Dissolution of altered tuffaceous rocks under conditions relevant for CO2 storage

We conducted CO₂–water–rock interaction experiments to elucidate the dissolution characteristics and geochemical trapping potential of three different altered andesitic to rhyolitic tuffaceous rocks (Tsugawa, Ushikiri and Daijima tuffaceous rock) relative to fresh mid-ocean ridge basalt. The experiments were performed under 1 MPa CO₂ pressure to reproduce the water–rock–CO₂ interactions in CO₂ storage situations. Basalt showed high acid neutralization potential and rapid dissolution of silicate minerals. Two of the tuffaceous rocks (Ushikiri and Daijima) showed relatively high solubility trapping potential, mainly due to the dissolution of carbonate minerals in the andesitic Ushikiri tuffaceous rock and the ion-exchange reaction with zeolite minerals in the rhyolitic Daijima tuffaceous rock. The mineral trapping potential of the Ushikiri tuffaceous rock was found to be relatively high, due to the rapid dissolution of Mg- and Ca-bearing silicate minerals. Our experimental results suggest that regions of porous and andesitic tuffaceous rock hold global promise as CO₂ storage sites.     

Assessing the potential consequences of CO2 leakage to freshwater resources: A batch-reaction experiment towards an isotopic tracing tool

The assessment of the environmental impacts of CO₂ geological storage requires the investigation of potential CO₂ leakages into fresh groundwater, particularly with respect to protected groundwater resources. The geochemical processes and perturbations associated with a CO₂ leak into fresh groundwater could alter groundwater quality: indeed, some of the reacting minerals may contain hazardous constituents, which might be released into groundwater. Since the geochemical reactions may occult direct evidence of intruding CO₂, it is necessary to characterize these processes and identify possible indirect indicators for monitoring CO₂ intrusion. The present study focuses on open questions: Can changes in water quality provide evidence of CO₂ leakage? Which parameters can be used to assess impact on freshwater aquifers? What is the time scale of water chemistry degradation in the presence of CO₂? The results of an experimental approach allow selecting pertinent isotope tracers as possible indirect indicators of CO₂ presence, opening the way to devise an isotopic tracing tool.The study area is located in the Paris Basin (France), which contains deep saline formations identified as targets by French national programs for CO₂ geological storage. The study focuses on the multi-layered Albian fresh water aquifer, confined in the central part of the Paris Basin a major strategic potable groundwater overlying the potential CO₂ storage formations. An experimental approach (batch reactors) was carried out in order to better understand the rock–water–CO₂ interactions with two main objectives. The first was to assess the evolution of the formation water chemistry and mineralogy of the solid phase over time during the interaction. The second concerned the design of an isotopic monitoring program for freshwater resources potentially affected by CO₂ leakage. The main focus was to select suitable environmental isotope tracers to track water rock interaction associated with small quantities of CO₂ leaking into freshwater aquifers.In order to improve knowledge on the Albian aquifer, and to provide representative samples for the experiments, solid and fluid sampling campaigns were performed throughout the Paris Basin. Albian groundwater is anoxic with high concentrations of Fe, a pH around 7 and a mineral content of 0.3 g L⁻¹. Macroscopic and microscopic solid analyses showed a quartz-rich sand with the presence of illite/smectite, microcline, apatite and glauconite. A water–mineral–CO₂ interaction batch experiment was used to investigate the geochemical evolution of the groundwater and the potential release of hazardous trace elements. It was complemented by a multi-isotope approach including δ¹³C_DIC and ⁸⁷Sr/⁸⁶Sr. Here the evolution of the concentrations of major and trace elements and isotopic ratios over batch durations from 1 day to 1 month are discussed. Three types of ion behavior are observed: Type I features Ca, SiO₂, HCO₃, F, PO₄, Na, Al, B, Co, K, Li, Mg, Mn, Ni, Pb, Sr, Zn which increased after initial CO₂ influx. Type II comprises Be and Fe declining at the start of CO₂ injection. Then, type III groups element with no variation during the experiments like Cl and SO₄. The results of the multi-isotope approach show significant changes in isotopic ratios with time. The contribution of isotope and chemical data helps in understanding geochemical processes involved in the system. The isotopic systems used in this study are potential indirect indicators of CO₂–water–rock interaction and could serve as monitoring tools of CO₂ leakage into an aquifer overlying deep saline formations used for C sequestration and storage.     

