CO2 Exsolution Research Articles

Geochemical impact of biomethane and natural gas blend injection in deep aquifer storage

Biomethane is one of the main renewable gases available today to offset fossil gas and decarbonize the energy system. The EU is seeking to rapidly replace part of the natural gas usually stored in deep saline aquifers by biomethane which contains up to 10000 ppm of O2. This study presents for the first-time assessment of deep aquifers’ resilience to biomethane and natural gas blend injection into two sandstone reservoirs with different mineral paragenesis from the Paris Basin (France). Multiphase reactive transport modeling allowed to evaluate changes in gas and water quality, mineralogy and moreover, to identify major resilience properties of storage facilities necessary to buffer the oxygen reactivity and induced acidification coming from gas–water–rock interactions. In the presence of pyrite, the injected oxygen is consumed during pyrite oxidation and contributes to the acidification of the formation water close to the injection point. However, the injection of 1% mole fraction of CO2(g) contained in the gas blend is found to be the main acidification factor. The analysis of reservoir resilience showed three protection levels of pH buffering capacity, which can effectively mitigate an increase in oxygen content up to 1000 ppm: (i) calcite dissolution, (ii) plagioclase and clay dissolution and (iii) clay sorption capacity. Models predict that the gas blend injection could induce minimal but long-term change in the mineralogy without significantly impacting the global porosity. Overall, both sandstones can preserve the quality of the gas plume despite partial CO2(g) exsolution induced by the geochemical perturbation of the formations. The developed models will be used as a basis for assessment of renewable natural gas storage facilities in the long-term.

Ascent rate of the Udachnaya-East kimberlite melts from olivine diffusion chronometry

Kimberlite melts are low volume, volatile-rich melts formed in the asthenosphere at depth >150-250 km. Due to their low viscosity and the exsolution of CO2 triggered by their interaction with the sub-cratonic mantle, these melts are highly buoyant and rise rapidly through the lithosphere, ultimately reaching the surface and erupting at sonic to super-sonic speed. Studies based on fluid/melt dynamics, fracturing and disaggregation of the transported mantle-derived cargo, as well as on H diffusion in olivine and Ar diffusion in magmatic phlogopite showed that the ascent rate of kimberlite melts can vary between 0.1 and 30 m/s. However, these rates are extrapolated from processes starting at different depths, and thus may not be representative for the whole ascent path of kimberlite melts through the sub-cratonic lithosphere. To shed light on this issue, we applied diffusion chronometry to the deepest feature ascribable to the liquid line of descent of the melts, i.e. early liquidus olivine formed around mantle-derived xenocrystic cores from the Udachnaya-East pipe (Siberian craton). Our results showed that the time elapsed between the formation of the Internal Zone II (early olivine on the liquidus) or rims (typical magmatic olivine) around xenocrystic cores and final cooling is between 5 and 74 days. Considering travelled distances of 80-110 km in accordance with thermobarometric calculations, this implies overall average ascent rates of kimberlite melts of 0.02 to 0.23 m/s, which are comparable to those of typical alkali basalts, albeit starting at much greater depth. This indicates that kimberlite melts start with moderate ascent rates and only later increase their speed, likely due to massive CO2 exsolution triggered by decarbonation reactions during melt/fluid-rock interactions in the lithospheric mantle. The overall timescales of 5 to 74 days obtained in our study overlap with the lowest values reported in the literature from chemical zoning of mantle-derived minerals (25 days to >100 years), confirming that the time elapsed between kimberlite-related mantle metasomatism and the ascent of kimberlite melts was short. Importantly, our data show that Fe-Mg interdiffusion in olivine can be used as a trustable chronometer in kimberlite systems, opening new perspectives on the study of ascent dynamics in kimberlites worldwide.

