Geological storage of CO2 within the oceanic crust by gravitational trapping

The rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long-term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep-sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment-basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment-covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions

Geological storage of carbon dioxide in oceanic crust

The rise of atmospheric carbon dioxide (CO2), due to decades of burning of fossil fuels, is a key driver of anthropogenic climate change. Carbon Capture and Storage (CCS) is one of the most promising mitigation strategies for long-term sequestration of CO2.Unlike most conventional CCS investigations targeting deep saline aquifers, this thesis focuses on the potential of the uppermost oceanic crust, inspired by the strong evidence that basaltic seafloor has acted, in the past, as a major sink for CO2.The study of temperature, pressure, and density of CO2 and seawater at the sediment-basement interface for the whole seafloor highlights the influence of water depth, sediment thickness, and oceanic crustal age on the relative gravitational stability of CO2. Consequently, 8% of the entire oceanic crust is recognised as suitable for gravitational and physical trapping of CO2 injected into the basement. Five potential targets are proposed, and even the smallest of these provides sufficient carbon dioxide sequestration capacity for the next centuries.Batch experiments on the mineral dissolution of submarine mafic rocks and ophiolitic gabbro, in CO2-rich solutions, contribute to improve the fundamental understanding of geochemical reactions at mid-ocean ridge flank temperatures (40 ?C). Concentrations of silicon and calcium in solution, and particle size are identified as the key factors to quantify the rock reactivity. Ca dissolution rates suggest calcite, plagioclase and amphibole are the principal sources of calcium at pH ~5.The attempted estimation of costs related to the transport and storage of 20 Mt/yr of CO2 in deep-sea basalts, as a function of distance from the shore, injection rate, and water depth, shows the economic feasibility of potential offshore CCS projects. Overall, the expenditures are dominated by the number of ships and wells required to deliver large volumes of CO2 to reservoirs located far from the coast, rather than by the water depth. These financial considerations could potentially improve if the CCS strategies conquered a significant place in the global market

Experimental study of epidote dissolution rates from pH 2 to 11 and temperatures from 25 to 200 °C

International audienceThe dissolution rates of Green Monster Mine epidote at temperatures of 25, 100 and 200 °C, and over the pH range 2–11, were determined from far-from equilibrium experiments performed in both batch and mixed-flow reactors. Epidote dissolution rates based on silicon release decrease with increasing pH to pH ∼ 8, then increase in response to further pH increases. The apparent relative metal release rates from epidote at chemical steady-state were found to vary with pH and temperature, in part due to secondary phase formation. At 100 and 200 °C, Al is preferentially released relative to Si at alkaline pH, but preferentially retained by the solid at 2 < pH < 8. This behaviour is interpreted to stem from the precipitation and re-dissolution of Al oxy-hydroxides such as boehmite, diaspore, and gibbsite, as observed in solids recovered after the experiments. Calcium tends to be preferentially released compared to Si and Al at acidic and near to neutral pH. Iron was found to be retained in the solid phase due to the prompt formation of secondary Fe minerals. Nevertheless, neither the dissolution of 20 wt% of the initial epidote mass nor the formation of secondary phases on the epidote surface substantially changed epidote dissolution rates. As epidote (i) is almost three times more Ca-rich than a typical basaltic glass, (ii) preferentially releases Ca at pH typical of CO2-charged water injections, and (iii) exhibits increasing dissolution rates as pH increases at alkaline conditions, it seems likely that the presence of epidote in altered basalts may be beneficial for the formation of calcite and other Ca-carbonates during mineral carbonation efforts. In addition, the relatively rapid rates of epidote dissolution at acidic conditions suggest that epidote has a significant role in the weathering of altered mafic rocks

Co2 Mineralization in Mafic Rocks: From Laboratory Experiments to Pilot Sites

To date, one of the safest long-term CO2 storage solutions is through carbon mineralization in mafic (or ultramafic) rocks containing high proportions of Mg, Ca, and Fe, which can react with dissolved CO2 to form carbonate-bearing minerals, ensuring its stability over time. The challenges still to be faced by this approach include i) the scalability to a worldwide scenario; ii) the adaptability to local geological contexts; and iii) the standardization of its application at industrial levels. A number of experimental and theoretical studies have been carried out to face these challenges, especially in relation to high temperature scenarios, but few have developed to date a consistent experimental procedure able to determine the in situ carbon mineralization potential in low-temperature geological settings that would, if effective, enhance industrial confidence in CCS/CCUS technologies, and possibly in its future applications. Within this context, we provide an overview of the experimental studies that have been conducted over the last 20 years, with an emphasis on the ongoing research aimed at improving the knowledge of the conditions and elementary processes that control the sequestration potential of mafic and ultramafic reservoirs. The results of these studies should direct the advancement of experimental and analytical protocols and will help the development and successful application of future CCUS actions