On the risk of hydraulic fracturing in CO2 geological storage

We present a contribution on the risk of hydraulic fracturing in CO₂ geological storage using an analytical model of hydraulic fracturing in weak formations. The work is based on a Mohr–Coulomb dislocation model that is extended to account for material with fracture toughness. The complete slip process that is distributed around the crack tip is replaced by superdislocations that are placed in the effective centers. The analytical model enables the identification of a dominant parameter, which defines the regimes of brittle to ductile propagation and the limit at which a mode‐1 fracture cannot advance. We examine also how the corrosive effect of CO₂ on rock strength may affect hydraulic fracture propagation. We found that a hydraulically induced vertical fracture from CO₂ injection is more likely to propagate horizontally than vertically, remaining contained in the storage zone. The horizontal fracture propagation will have a positive effect on the injectivity and storage capacity of the formation. The containment in the vertical direction will mitigate the risk of fracturing and migration of CO₂ to upper layers and back to the atmosphere. Although the corrosive effect of CO₂ is expected to decrease the rock toughness and the resistance to fracturing, the overall decrease of rock strength promotes ductile behavior with the energy dissipated in plastic deformation and hence mitigates the mode‐1 fracture propagation. Copyright © 2016 John Wiley & Sons, Ltd.     

Reactive transport simulation study of geochemical CO2 trapping on the Tokyo Bay model – With focus on the behavior of dawsonite

A long-term (up to 10 ka) geochemical change in saline aquifer CO₂ storage was studied using the TOUGHREACT simulator, on a 2-dimensional, 2-layered model representing the underground geologic and hydrogeologic conditions of the Tokyo Bay area that is one of the areas of the largest CO₂ emissions in the world. In the storage system characterized by low permeability of reservoir and cap rock, the dominant storage mechanism is found to be solubility trapping that includes the dissolution and dissociation of injected CO₂ in the aqueous phase followed by geochemical reactions to dissolve minerals in the rocks. The CO₂–water–rock interaction in the storage system (mainly in the reservoir) changes the properties of water in a mushroom-like CO₂ plume, which eventually leads to convective mixing driven by gravitational instability. The geochemically evolved aqueous phase precipitates carbonates in the plume front due to a local rise in pH with mixing of unaffected reservoir water. The carbonate precipitation occurs extensively within the plume after the end of its enlargement, fixing injected CO₂ in a long, geologic period.Dawsonite, a Na–Al carbonate, is initially formed throughout the plume from consumption of plagioclase in the reservoir rock, but is found to be a transient phase finally disappearing from most of the CO₂-affected part of the system. The mineral is unstable relative to more common types of carbonates in the geochemical evolution of the CO₂ storage system initially having formation water of relatively low salinity. The exception is the reservoir-cap rock boundary where CO₂ saturation remains very high throughout the simulation period.     

Internally consistent data for sulphur‐bearing phases and application to the construction of pseudosections for mafic greenschist facies rocks in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–CO2–O–S–H2O