Oceanic intraplate explosive eruptions fed directly from the mantle

Constraining the volatile content of magmas is critical to our understanding of eruptive processes and their deep Earth cycling essential to planetary habitability [R. Dasgupta, M. M. Hirschmann, Earth Planet. Sci. Lett. 298, 1 (2010)]. Yet, much of the work thus far on magmatic volatiles has been dedicated to understanding their cycling through subduction zones. Further, studies of intraplate mafic volcanism have disproportionately focused on Hawaii [P. E. Wieser et al., Geochem. Geophys. Geosyst. 22, e2020GC009364 (2021)], making assessments of the overall role of intraplate volcanoes in the global volatile cycles a challenge. Additionally, while mafic volcanoes are the most common landform on Earth and the Solar System [C. A. Wood, J. Volcanol. Geotherm. Res. 7, 387-413 (1980)], they tend to be overlooked in favor of silicic volcanoes when it comes to their potential for explosivity. Here, we report primitive (olivine-hosted, with host Magnesium number - Mg# 78 to 88%) melt inclusion (MI) data from Fogo volcano, Cabo Verde, that suggest that oceanic intraplate silica-undersaturated explosive eruptions sample volatile-rich sources. Primitive MI (melt Mg# 70 to 71%) data suggest that these melts are oxidized (NiNiO to NiNiO+1) and very high in volatiles (up to 2 wt% CO2, 2.8 wt% H2O, 6,000 ppm S, 1,900 ppm F, and 1,100 ppm Cl) making Fogo a global endmember. Storage depths calculated from these high volatile contents also imply that magma storage at Fogo occurs at mantle depths (~20 to 30 km) and that these eruptions are fed from the mantle. Our results suggest that oceanic intraplate mafic eruptions are sustained from the mantle by high volatile concentrations inherited from their source and that deep CO2 exsolution (here up to ~800 MPa) drives their ascent and explosivity.

Assessment of the oxygen reactivity in a gas storage facility by multiphase reactive transport modeling of field data for air injection into a sandstone reservoir in the Paris Basin, France

The first objective of this study is to present unique field data on a three-year pilot test during which air containing 8 mol% O2(g) was injected as a cushion gas into a natural gas reservoir, a carbonate-cemented sandstone aquifer located in the Paris Basin (France). 10-year system survey showed that: the oxygen was fully depleted several months after injection completion, meanwhile CO2(g) was detected around 2–6 mol%; the pH decreased from 8 to 6, while reducing conditions shifted to mildly oxidizing ones with increasing concentration of sulfates in equilibrium with gypsum. 3 years after injection completion, the pH gradually returned to its near initial state and sulfates were reduced by 2 to 3 times. The second objective is to develop a multiphase reactive transport model based on the field data. Simulations were constructed using the HYTEC reactive transport code, progressing from 0D-batch to 2D-reservoir configurations. The model reproduced the gas-water-rock reactive sequence: 1/ full depletion of the injected O2(g) due to pyrite oxidation, 2/ leading to acidity production and dissolved sulfates, 3/ acidity buffering by calcite dissolution, 4/ followed by gypsum precipitation and CO2(g) exsolution. The model demonstrated that pyrite kinetics was the most significant factor governing not only the amount of O2(g), CO2(g) and dissolved minerals, but also the spatial extent of these chemical reactions and, hence, the gas spread inside the reservoir. The formulated advective Damköhler number for oxygen consumption indicated advection- and reaction-dominant regimes explaining the gas composition and extension. The developed field-based model could be used as a workflow for other gas storage facilities, e.g. biomethane, compressed air, and CO2. For underground biomethane storage, the O2(g) contents recommended in Europe, i.e. the EASEE-gas specification 2005-001-02, should have a low impact on gas composition and reservoir geochemistry when the reservoir contains efficient pH-buffers such as calcite.

Violent Groundwater Eruption Triggered by a Distant Earthquake

AbstractIt is now well established that earthquakes cause various hydrogeological responses at distances thousands of kilometers from the epicenter. What remains unexplained is the large amplitude and intensity of some responses. Following the 2004 Mw 9.1 Sumatra earthquake, groundwater 3,200 km from the epicenter erupted violently from a well and formed a water fountain reaching a height exceeding 60 m. We model the relevant processes by combining tidal analysis of groundwater level with numerical simulations using a two‐dimensional finite‐element model. We suggest that the eruption resulted from a combination of factors, including a rapid increase of crustal permeability and runaway CO2 exsolution and bubble nucleation induced by the passage of seismic waves. Our results may have implications for some engineering applications such as oil production and CO2 sequestration, and the eruption of hydrothermal features such as geysers.

