Dynamics of CO<Subscript>2</Subscript> fluxes and concentrations during a shallow subsurface CO<Subscript>2</Subscript> release

A field facility located in Bozeman, Montana provides the opportunity to test methods to detect, locate, and quantify potential CO₂ leakage from geologic storage sites. From 9 July to 7 August 2008, 0.3 t CO₂ day⁻¹ were injected from a 100-m long, ~2.5-m deep horizontal well. Repeated measurements of soil CO₂ fluxes on a grid characterized the spatio-temporal evolution of the surface leakage signal and quantified the surface leakage rate. Infrared CO₂ concentration sensors installed in the soil at 30 cm depth at 0–10 m from the well and at 4 cm above the ground at 0 and 5 m from the well recorded surface breakthrough of CO₂ leakage and migration of CO₂ leakage through the soil. Temporal variations in CO₂ concentrations were correlated with atmospheric and soil temperature, wind speed, atmospheric pressure, rainfall, and CO₂ injection rate.     

Feasibility of CO<Subscript>2</Subscript> migration detection using pressure and CO<Subscript>2</Subscript> saturation monitoring above an imperfect primary seal of a geologic CO<Subscript>2</Subscript> storage formation: a numerical investigation

A numerical model was developed to investigate the potential to detect fluid migration in a (homogeneous, isotropic, with constant pressure lateral boundaries) porous and permeable interval overlying an imperfect primary seal of a geologic CO₂ storage formation. The seal imperfection was modeled as a single higher-permeability zone in an otherwise low-permeability seal, with the center of that zone offset from the CO₂ injection well by 1400 m. Pressure response resulting from fluid migration through the high-permeability zone was detectable up to 1650 m from the centroid of that zone at the base of the monitored interval after 30 years of CO₂ injection (detection limit = 0.1 MPa pressure increase); no pressure response was detectable at the top of the monitored interval at the same point in time. CO₂ saturation response could be up to 774 m from the center of the high-permeability zone at the bottom of the monitored interval, and 1103 m at the top (saturation detection limit = 0.01). More than 6% of the injected CO₂, by mass, migrated out of primary containment after 130 years of site performance (including 30 years of active injection) in the case where the zone of seal imperfection had a moderately high permeability (10^??17 m² or 0.01 mD). Free-phase CO₂ saturation monitoring at the top of the overlying interval provides favorable spatial coverage for detecting fluid migration across the primary seal. Improved sensitivity of detection for pressure perturbation will benefit time of detection above an imperfect seal.     

Carbonate cements in Miller field of the UK North Sea: a natural analog for mineral trapping in CO<Subscript>2</Subscript> geological storage

Miller field of the North Sea has had high concentrations of natural CO₂ for ~70 Ma. It is an ideal analog for the long-term fate of CO₂ during engineered storage, particularly for formation of carbonate minerals that permanently lock up CO₂ in solid form. The Brae Formation reservoir sandstone contains an unusually high quantity of calcite concretions; however, C and O stable isotopic signatures suggest that these are not related to the present-day CO₂ charge. Margins of the concretions are corroded, probably because of reduced pH due to CO₂ influx. Dispersed calcite cements are also present, some of which postdate the CO₂ charge and, therefore, are the products of mineral trapping. It is calculated that only a minority of the reservoired CO₂ in Miller (6–24%) has been sequestrated in carbonates, even after 70 Ma of CO₂ emplacement. Most of the CO₂ accumulation is dissolved in pore fluids. Therefore, in a reservoir similar to the Brae Formation, engineered CO₂ storage must rely on physical retention mechanisms because mineral trapping is both incomplete and slow.     

The Role of CaCO<Subscript>3</Subscript> Reactions in the Contemporary Oceanic CO<Subscript>2</Subscript> Cycle