The Holland and Powell internally consistent data set version 5.5 has been augmented to include pyrite, troilite, trov (Fe_0.875S), anhydrite, H₂S, elemental S and S₂ gas. Phase changes in troilite and pyrrhotite are modelled with a combination of multiple end‐members and a Landau tricritical model. Pyrrhotite is modelled as a solid solution between hypothetical end‐member troilite (trot) and Fe_0.875S (trov); observed activity–composition relationships fit well to a symmetric formalism model with a value for w_trot?trov of ?3.19 kJ mol^?1. The hypothetical end‐member approach is required to compensate for iron distribution irregularities in compositions close to troilite. Mixing in fluids is described with the van Laar asymmetric formalism model with a_ij values for H₂O–H₂S, H₂S–CH₄ and H₂S–CO₂ of 6.5, 4.15 and 0.045 kJ mol^?1 respectively. The derived data set is statistically acceptable and replicates the input data and data from experiments that were not included in the initial regression. The new data set is applied to the construction of pseudosections for the bulk composition of mafic greenschist facies rocks from the Golden Mile, Kalgoorlie, Western Australia. The sequence of mineral assemblages is replicated successfully, with observed assemblages predicted to be stable at X(CO₂) increasing with increasing degree of hydrothermal alteration. Results are compatible with those of previous work. Assemblages are insensitive to the S bulk content at S contents of less than 1 wt%, which means that volatilization of S‐bearing fluids and sulphidation are unlikely to have had major effects on the stable mineral assemblage in less metasomatized rocks. The sequence of sulphide and oxide phases is predicted successfully and there is potential to use these phases qualitatively for geobarometry. Increases in X(CO₂) stabilized, in turn, pyrite–magnetite, pyrite–hematite and anhydrite–pyrite. Magnetite–pyrrhotite is predicted at temperatures greater than 410 °C. The prediction of a variety of sulphide and oxide phases in a rock of fixed bulk composition as a function of changes in fluid composition and temperature is of particular interest because it has been proposed that such a variation in phase assemblage is produced by the infiltration of multiple fluids with contrasting redox state. The work presented here shows that this need not be the case.     

The application of P–T–X(CO2) modelling in constraining metamorphism and hydrothermal alteration at the Damang gold deposit,Ghana

Orogenic gold mineralization at the Damang deposit, Ghana, is associated with hydrothermal alteration haloes around gold‐bearing quartz veins, produced by the infiltration of a H₂O–CO₂–K₂O–H₂S fluid following regional metamorphism. Alteration assemblages are controlled by the protoliths with sedimentary rocks developing a typical assemblage of muscovite, ankerite and pyrite, while intrusive dolerite bodies contain biotite, ankerite and pyrrhotite, accompanied by the destruction of hornblende. Mineral equilibria modelling was undertaken with the computer program thermocalc , in subsets of the model system MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O–TiO₂–Fe₂O₃, to constrain conditions of regional metamorphism and the subsequent gold mineralization event. Metapelites with well‐developed amphibolite facies assemblages reliably constrain peak regional metamorphism at ~595 °C and 5.5 kbar. Observed hydrothermal alteration assemblages associated with gold mineralization in a wide compositional range of lithologies are typically calculated to be stable within P–T–X(CO₂) arrays that trend towards lower temperatures and pressures with increasing equilibrium fluid X(CO₂). These independent P–T–X(CO₂) arrays converge and the region of overlap at ~375–425 °C and 1–2 kbar is taken to represent the conditions of alteration approaching equilibrium with a common infiltrating fluid with an X(CO₂) of ~0.7. Fluid‐rock interaction calculations with M–X(CO₂) diagrams indicate that the observed alteration assemblages are consistent with the addition of a single fluid phase requiring minimum fluid/rock ratios on the order of 1.     

Quick-look assessments to identify optimal CO2 EOR storage sites

A newly developed, multistage quick-look methodology allows for the efficient screening of an unmanageably large number of reservoirs to generate a workable set of sites that closely match the requirements for optimal CO₂ enhanced oil recovery (EOR) storage. The objective of the study is to quickly identify miscible CO₂ EOR candidates in areas that contain thousands of reservoirs and to estimate additional oil recovery and sequestration capacities of selected top options through dimensionless modeling and reservoir characterization. Quick-look assessments indicate that the CO₂ EOR resource potential along the US Gulf Coast is 4.7 billion barrels, and CO₂ sequestration capacity is 2.6 billion metric tons. In the first stage, oil reservoirs are screened and ranked in terms of technical and practical feasibility for miscible CO₂ EOR. The second stage provides quick estimates of CO₂ EOR potential and sequestration capacities. In the third stage, a dimensionless group model is applied to a selected set of sites to improve the estimates of oil recovery and storage potential using appropriate inputs for rock and fluid properties, disregarding reservoir architecture and sweep design. The fourth stage validates and refines the results by simulating flow in a model that describes the internal architecture and fluid distribution in the reservoir. The stated approach both saves time and allows more resources to be applied to the best candidate sites.     