Sealing fractures to increase underground storage security: Lessons learned from a multiscale multimodal imaging study of a syntaxial vein in a mudrock

Veins recovered from deep sedimentary formations offer insights into mineral precipitates and processes that lead to sealing of underground fractures. These processes are of interest in the context of subsurface negative emissions technologies such as geologic carbon storage, where fractures are potential pathways for unwanted fluid migration. In this study, we characterized the mineralogy and porosity of a syntaxial vein in a mudrock sample from the Wolfcamp formation in Texas. The original fracture had an aperture of 5 mm and is now filled with distinct zones of minerals and vuggy regions. Thin sections from cuts across the vein were examined at micron scale resolution using scanning electron microscopy, energy dispersive X-ray spectroscopy, QEMSCAN, and polarized light microscopy. Larger-scale analyses were done using synchrotron X-ray fluorescence. Collectively, these methods reveal elongated crystals of dolomite as large as 900 μm, overlain with a mixture of smaller crystals including calcite and ferroan dolomite. Silica fills some of the void space. Mineral identification was corroborated using powder X-ray diffraction. Quantitative analysis of a 3D X-ray computed tomography image indicates that the vein volume contains 62% elongate dolomite crystals, 33% mixed ferroan dolomite and calcite, 1% silica, and 4% vuggy void space. Synchrotron small and ultra-small angle X-ray scattering reveals that the vein mineral precipitates have ~1% porosity. This is much smaller than the porosity of the mudrock matrix. The findings in this study suggest that as the formation formed and subsided, fracture fluids migrated vertically and experienced pressure reduction causing exsolution of CO2. A geochemical simulation demonstrated how this could have led to carbonate precipitation in the veins. A fundamental understanding of the sequence of vein mineral precipitation and the associated reduction in porosity may inspire strategies designed to induce fracture sealing, thus preserving the integrity of underground fluid storage. Changes that would lead to CO2 exsolution, carbonate supersaturation, and mineral precipitation include increasing the pH, addition of divalent cations, enabling vertical migration with subsequent depressurization, and heating to reduce carbonate solubility.

Physicochemical patterns observed in a groundwater well with CO2 stratification: Learnings from an automated monitoring from South Korean national groundwater monitoring network

Unusual episodic fluctuations of electrical conductivity (EC) were observed twice a year in a national groundwater monitoring network well in South Korea where EC was automatically monitored at a depth of 20 m below ground level (bgl). To address the causes of the observed EC fluctuations, this study examined the depth profile of wellbore water in the 70-meter-deep monitoring well screened between 50 and 70 m bgl and cased down to 50 m bgl. The results of well logging, borehole video recording, and hydrochemical analysis of wellbore water indicated that the CO2-rich groundwater entering through the screened zones below 50 m bgl was physicochemically stratified into three layers with distinct EC that were separated by two transition zones in the well: a bottom layer (70–43 m bgl) with an EC of ∼3900 μS/cm, intermediate layer (35–24 m bgl) of ∼1800 μS/cm, and top layer (16–3 m bgl) of ∼300 μS/cm. The first transition zone at depths of 43–35 m bgl was attributed to CO2 exsolution in the open system and the subsequent physicochemical changes of wellbore water, while the second transition zone at depths of 24–16 m bgl was formed by the precipitation of hydrous ferric oxides with consequent sorption of remaining ions due to a sudden change toward the oxidizing environment. The monitoring probe installed at a depth of 20 m bgl was found to be located within the upper transition zone, which caused EC peaks when the well was purged at a depth of 25 m bgl for well maintenance twice a year. This study shows that automated groundwater monitoring systems may misguide one about the groundwater quality if an unexpected physicochemical variation (such as stratification) occurs in a monitoring well. Therefore, the presence of interface zones caused by abrupt changes in EC must be carefully considered when an automated monitoring well is designed. The screened zone is a suitable location for installing an automated monitoring probe to measure the representative water quality (i.e., EC), in particular, in an aquifer that is under the influence of inputs of low-pH and high-TDS fluids (e.g., CO2-rich groundwater and acid mine drainage).