The present analysis adjusts previous estimates of global ocean CaCO₃ production rates substantially upward, to 133 × 10¹² mol yr^?1 plankton production and 42 × 10¹² mol yr^?1 shelf benthos production. The plankton adjustment is consistent with recent satellite-based estimates; the benthos adjustment includes primarily an upward adjustment of CaCO₃ production on so-called carbonate-poor sedimentary shelves and secondarily pays greater attention to high CaCO₃ mass (calcimass) and turnover of shelf communities on temperate and polar shelves. Estimated CaCO₃ sediment accumulation rates remain about the same as they have been for some years: ~20 × 10¹² mol yr^?1 on shelves and 11 × 10¹² mol yr^?1 in the deep ocean. The differences between production and accumulation of calcareous materials call for dissolution of ~22 × 10¹² mol yr^?1 (~50 %) of shelf benthonic carbonate production and 122 × 10¹² mol yr^?1 (>90 %) of planktonic production. Most CaCO₃ production, whether planktonic or benthonic, is assumed to take place in water depths of <100 m, while most dissolution is assumed to occur below this depth. The molar ratio of CO₂ release to CaCO₃ precipitation (CO₂↑/CaCO₃↓) is <1.0 and varies with depth. This ratio, Ψ, is presently about 0.66 in surface seawater and 0.85 in ocean waters deeper than about 1000 m. The net flux of CO₂ associated with CaCO₃ reactions in the global ocean in late preindustrial time is estimated to be an apparent influx from the atmosphere to the ocean, of +7 × 10¹² mol C yr^?1, at a time scale of 10²–10³ years. The CaCO₃-mediated influx of CO₂ is approximately offset by CO₂ release from organic C oxidation in the water column. Continuing ocean acidification will have effects on CaCO₃ and organic C metabolic responses to the oceanic inorganic C cycle, although those responses remain poorly quantified.     

Pressure transient technique to constrain CO<Subscript>2</Subscript> plume boundaries

Carbon dioxide (CO₂) has been injected in the subsurface permeable formations as a means to cut atmospheric CO₂ emissions and/or enhance oil recovery (EOR). It is important to constrain the boundaries of the CO₂ plume in the target formation and/or other formations hosting the CO₂ migrated from the target formation. Monitoring methods and technologies to assess the CO₂ plume boundaries over time within a reservoir of interest are required. Previously introduced methods and technologies on pressure monitoring to detect the extent of the CO₂ plume require at least two wells, i.e. pulser and observation wells. We introduce pressure transient technique requiring single well only. Single well pressure transient testing (drawdown/buildup/injection/falloff) is widely used to determine reservoir properties and wellbore conditions. Pressure diagnostic plots are used to identify different flow regimes and determine the reservoir/well characteristics. We propose a method to determine the plume extent for a constant rate pressure transient test at a single well outside the CO₂ plume. Due to the significant contrast between mobility and storativity of the CO₂ and native fluids (oil or brine), the CO₂ boundary causes deviation in the pressure diagnostic response from that corresponding to previously identified heterogeneities. Using the superposition principle, we develop a relationship between the deviation time and the plume boundary. We demonstrate the applicability of the proposed method using numerically generated synthetic data corresponding to homogeneous, heterogeneous, and anisotropic cases to evaluate its potential and limitations. We discuss ways to identify and overcome the potential limitations for application of the method in the field.     

Analytical model for screening potential CO<Subscript>2</Subscript> repositories

Assessing potential repositories for geologic sequestration of carbon dioxide using numerical models can be complicated, costly, and time-consuming, especially when faced with the challenge of selecting a repository from a multitude of potential repositories. This paper presents a set of simple analytical equations (model), based on the work of previous researchers, that could be used to evaluate the suitability of candidate repositories for subsurface sequestration of carbon dioxide. We considered the injection of carbon dioxide at a constant rate into a confined saline aquifer via a fully perforated vertical injection well. The validity of the analytical model was assessed via comparison with the TOUGH2 numerical model. The metrics used in comparing the two models include (1) spatial variations in formation pressure and (2) vertically integrated brine saturation profile. The analytical model and TOUGH2 show excellent agreement in their results when similar input conditions and assumptions are applied in both. The analytical model neglects capillary pressure and the pressure dependence of fluid properties. However, simulations in TOUGH2 indicate that little error is introduced by these simplifications. Sensitivity studies indicate that the agreement between the analytical model and TOUGH2 depends strongly on (1) the residual brine saturation, (2) the difference in density between carbon dioxide and resident brine (buoyancy), and (3) the relationship between relative permeability and brine saturation. The results achieved suggest that the analytical model is valid when the relationship between relative permeability and brine saturation is linear or quasi-linear and when the irreducible saturation of brine is zero or very small.     