Contribution of carbonate rock weathering to the atmospheric CO2 sink

To accurately predict future CO₂ levels in the atmosphere, which is crucial in predicting global climate change, the sources and sinks of the atmospheric CO₂ and their change over time must be determined. In this paper, some typical cases are examined using published and unpublished data. Firstly, the sensitivity of carbonate rock weathering (including the effects by both dissolution and reprecipitation of carbonate) to the change of soil CO₂ and runoff will be discussed, and then the net amount of CO₂ removed from the atmosphere in the carbonate rock areas of mainland China and the world will be determined by the hydrochem-discharge and carbonate-rock-tablet methods, to obtain an estimate of the contribution of carbonate rock weathering to the atmospheric CO₂ sink. These contributions are about 0.018 billion metric tons of carbon/a and 0.11 billion metric tons of carbon/a for China and the world, respectively. Further, by the DBL (Diffusion Boundary Layer)-model calculation, the potential CO₂ sink by carbonate rock dissolution is estimated to be 0.41 billion metric tons of carbon/a for the world. Therefore, the potential CO₂ source by carbonate reprecipitation is 0.3 billion metric tons of carbon/a. Received: 12 May 1999 · Accepted: 16 August 1999     

Thermo‐hydro‐mechanical modeling with Langmuir's adsorption isotherm of the CO2 injection in coal

In this paper, we used a theoretical model for the variation of Eulerian porosity, which takes into account the adsorption process known to be the main mechanism of production or sequestration of gas in many reservoir of coal. This process is classically modeled using Langmuir's isotherm. After implementation in Code_Aster, a fully coupled thermo‐hydro‐mechanical analysis code for structures calculations, we used numerical simulations to investigate the influence of coal's hydro‐mechanical properties (Biot's coefficient, bulk modulus), Langmuir's adsorption parameters, and the initial liquid pressure in rock mass during CO₂ injection in coal. These simulations showed that the increase in the values of Langmuir's parameters and Biot's coefficient promotes a reduction in porosity because of the adsorption process when the gas pressure increases. Low values of bulk modulus increase the positive effect (i.e., increase) of hydro‐mechanical coupling on the porosity evolution. The presence of high initial liquid pressure in the rock mass prevents the progression of injected gas pressure when CO₂ dissolution in water is taken into account. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of in-situ conditions on relative permeability characteristics of CO2-brine systems

Carbon dioxide capture and geological storage (CCGS) is an emerging technology that is increasingly being considered for reducing greenhouse gas emissions to the atmosphere. Deep saline aquifers provide a very large capacity for CO₂ storage and, unlike hydrocarbon reservoirs and coal beds, are immediately accessible and are found in all sedimentary basins. Proper understanding of the displacement character of CO₂-brine systems at in-situ conditions is essential in ascertaining CO₂ injectivity, migration and trapping in the pore space as a residual gas or supercritical fluid, and in assessing the suitability and safety of prospective CO₂ storage sites. Because of lack of published data, the authors conducted a program of measuring the relative permeability and other displacement characteristics of CO₂-brine systems for sandstone, carbonate and shale formations in central Alberta in western Canada. The tested formations are representative of the in-situ characteristics of deep saline aquifers in compacted on-shore North American sedimentary basins. The results show that the capillary pressure, interfacial tension, relative permeability and other displacements characteristics of CO₂-brine systems depend on the in-situ conditions of pressure, temperature and water salinity, and on the pore size distribution of the sedimentary rock. This paper presents a synthesis and interpretation of the results.     