Addressing the root cause of calcite precipitation that leads to energy loss in geothermal systems

Geothermal sources are an essential energy resource in many cities around the world, in which a deep hot water reservoir has two energetic bases: thermal and hydraulic. The use of this energy can produce both electricity and heat. A geothermal well is a way in which the energy flows from the reservoir to the surface. A known operational problem can directly consume hydraulic energy and indirectly consume thermal energy. Calcium carbonate (CaCO3) precipitation and scaling can reduce and even block the flow area of a geothermal well, deferring, degrading, or even closing the production of water, steam, heat, and power. CO2 degassing, which is intrinsic to the deepwater system, is often the cause of this problem. This relationship is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). With the head loss provided by the water flow, CO2(aq) changes to the gaseous state (CO2(aq) → CO2(g)), which disfavors the CaCO3 dissolution and implies its mineralization. The CO2(aq) closely links the two mentioned chemical reactions and makes them mutually dependent. Therefore, two causes favor the CaCO3 precipitation: changes in thermodynamic conditions (pressure and temperature) and CO2 exsolution. It takes place even if the hot water is not boiling or flashing. This paper aims to quantify these two causes by applying our techniques in some geothermal fields in different locations in the world. In all scenarios investigated, the release of CO2 contributes from 66.1% to 92.6% of the CaCO3 precipitation. These results can lead engineers to adequately remedy the problem and suitably design and operate a geothermal system as a whole.

Do Oceanic Convection and Clathrate Dissociation Drive Europa’s Geysers?

Abstract Water vapor geysers on Europa have been inferred from observations made by the Galileo spacecraft, the Hubble Space Telescope, and the Keck Observatory. Unlike the water-rich geysers observed on Enceladus, Europa’s geysers appear to be an intermittent phenomenon, and the dynamical mechanism permitting water to sporadically erupt through a kilometers-thick ice sheet is not well understood. Here we outline and explore the hypothesis that the Europan geysers are driven by CO2 gas released by dissociation and depressurization of CO2 clathrate hydrates initially sourced from the subsurface ocean. We show that CO2 hydrates can become buoyant to the upper ice–water interface under plausible oceanic conditions, namely, if the temperature or salinity conditions of a density-stratified two-layer water column evolve to permit the onset of convection that generates a single mixed layer. To quantitatively describe the eruptions once the CO2 has been released from the hydrate state, we extend a one-dimensional hydrodynamical model that draws from the literature on volcanic magma explosions on Earth. Our results indicate that for a sufficiently high concentration of exsolved CO2, these eruptions develop vertical velocities of ∼700 m s−1. These high velocities permit the ejecta to reach heights of ∼200 km above the Europan surface, thereby explaining the intermittent presence of water vapor at these high altitudes. Molecules ejected via this process will persist in the Europan atmosphere for a duration of about 10 minutes, limiting the timescale over which geyser activity above the Europan surface may be observable. Our proposed mechanism requires Europa’s ice shell thickness to be d ≲ 10 km.

Exsolution effects in CO2 huff-n-puff enhanced oil recovery: Water-Oil-CO2 three phase flow visualization and measurements by micro-PIV in micromodel

CO2 huff-n-puff is one of the most important methods to enhanced oil recovery, which can simultaneously improve oil production and achieve CO2 geological storage. Due to the fluctuation of formation pressure, water-oil-CO2 three phase migration in porous structures was affected significantly by supercritical CO2 exsolution and expansion. However, the existing research on phase distribution is not enough to understand the mechanism of three-phase migration and the essential processes beneath the phase distribution are still not clear. Here, we developed a high-temperature and high-pressure micro-PIV (Particle Image Velocimetry) and fluorescence visualization experimental system (10 MPa, 50°C), which realized the simultaneous measurement of the velocity field and phase distribution of the water-oil-CO2 three-phase. The multiphase migration process under different wettability conditions during the supercritical-subcritical CO2 exsolution were captured. The results showed that the exsolution induced CO2 bubbles expansion in aqueous and oil phase blocked the preferential migration passage and dominated different multiphase migration mechanisms under different wettability. It was found that in water-wet condition, the aqueous phase can still migrate downstream through the water film between the CO2/oil and the wall, unable to further drive CO2/oil downstream. While in oil-wet condition, the aqueous phase eventually broke through the oil phase blocked at throats and formed a new preferential migration passage, pushing part of the oil phase encapsulating CO2 bubbles downstream. Our research reveals the multiphase migration mechanism beneath the phase distribution under different wettability during the exsolution process of the huff-n-puff, which provides a new insight for CO2 enhanced oil recovery and geological storage.