Carbonate geochemistry and its role in geologic carbon storage

Massive quantities of CO 2 need to be captured and stored to address the potential consequences of global warming. Geologic storage of CO 2 may be the only realistic option available to store the bulk of this CO 2 due to the required storage volumes. Geologic storage involves the injection of CO 2 into the subsurface. This injection will lead to the acidification of the formation fluids and provoke a large number of fluid-mineral reactions in the subsurface. Of these reactions, those among CO 2-rich fluids and carbonate minerals may be the most significant as these reactions are relatively rapid and have the potential to alter the integrity of caprocks and well bore cements. This review provides a detailed summary of field, laboratory and modeling results illuminating the potential impacts of the injection of large quantities of CO 2 into the subsurface as part of geologic storage efforts

Experimental study on mafic rock dissolution rates within CO2-seawater-rock systems

Far-from-equilibrium batch experiments have been performed to study the low temperature dissolution potential of crystalline submarine basalts (from Juan de Fuca Plate and Mid-Atlantic Ridges) and of a highly altered gabbro from the Troodos ophiolite (Cyprus) in presence of seawater and carbon dioxide (CO2). The experiments have been carried out at 40 °C for up to 20 days with initial pH of ∼4.8 and under ∼1 bar pCO2 to identify the progressive water-rock interactions. Elemental steady-state release rates from the rock samples have been determined for silicon and calcium, the solution concentrations of which were found to be the most effective monitors of rock dissolution. Mass balance calculations based on dissolved Si and Ca concentrations suggest the operation of reaction mechanisms focussed on the grain surfaces that are characteristic of incongruent dissolution. Also, basic kinetic modelling highlights the role of mass-transport limitations during the experiments. Ca release rates at pH ∼ 5 indicate significant contributions of plagioclase dissolution in all the rocks, with an additional contribution of amphibole dissolution in the altered gabbro. Si release rates of all solids are found to be similar to previously studied reactions between seawater and basaltic glass and crystalline basalt from Iceland, but are higher than rates measured for groundwater-crystalline basalt interaction systems. This comparison with previous experimental results resumes the debate on the role of experimental variables, such initial rock mass and crystallinity, pCO2, and fluid chemistry on dissolution processes. Our new data suggest that CO2-rich saline solutions react with mafic rocks at higher rates than fresh water with low pCO2, at the same pH. Most significantly, both ophiolitic gabbro and Juan de Fuca basalts show Si and Ca release rates similar or higher than unaltered crystalline basalt from Iceland, highlighting the potential substantial role that ophiolitic rocks and offshore mafic reservoirs could play for the geological storage of CO2

Mineralization potential of water-dissolved CO2 and H2S injected into basalts as function of temperature: Freshwater versus Seawater

Highlights • Reaction path models quantified gas-charged waters/basalt interactions • Gas-charged freshwater and seawater compared • Geochemical reactions modelled at temperatures from 25 to 260°C • Optimal conditions for subsurface mineralization of CO2 and H2S identified Mineralization of freshwater-dissolved gases, such as CO2 and H2S, in subsurface mafic rocks is a successful permanent gas storage strategy. To apply this approach globally, the composition of locally available water must be considered. In this study, reaction path models were run to estimate the rate and extent of gas mineralization reactions during gas-charged freshwater and seawater injection into basalts at temperatures of 260, 170, 100, and 25°C. The calculations were validated by comparison to field observations of gas-charged freshwater injections at the CarbFix2 site (Iceland). The results show that more than 80% of the injected CO2 dissolved in freshwater or seawater mineralizes as Ca and Fe carbonates at temperatures ≤170°C after reaction of 0.2 mol/kgw of basalt, whereas at 260°C much lower carbon mineralization rates are observed in response to the same amount of basalt dissolution. This difference is due to the competition between carbonate versus non-carbonate secondary minerals such as epidote, prehnite, and anhydrite for Ca. In contrast, from 80 to 100% of the injected H2S is predicted to be mineralized as pyrite in all fluid systems at all considered temperatures. Further calculations with fluids having higher CO2 contents (equilibrated with 9 bar pCO2) reveal that i) the pH of gas-charged seawater at temperatures ≤170°C is buffered at ≤6 due to the precipitation of Mg-rich aluminosilicates, which delays CO2 carbonation; and ii) the most efficient carbonation in seawater systems occurs at temperatures <150°C as anhydrite formation is likely significant at higher temperatures