Numerical analysis of the effect of natural microcracks on the supercritical CO2 fracturing crack network of shale rock based on bonded particle models

The problem of predicting the geometric structure of induced fractures is highly complex and significant in the fracturing stimulation of rock reservoirs. In the traditional continuous fracturing models, the mechanical properties of reservoir rock are input as macroscopic quantities. These models neglect the microcracks and discontinuous characteristics of rock, which are important factors influencing the geometric structure of the induced fractures. In this paper, we simulate supercritical CO₂ fracturing based on the bonded particle model to investigate the effect of original natural microcracks on the induced‐fracture network distribution. The microcracks are simulated explicitly as broken bonds that form and coalesce into macroscopic fractures in the supercritical CO₂ fracturing process. A calculation method for the distribution uniformity index (DUI) is proposed. The influence of the total number and DUI of initial microcracks on the mechanical properties of the rock sample is studied. The DUI of the induced fractures of supercritical CO₂ fracturing and hydraulic fracturing for different DUIs of initial microcracks are compared, holding other conditions constant. The sensitivity of the DUI of the induced fractures to that of initial natural microcracks under different horizontal stress ratios is also probed. The numerical results indicate that the distribution of induced fractures of supercritical CO₂ fracturing is more uniform than that of common hydraulic fracturing when the horizontal stress ratio is small.     

Potential and Suitability Evaluation of CO2 Geological Storage in Major Sedimentary Basins of China, and the Demonstration Project in Ordos Basin

From 2010 to 2012, the China Geological Survey Center for Hydrogeology and Environmental Geology Survey (CHEGS) carried out the project “Potential evaluation and demonstration project of CO2 Geological Storage in China”. During this project, we developed an evaluation index system and technical methods for the potential and suitability of CO2 geological storage based on China’s geological conditions, and evaluated the potential and suitability of the primary basins for CO2 geological storage, in order to draw a series of regional scale maps (at a scale of 1:5000000) and develop an atlas of the main sedimentary basins in China. By using these tools, we delineated many potential targets for CO2 storage. We also built techniques and methods for site selection and the exploration and assessment of CO2 geological storage in deep saline aquifers. Furthermore, through cooperation with the China Shenhua Coal to Liquid and Chemical Co., Ltd., we successfully constructed the first coal-based demonstration project for CO2 geological storage in deep saline aquifers in the Yijinhuoluo Banner of Ordos in the Inner Mongolia Autonomous Region, which brought about the basic preliminary theories, techniques, and methods of geological CO2 storage in deep saline aquifers under China’s geological conditions.     

Chemistry of fluids from a natural analogue for a geological CO2 storage site (Montmiral, France): Lessons for CO2–water–rock interaction assessment and monitoring

Chemical and isotope studies of natural CO₂ accumulations aid in assessing the chemical effects of CO₂ on rock and thus provide a potential for understanding the long-term geochemical processes involved in CO₂ geological storage. Several natural CO₂ accumulations were discovered during gas and oil exploration in France’s carbogaseous peri-Alpine province (south-eastern France) in the 1960s. One of these, the Montmiral accumulation at a depth of more than 2400 m, is currently being exploited. The chemical composition of the water collected at the wellhead has changed in time and the final salinity exceeds 75 g/L. These changes in time can be explained by assuming that the fraction of the reservoir brine in the recovered brine–CO₂–H₂O mixture varies, resulting in variable proportions of H₂O and brine in the sampled water. The proportions can be estimated in selected samples due to the availability of gas and water flowrate data. These data enabled the reconstruction of the chemical and isotope composition of the brine. The proportions of H₂O and brine can also be estimated from isotope (δ²H, δ¹⁸O) composition of collected water and δ¹⁸O of the sulfates or CO₂. The reconstituted brine has a salinity of more than 85 g/L and, according to its Br⁻ content and isotope (δ²H, δ¹⁸O, δ³⁴S) composition, originates from an evaporated Triassic seawater that underwent dilution by meteoric water. The reconstitution of the brine’s chemical composition enabled an evaluation of the CO₂–water–rock interactions based on: (1) mineral saturation indices; and (2) comparison with initial evaporated Triassic seawater. Dissolution of K- and SO₄-containing minerals such as K-feldspar and anhydrite, and precipitation of Ca and Mg containing minerals that are able to trap CO₂ (carbonates) are highlighted. The changes in concentration of these elements in the brine, which are attributed to CO₂ interactions, illustrate the relevance of monitoring the water quality at future industrial CO₂ storage sites.     