CO<Subscript>2</Subscript> fluid inclusions in Jack Hills zircons

The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO₂ inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO₂ densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO₂ inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO₂ prior to analysis. This indicates a close relationship between CO₂ and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO₂-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills.     

Planktonic Foraminiferal and Alkenone Records of the Last Deglaciation from the Eastern Arabian Sea

The oxygen isotope record of planktonic foraminifera from tropical core MD77194 (Eastern Arabian Sea) exhibits a clear two-step deglaciation with a brief δ¹⁸O transient event. In the tropics, this δ¹⁸O maximum could correspond to a cooling or to a change in the δ¹⁸O content of sea water. In this study, past sea-surface temperature (SST) and primary production (PP) are reconstructed from foraminiferal transfer functions and compared to values estimated from alkenone measurements. SST and PP records from both proxies indicate a 1.5–2.5°C deglacial warming, coupled with a PP decrease, and a 0.5–1°C cooling during the Younger Dryas (YD). A detailed comparison between independent micropaleontological and geochemical proxies helps us identify potential biases and thus strengthen the paleo-reconstructions.     

Using hyperspectral plant signatures for CO<Subscript>2</Subscript> leak detection during the 2008 ZERT CO<Subscript>2</Subscript> sequestration field experiment in Bozeman,Montana

Hyperspectral plant signatures can be used as a short-term, as well as long-term (100-year timescale) monitoring technique to verify that CO₂ sequestration fields have not been compromised. An influx of CO₂ gas into the soil can stress vegetation, which causes changes in the visible to near-infrared reflectance spectral signature of the vegetation. For 29 days, beginning on July 9, 2008, pure carbon dioxide gas was released through a 100-m long horizontal injection well, at a flow rate of 300 kg day⁻¹. Spectral signatures were recorded almost daily from an unmown patch of plants over the injection with a “FieldSpec Pro” spectrometer by Analytical Spectral Devices, Inc. Measurements were taken both inside and outside of the CO₂ leak zone to normalize observations for other environmental factors affecting the plants. Four to five days after the injection began, stress was observed in the spectral signatures of plants within 1 m of the well. After approximately 10 days, moderate to high amounts of stress were measured out to 2.5 m from the well. This spatial distribution corresponded to areas of high CO₂ flux from the injection. Airborne hyperspectral imagery, acquired by Resonon, Inc. of Bozeman, MT using their hyperspectral camera, also showed the same pattern of plant stress. Spectral signatures of the plants were also compared to the CO₂ concentrations in the soil, which indicated that the lower limit of soil CO₂ needed to stress vegetation is between 4 and 8% by volume.     

CO<Subscript>2</Subscript> leakage through an abandoned well: problem-oriented benchmarks

The efficiency and sustainability of carbon dioxide (CO₂) storage in deep geological formations crucially depends on the integrity of the overlying cap-rocks. Existing oil and gas wells, which penetrate the formations, are potential leakage pathways. This problem has been discussed in the literature, and a number of investigations using semi-analytical mathematical approaches have been carried out by other authors to quantify leakage rates. The semi-analytical results are based on a number of simplifying assumptions. Thus, it is of great interest to assess the influence of these assumptions. We use a numerical model to compare the results with those of the semi-analytical model. Then we ease the simplifying restrictions and include more complex thermodynamic processes including sub- and supercritical fluid properties of CO₂ and non-isothermal as well as compositional effects. The aim is to set up problem-oriented benchmark examples that allow a comparison of different modeling approaches to the problem of CO₂ leakage.     

The effect of alkalis and polymerization on the solubility of H<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> in alkali-rich silicate melts