Dissolution of mantle orthopyroxene in kimberlitic melts: Petrographic, geochemical and melt inclusion constraints from an orthopyroxenite xenolith from the Udachnaya-East kimberlite (Siberian Craton, Russia)

Reconstructing the original composition of kimberlite melts in the mantle and delineating the processes that modify them during magmatic ascent and emplacement in the crust remains a significant challenge in kimberlite petrology. One of the most significant processes commonly cited to drive initial kimberlite melts towards more Si-Mg-rich compositions and decrease the solubility of CO2 is the assimilation of mantle orthopyroxene. However, there is limited direct evidence to show the types of reactions that may occur between mantle orthopyroxene and the host kimberlite melt.To provide new constraints on the interaction between orthopyroxene and parental kimberlite melts, we examined a fresh (i.e. unmodified by secondary/post-magmatic alteration) orthopyroxenite xenolith, which was recovered from the serpentine-free units of the Udachnaya-East kimberlite (Siberian Craton, Russia). This xenolith is composed largely of orthopyroxene (~ 90%), along with lesser olivine and clinopyroxene and rare aluminous magnesian chromite. We can show that this xenolith was invaded by the host kimberlite melt along grain interstices and fractures, where it partially reacted with orthopyroxene along the grain boundaries and replaced it with aggregates of compositionally distinct clinopyroxene, olivine and phlogopite, along with subordinate Fe-Cr-Mg spinel, FeNi sulphides and djerfisherite (K6(Fe,Ni,Cu)25S26Cl).Primary melt inclusions in clinopyroxene replacing xenolith-forming orthopyroxene, as well as secondary melt inclusion trails in xenolith orthopyroxene, clinopyroxene and olivine are composed of similar daughter mineral assemblages that consist largely of: NaK chlorides, along with varying proportions of phlogopite, Fe-Cu-Ni sulphides, djerfisherite, rasvumite (KFe2S3), Cr-Fe-Mg spinel, nepheline and apatite, and rare rutile, sodalite, barite, olivine, Ca-K-Na carbonates and NaK sulphates. The melt entrapped by these inclusions likely represent the hybrid products produced by the invading kimberlite melt reacting with orthopyroxene in the xenolith.The mechanism that could explain the partial replacement of orthopyroxene in this xenolith by clinopyroxene, olivine and phlogopite could be attributed to the following reaction:Orthopyroxene + Carbonatitic (melt) ➔ Olivine + Clinopyroxene + Phlogopite + CO2.This reaction is supported by theoretical and experimental studies that advocate the dissolution of mantle orthopyroxene within an initially silica-poor and carbonate-rich kimberlite melt. The mineral assemblages replacing orthopyroxene in the xenolith, together with hosted melt inclusions, suggests that the kimberlitic melt prior to reaction with orthopyroxene was likely carbonate-rich and Na-K-Cl-S bearing. The paucity of carbonate in the reaction zones around orthopyroxene and in melt inclusions in clinopyroxene replacing xenolith-forming orthopyroxene and xenolith minerals (orthopyroxene, clinopyroxene and olivine) is attributed to the consumption of carbonates and subsequent exsolution of CO2 by the proposed decarbonation reaction.Concluding, we propose that this orthopyroxenite xenolith provides a rare example of the types of reactions that can occur between mantle orthopyroxene and the host kimberlite melt. The preservation of this xenolith and zones around orthopyroxene present new insights into the composition and evolution of parental kimberlite melts and CO2 exsolution.