Author Correction: Carbon dioxide storage through mineral carbonation

International audienceAn amendment to this paper has been published and can be accessed via a link at the top of the paper

An experimental study of basalt–seawater–CO2 interaction at 130 °C

International audienceOver millions of years, the interaction of marine basalt with percolating seawater in low-temperature ocean floor hydrothermal systems leads to the formation of calcite and aragonite. The presence of these minerals in marine basalts provides evidence for substantial CO2 fixation in these rocks. Here, we report on laboratory experiments to study this process under enhanced CO2 partial pressures (pCO2) at 130 °C. Mid-ocean-ridge-basalt (MORB) glass was reacted with North Atlantic Seawater charged with CO2 in batch experiments lasting up to 7 months. For experiments initiated with seawater charged with ~ 2.5 bar pCO2, calcite and aragonite are the first carbonate minerals to form, later followed by only aragonite (± siderite and ankerite). For experiments initiated with seawater charged with ~ 16 bar pCO2, magnesite was the only carbonate mineral observed to form. In total, approximately 20 % of the initial CO2 in the reactors was mineralized within five months. This carbonation rate is similar to corresponding rates observed in freshwater-basalt-CO2 interaction experiments and during field experiments of the carbonation of basalts in response to CO2-charged freshwater injections in SW-Iceland. Our experiments thus suggest that CO2-charged seawater injected into submarine basalts will lead to rapid CO2 mineralization. Notably, at pCO2 of tens of bars, magnesite will form, limiting the formation of Mg-rich clays, which might otherwise compete for the Mg cation and pore-space in the submarine basaltic crust. This suggests that the injection of CO2-charged seawater into subsurface basalts can be an efficient and effective approach to the long-term safe mineral storage of anthropogenic carbon

Paleolatitudes of Late Triassic radiolarian cherts from Argolis, Greece: Insights on the paleogeography of the western Tethys

The Hellenides fold-and-thrust range comprises two subparallel ophiolite belts of Triassic to Jurassic age – the external ophiolite belt (e.g., Mirdita, Pindos, Argolis) in the west, and the internal ophiolite belt in the east – broadly separated by the continental crust units of the Korabi–Pelagonian Zone. It is still a matter of debate whether these ophiolites derived from a single ocean (Meliata–Maliac–Vardar) or mark the suture of two distinct oceanic seaways (Pindos and Meliata–Maliac–Vardar). We contribute to the resolution of this controversy by studying the Migdalitsa Ophiolitic Complex in Argolis (Greece), which contains Triassic oceanic basalts and pertains to the external (western) ophiolite belt. Three key areas were mapped and several sites were targeted for structural, biostratigraphic, and paleomagnetic analyses. Radiolarian cherts in primary or tectonic contact with oceanic basalts were dated to the Late Triassic (Carnian–Norian) using radiolarians, and provided paleomagnetic directions of primary origin carried by magnetite and hematite. The derived mean direction was corrected for sedimentary inclination shallowing and yielded a paleolatitude of ~ 22°N that was placed in a broader paleogeographic context by reconstructing Pangea at ~ 225 ± 5 Ma using a recent apparent polar wander path corrected for sedimentary inclination shallowing. The Late Triassic paleogeography of the Tethys Ocean was constrained using additional paleolatitude estimates from the literature, which we checked and corrected (when possible) for sedimentary inclination shallowing. According to our reconstruction, the Meliata–Maliac–Vardar Ocean between Adria – the promontory of Africa – and Europe represents the locus of origin of the Late Triassic Argolis ophiolitic rocks of the external ophiolite belt, presently resting in tectonic contact on the Pindos–Subpelagonian zones, which represent a deep-water trough with no substantial evidence of in situ oceanization in the Triassic