Groundwater geochemistry of a small reservoir catchment in Central Tunisia

Due to the scarcity of water resources in semiarid sedimentary basins, hill reservoirs are often constructed to recharge groundwater and limit runoff induced water loss. The impact of such reservoirs on groundwater chemistry is investigated in the aquifers of the El Gouazine watershed, Central Tunisia. Three groundwater types are recognised, Ca–HCO₃, Na–Cl and Ca–SO₄. The strong similarity between host rock and groundwater chemistries indicates significant rock–water interaction. A flowpath, along which the chemical composition of the groundwater evolves, can be identified using the contrast in stable isotope signature between upstream and downstream groundwater. Shallow upstream groundwater is recharged by the infiltration of rainwater with the rate of recharge strongly linked to the permeability of the host lithology. Calcium and HCO₃ are supplied to an alluvial aquifer from a more rapidly recharged limestone aquifer with the concentration of Ca and HCO₃ ions decreasing by dilution. The alluvial aquifer is also enriched in Ca and SO₄ during the downstream flow of groundwater through gypsiferous materials. There is evidence of mixing between meteoric groundwater and evaporated reservoir water. Below the reservoir and partly responsible for reservoir leakage is a sandy aquifer, formed by weathering and erosion of a sandstone host which also supplies water to the alluvial aquifer.     

Changes in the chemistry of shallow groundwater related to the 2008 injection of CO2 at the ZERT field site, Bozeman, Montana

Approximately 300 kg/day of food-grade CO₂ was injected through a perforated pipe placed horizontally 2–2.3 m deep during July 9–August 7, 2008 at the MSU-ZERT field test to evaluate atmospheric and near-surface monitoring and detection techniques applicable to the subsurface storage and potential leakage of CO₂. As part of this multidisciplinary research project, 80 samples of water were collected from 10 shallow monitoring wells (1.5 or 3.0 m deep) installed 1–6 m from the injection pipe, at the southwestern end of the slotted section (zone VI), and from two distant monitoring wells. The samples were collected before, during, and following CO₂ injection. The main objective of study was to investigate changes in the concentrations of major, minor, and trace inorganic and organic compounds during and following CO₂ injection. The ultimate goals were (1) to better understand the potential of groundwater quality impacts related to CO₂ leakage from deep storage operations, (2) to develop geochemical tools that could provide early detection of CO₂ intrusion into underground sources of drinking water (USDW), and (3) to test the predictive capabilities of geochemical codes against field data. Field determinations showed rapid and systematic changes in pH (7.0–5.6), alkalinity (400–1,330 mg/l as HCO₃), and electrical conductance (600–1,800 μS/cm) following CO₂ injection in samples collected from the 1.5 m-deep wells. Laboratory results show major increases in the concentrations of Ca (90–240 mg/l), Mg (25–70 mg/l), Fe (5–1,200 ppb), and Mn (5–1,400 ppb) following CO₂ injection. These chemical changes could provide early detection of CO₂ leakage into shallow groundwater from deep storage operations. Dissolution of observed carbonate minerals and desorption-ion exchange resulting from lowered pH values following CO₂ injection are the likely geochemical processes responsible for the observed increases in the concentrations of solutes; concentrations generally decreased temporarily following four significant precipitation events. The DOC values obtained are 5 ± 2 mg/l, and the variations do not correlate with CO₂ injection. CO₂ injection, however, is responsible for detection of BTEX (e.g. benzene, 0–0.8 ppb), mobilization of metals, the lowered pH values, and increases in the concentrations of other solutes in groundwater. The trace metal and BTEX concentrations are all significantly below the maximum contaminant levels (MCLs). Sequential leaching of core samples is being carried out to investigate the source of metals and other solutes.     