Constraining carbonation freezing and petrography of the carbonated cratonic mantle with natural samples

Peridotite xenoliths from the Cretaceous Chidliak kimberlite province (SE Baffin Island, Canada) were recently studied by Kopylova et al. (2019). Here, we focus on rare textures, with orthopyroxene grains invariably rimmed by 3–20 μm coronas of clinopyroxene, while all clinopyroxenes are rimmed by equally thin monticellite coronas. Thicker, 0.1–0.5 mm texturally equilibrated clinopyroxene also mantles garnet, and there is a gradual transition from micron- to millimeter-thick clinopyroxene mantles. We investigated the origin of these rarely preserved textures using major and trace element zoning in minerals, and measured and reconstructed bulk compositions of xenoliths. Fluxes of major elements were identified based on the conserved element ratios while accounting for the closure effect due to normalization of bulk compositions to 100%. Ca dominates the absolute elemental gain, expressed in moles per 1000 mol of Fe. The observed mineralogical and compositional changes are associated with the significant metasomatic removal of Na (70% of its budget) Al, and Cr (35% loss), minor removal of Si, Mn, Mg and Ni and the gain of Ca (~ 20%), Ti, K and incompatible trace elements. The metasomatic fluid addition beneath Chidliak was likely below 10%. The fluid was very enriched and fractionated resembling volatile-rich low-degree melts like carbonatites or kimberlites. The Chidliak peridotites were affected by “carbonation freezing”, i.e. immobilization of a carbonate-rich metasomatic agent via reactions with pyroxenes. Clinopyroxene and monticellite coronas formed in decarbonation reactions, whereby ephemeral carbonatitic fluid readily gave away Ca to silicate minerals and exsolved CO2. Chidliak peridotites highlight that it would be deceptive to imagine “carbonated peridotites” storing carbon in a normal assemblage of peridotite plus carbonate. “Carbonated peridotites” are coarse peridotites with elevated modes of clinopyroxene, garnet and olivine, and with thin rims of calcic silicate minerals storing incompatible elements. The CO2-rich magmatism on cratons and the match between the temporal Ca addition to the cratonic mantle and the observed fluxes from the carbonate-rich metasomatism underscores the importance of the latter process in shaping up the lithospheric mantle and its melts.

COMPUTATIONAL METHODOLOGY TO ANALYZE THE EFFECT OF MASS TRANSFER RATE ON ATTENUATION OF LEAKED CARBON DIOXIDE IN SHALLOW AQUIFERS

Exsolution and re-dissolution of CO2 gas within heterogeneous porous media are investigated using experimental data and mathematical modeling. In a set of bench-scale experiments, water saturated with CO2 under a given pressure is injected into a 2-D water-saturated porous media system, causing CO2 gas to exsolve and migrate upwards. A layer of fine sand mimicking a heterogeneity within a shallow aquifer is present in the tank to study accumulation and trapping of exsolved CO2. Then, clean water is injected into the system and the accumulated CO2 dissolves back into the flowing water. Simulated exsolution and dissolution mass transfer processes are studied using both nearequilibrium and kinetic approaches and compared to experimental data under conditions that do and do not include lateral background water flow. The mathematical model is based on the mixed hybrid finite element method that allows for accurate simulation of both advection- and diffusion- dominated processes.

Highly explosive basaltic eruptions driven by CO2 exsolution

The most explosive basaltic scoria cone eruption yet documented (>20 km high plumes) occurred at Sunset Crater (Arizona) ca. 1085 AD by undetermined eruptive mechanisms. We present melt inclusion analysis, including bubble contents by Raman spectroscopy, yielding high total CO2 (approaching 6000 ppm) and S (~2000 ppm) with moderate H2O (~1.25 wt%). Two groups of melt inclusions are evident, classified by bubble vol%. Modeling of post-entrapment modification indicates that the group with larger bubbles formed as a result of heterogeneous entrapment of melt and exsolved CO2 and provides evidence for an exsolved CO2 phase at magma storage depths of ~15 km. We argue that this exsolved CO2 phase played a critical role in driving this explosive eruption, possibly analogous to H2O exsolution driving silicic caldera-forming eruptions. Because of their distinct gas compositions relative to silicic magmas (high S and CO2), even modest volume explosive basaltic eruptions could impact the atmosphere.

