci.306 (in press)]. Results show a similar trend of atmospheric CO₂ over the Phanerozoic to the results of GEOCARB III, but with some differences during the early Paleozoic and, for variable volcanic rock weathering, lower CO₂ values during the Mesozoic. Atmospheric oxygen shows a major broad late Paleozoic peak with a maximum value of about 30% O₂ in the Permian, a secondary less-broad peak centered near the Silurian/Devonian boundary, variation between 15% and 20% O₂ during the Cambrian and Ordovician, a very sharp drop from 30% to 15% O₂ at the Permo-Triassic boundary, and a more-or less continuous rise in O₂ from the late Triassic to the present.     

Carbon isotope evidence implying high O2/CO2 ratios in the Permo-Carboniferous atmosphere

Theoretical models predict a marked increase in atmospheric O₂ to ∼35% during the Permo-Carboniferous (∼300 Ma) occurring against a low (∼0.03%) CO₂ level. An upper O₂ value of 35%, however, remains disputed because ignition data indicate that excessive global forest fires would have ensued. This uncertainty limits interpretation of the role played by atmospheric oxygen in Late Paleozoic biotic evolution. Here, we describe new results from laboratory experiments with vascular land plants that establish that a rise in O₂ to 35% increases isotopic fractionation (Δ¹³C) during growth relative to control plants grown at 21% O₂. Despite some effect of the background atmospheric CO₂ level on the magnitude of the increase, we hypothesize that a substantial Permo-Carboniferous rise in O₂ could have imprinted a detectable geochemical signature in the plant fossil record. Over 50 carbon isotope measurements on intact carbon from four fossil plant clades with differing physiological ecologies and ranging in age from Devonian to Cretaceous reveal a substantial Δ¹³C anomaly (5‰) occurring between 300 and 250 Ma. The timing and direction of the Δ¹³C excursion is consistent with the effects of a high O₂ atmosphere on plants, as predicted from photosynthetic theory and observed in our experiments. Preliminary calibration of the fossil Δ¹³C record against experimental data yields a predicted O₂/CO₂ mixing ratio of the ancient atmosphere consistent with that calculated from long-term models of the global carbon and oxygen cycles. We conclude that further work on the effects of O₂ in the combustion of plant materials and the spread of wildfire is necessary before existing data can be used to reliably set the upper limit for paleo-O₂ levels.