The effect of alkalis on the solubility of H₂O and CO₂ in alkali-rich silicate melts was investigated at 500 MPa and 1,250 °C in the systems with H₂O/(H₂O + CO₂) ratio varying from 0 to 1. Using a synthetic analog of phonotephritic magma from Alban Hills (AH1) as a base composition, the Na/(Na + K) ratio was varied from 0.28 (AH1) to 0.60 (AH2) and 0.85 (AH3) at roughly constant total alkali content. The obtained results were compared with the data for shoshonitic and latitic melts having similar total alkali content but different structural characteristics, e.g., NBO/T parameter (the ratio of non-bridging oxygens over tetrahedrally coordinated cations), as those of the AH compositions. Little variation was observed in H₂O solubility (melt equilibrated with pure H₂O fluid) for the whole compositional range in this study with values ranging between 9.7 and 10.2 wt. As previously shown, the maximum CO₂ content in melts equilibrated with CO₂-rich fluids increases strongly with the NBO/T from 0.29 wt % for latite (NBO/T = 0.17) to 0.45 wt % for shoshonite (NBO/T = 0.38) to 0.90 wt % for AH2 (NBO/T = 0.55). The highest CO₂ contents determined for AH3 and AH1 are 1.18 ± 0.05 wt % and 0.86 ± 0.12 wt %, respectively, indicating that Na is promoting carbonate incorporation stronger than potassium. At near constant NBO/T, CO₂ solubility increases from 0.86 ± 0.12 wt % in AH1 [Na/(Na + K)] = 0.28, to 1.18 ± 0.05 wt % in AH3 [Na/(Na + K)] = 0.85, suggesting that Na favors CO₂ solubility on an equimolar basis. An empirical equation is proposed to predict the maximum CO₂ solubility at 500 MPa and 1,100–1,300 °C in various silicate melts as a function of the NBO/T, (Na + K)/∑cations and Na/(Na + K) parameters: \({\text{wt}}\% \;{\text{CO}}_{2} = - 0.246 + 0.014\exp \left( {6.995 \cdot \frac{\text{NBO}}{T}} \right) + 3.150 \cdot \frac{{{\text{Na}} + {\text{K}}}}{{\varSigma {\text{cations}}}} + 0.222 \cdot \frac{\text{Na}}{{{\text{Na}} + {\text{K}}}}.\) This model is valid for melt compositions with NBO/T between 0.0 and 0.6, (Na + K)/∑cation between 0.08 and 0.36 and Na/(Na + K) ratio from 0.25 to 0.95 at oxygen fugacities around the quartz–fayalite–magnetite buffer and above.     

Delineating hazardous CO<Subscript>2</Subscript> fluxes from acid mine drainage

Incidents of hazardous accumulations of CO₂ in homes built on or near reclaimed mine land, in the last decade, have been shown to be linked to neutralization reactions between acidic mine drainage and carbonate material. Recent research has shown that CO₂ fluxes on reclaimed mine land with this hazard are, sometimes, spatially autocorrelated (i.e., the spatial variability is not random). This result implies geostatistics can be used to delineate hazardous areas where fluxes are likely to exceed established thresholds. This study applies sequential Gaussian simulation to delineate this emerging hazard on a site in southwestern Indiana, USA. Due to lack of regulatory threshold limits for CO₂ flux at the current time, the authors conduct a sensitivity analysis of the threshold limit using the 75th, 90th and 95th percentiles of the measured fluxes for the first day of monitoring. These limits are used to produce hazard maps, which are validated with the known hazard at the site. This work further shows the potential of surface CO₂ flux monitoring as a cheap and effective strategy to monitor and delineate such hazards to avoid residential and commercial real estate development in high risk zones.     

Preparation of carbon molecular sieves and its impregnation with Co and Ni for CO<Subscript>2</Subscript>/N<Subscript>2</Subscript> separation

The carbon molecular sieves (CMSs) prepared by carbonaceous materials as precursors are effective in CO₂/N₂ separation. However, selectivity of these materials is too low, since hydrocarbon cracking for developing the desired microporosity in carbonaceous materials has not been done effectively. Hence, in this study, cobalt and nickel impregnation on the precursor was conducted to introduce catalysts for hydrocarbon cracking. Cobalt and nickel impregnation, carbonization under N₂ atmosphere, and chemical vapor deposition (CVD) by benzene were conducted on the extruded mixtures of activated carbon and coal tar pitch under different conditions to prepare CMSs. The best CMS prepared by carbon deposition on the cobalt-impregnated samples exhibited CO₂ adsorption capacity of 54.79 mg/g and uptake ratio of 28.9 at 0 °C and 1 bar. In terms of CO₂ adsorption capacity and uptake ratio, CMSs prepared by carbon deposition on non-impregnated and cobalt-impregnated samples presented the best results, respectively. As benzene concentration and CVD time increased, equilibrium adsorption capacity of CO₂ decreased, and uptake ratio increased. Cobalt was found to be the best catalyst for benzene cracking in the CVD process.