Monitoring of fluid–rock interaction and CO2 storage through produced fluid sampling at the Weyburn CO2-injection enhanced oil recovery site,Saskatchewan, Canada

The Weyburn Oil Field is a carbonate reservoir in south central Saskatchewan, Canada and is the site of a large CO₂ injection project for purposes of enhanced oil recovery. The Weyburn Field, in the Mississippian Midale Formation, was discovered in 1954 and was under primary production until secondary recovery by water flood began in 1964. The reservoir comprises two units, the Vuggy and the Marly, and primary and secondary recovery are thought to only have significantly depleted the Vuggy zone, leaving the Marly with higher oil saturations. In 2000, PanCanadian Resources (now EnCana), the operator of the field, began tertiary recovery by injection of CO₂ and water, primarily into the Marly. The advent of this project was an opportunity to study the potential for geological storage of CO₂.Using 43 Baseline samples collected in August 2000, before CO₂ injection at Weyburn, and 44 monitoring samples collected in March 2001, changes in the fluid chemistry and isotope composition have been tracked. The initial fluid distribution showed water from discovery through water flood in the Midale Formation with Cl ranging from 25,000 to 60,000 mg/L, from the NW to the SE across the Phase 1A area. By the time of Baseline sampling the produced water had been diluted to Cl of 25,000–50,000 mg/L as a result of the addition of make up water from the low TDS Blairmore Formation, but the pattern of distribution was still present. The Cl distribution is mimicked by the distribution of other dissolved ions and variables, with Ca (1250–1500 mg/L) and NH_{3 (aq)} increasing from NW to SE, and alkalinity (700–300 mg/L), resistivity, and H₂S (300–100 mg/L) decreasing. Based on chemical and isotopic data, the H₂S is interpreted to result from bacterial SO₄ reduction. After 6 months of injection of CO₂, the general patterns are changed very little, except that the pH has decreased by 0.5 units and alkalinity has increased, with values over 1400 mg/L in the NW, decreasing to 500 mg/L in the SE. Calcium has increased to range from 1250 to 1750 mg/L, but the pattern of NW–SE distribution is altered. Chemical and isotopic data suggest this change in distribution is caused by the dissolution of calcite due to water–rock reactions driven by CO₂. The Baseline samples varied from −22 to −12‰ δ¹³C (V-PDB) for CO₂ gas. The injected CO₂ has an isotope ratio of −20‰. The Monitor-1 samples of produced CO₂ ranged from −18 to −13‰, requiring a heavy source of C, most easily attributed to dissolution of carbonate minerals. Field measured pH had increased and alkalinity had decreased by the second monitoring trip (July 2001) to near Baseline values, suggesting continued reaction with reservoir minerals.Addition of CO₂ to water–rock mixtures comprising carbonate minerals causes dissolution of carbonates and production of alkalinity. Geochemical modeling suggests dissolution is taking place, however more detail on water–oil–gas ratios needs to be gathered to obtain more accurate estimates of pH at the formation level. Geological storage of CO₂ relies on the potential that, over the longer term, silicate minerals will buffer the pH, causing any added CO₂ to be precipitated as calcite. Some initial modeling of water–rock reactions suggests that silica sources are available to the water resident in the Midale Formation, and that clay minerals may well be capable of acting as pH buffers, allowing injected CO₂ to be stored as carbonate minerals. Further work is underway to document the mineralogy of the Midale Formation and associated units so as to define more accurately the potential for geological storage.     

Deep groundwater in the crystalline basement of the Black Forest region

Two major types of groundwater can be readily distinguished in the Variscian crystalline basement of the Black Forest in S–W Germany. Saline thermal water utilized in spas has its origin in 3–4 km deep reservoirs and developed its composition by 3 component mixing of surface freshwater, saltwater (of ultimately marine origin) and a water–rock reaction component. In contrast to the thermal water, CO₂-rich mineral water, tapped and bottled from many wells in the Black Forest, has low salinities but a TDS distribution similar to that of thermal water. It developed its chemical composition entirely by reaction of CO₂-rich water with the gneissic or granitic aquifer rock matrix. Particularly important is the contribution of various plagioclase dissolution and weathering reactions that may, at some locations, involve precipitation and dissolution of secondary calcite. Sodium/Ca ratios of water and of rock forming plagioclase in the basement rocks suggests that plagioclase weathering is strongly incongruent. Calcium is released to the water, whereas Na remains fixed to the albite feldspar component.The major element composition of 192 water samples used in this study also indicates a clear vertical stratification of the type of water chemistry; Ca–HCO₃ near the surface, Na–Ca–HCO₃–SO₄ at intermediate depth and Na–Ca–Cl at great depth.The mean permeability of Black Forest granite is about K=10⁻⁶ m/s; it is significantly lower in gneisses (gneiss: mean K=5×10⁻⁸ m/s) leading to focused flow through granite. Highly permeable fracture and fault zones, particularly in granite, are utilized by high-TDS saline deep groundwater as ascent channels and flow paths. Although spatially closely associated, the topography driven upwelling system of saline deep water and the near-surface flow system of CO₂-rich mineral waters are hydraulically and chemically unconnected.     

Natural and industrial analogues for leakage of CO2 from storage reservoirs: identification of features, events, and processes and lessons learned

Instances of gas leakage from naturally occurring CO₂ reservoirs and natural gas storage sites serve as analogues for the potential release of CO₂ from geologic storage sites. This paper summarizes and compares the features, events, and processes that can be identified from these analogues, which include both naturally occurring releases and those associated with industrial processes. The following conclusions are drawn: (1) carbon dioxide can accumulate beneath, and be released from, primary and secondary shallower reservoirs with capping units located at a wide range of depths; (2) many natural releases of CO₂ are correlated with a specific event that triggered the release; (3) unsealed fault and fracture zones may act as conduits for CO₂ flow from depth to the surface; (4) improperly constructed or abandoned wells can rapidly release large quantities of CO₂; (5) the types of CO₂ release at the surface vary widely between and within different leakage sites; (6) the hazard to human health was small in most cases, possibly because of implementation of post-leakage public education and monitoring programs; (7) while changes in groundwater chemistry were related to CO₂ leakage, waters often remained potable. Lessons learned for risk assessment associated with geologic carbon sequestration are discussed. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

The noble gas geochemistry of natural CO2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

Identification of the source of CO₂ in natural reservoirs and development of physical models to account for the migration and interaction of this CO₂ with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO₂ in the subsurface. We present the results of 57 noble gas determinations in CO₂ rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO₂/³He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO₂ can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO₂ phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO₂. We show that coherent fractionation of groundwater ²⁰Ne/³⁶Ar with crustal radiogenic noble gases (⁴He, ²¹Ne, ⁴⁰Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO₂ injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO₂/water phase partitioning. The CO₂ containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO₂ provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO₂. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO₂ filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm³ groundwater/cm³ gas (STP) in one Bravo Dome sample, to 2.56 cm³ groundwater/cm³ gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm³ water/cm³ gas (at reservoir pressure and temperature).     

The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO₂ and Fe₂O₃ on greenschist and amphibolite facies mineral equilibria in metapelites. The end‐member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO₂ and Fe₂O₃ on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO₂ and Fe₂O₃ between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO₂ and Fe₂O₃ in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO₂ to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe₂O₃ develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe₂O₃‐rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.