Experimentally Induced Volumetric Re‐equilibration of Plagioclase‐Hosted Melt Inclusions

AbstractThe application of melt inclusions (MI) to infer magmatic processes assumes the MI have remained as constant mass, constant volume systems since the time of trapping. Understanding the effects of both compositional and volumetric re‐equilibration is key for the interpretation of MI data. Although the re‐equilibration behavior MI in quartz and olivine has been studied in some detail, the process is less understood for other MI host phases such as plagioclase, a common phase in igneous rocks. A MI can re‐equilibrate when it experiences pressure and temperature (PT) conditions that differ from formation PT conditions. During laboratory heating, irreversible MI expansion may occur. As a result, the internal pressure within the MI decreases, resulting in chemical and structural changes to the MI and host. We present results of heating experiments on plagioclase‐hosted MI designed to induce volumetric re‐equilibration. The experiments consisted of incrementally heating the MI to temperatures above the homogenization temperatures. At ∼40°C above, the temperature at which the daughter minerals melted, irreversible volume expansion lowered the pressure in the MI, and led to exsolution of CO2 into vapor bubbles. With each additional few degrees of heating, additional episodes of CO2 exsolution, bubble nucleation and expansion of the vapor bubbles occurred. Re‐equilibration of MI in plagioclase occurred through a combination of ductile and brittle deformation of the host surrounding the MI, whereas previous studies have shown that MI in olivine re‐equilibrate dominantly through ductile deformation associated with movement along dislocations. This behavior is consistent with the differing rheological properties of these phases.

Quantitative vesicle analyses and total CO2 reconstruction in mid-ocean ridge basalts

Vesicle textures in submarine lavas have been used to calculate total (pre-eruption) volatile concentrations in mid-ocean ridge basalts (MORB), which provide constraints on upper mantle volatile concentrations and global mid-ocean ridge CO2 flux. In this study, we evaluate vesicle size distributions (VSDs) and volatile concentrations in a suite of 20 MORB samples that span the range of vesicularities and vesicle number densities observed in MORB globally. We provide recommended best practices for quantifying vesicularity, vesicle number densities, and VSDs based on synthetic vesicle populations and comparisons between traditional 2D estimates and X-ray computed micro-tomography results. For 2D measurements, we recommend analyzing multiple polished fragments with a cumulative area >100 times the area of the largest observed vesicle and including >200 vesicles in stereological VSD reconstructions. For 3D measurements, we recommend analyzing sample volumes >0.01 cm3 at resolutions <2.0 μm/pixel for low vesicularity MORB (i.e., <4 vol.%) and sample volumes >0.1 cm3 with resolutions <5 μm/pixel for higher vesicularity samples. Our validation of vesicularity measurements allows reconstructions of total CO2 concentrations in MORB using dissolved volatile concentrations, vesicularities, and equations of state. We assess approaches for estimating the exsolved CO2 concentration in MORB vesicles and find that CO2(g) density is ~40% lower than previously suggested, likely due to melt contraction during quenching. Based on these results, we recommend using sample eruption pressures, magmatic temperatures, and an equation of state that accounts for non-ideality at high temperatures to calculate exsolved CO2 concentrations when independent constraints from Raman spectroscopy or laser ablation are unavailable. Our results suggest that some previous studies may have overestimated MORB CO2 concentrations by as much as 50%, with the greatest differences in samples with the highest vesicularities. These new results imply lower CO2/Ba of undegassed, enriched-MORB and lower integrated global ridge CO2 flux than previously inferred.

Sulphur behaviour and redox conditions in etnean magmas during magma differentiation and degassing

Abstract Sulphur behaviour and variations in redox conditions during magma differentiation and degassing in Mt. Etna (Italy) volcanic system have been explored by integrating the study of olivine-hosted melt inclusions (MIs) with an experimental survey of sulphur solubility in hydrous basaltic magmas. Sulphur solubility experiments were performed at conditions relevant to the Etnean plumbing system (1200 °C, 200 MPa and oxygen fugacity between NNO + 0.2 and NNO + 1.7, with NNO being the Nickel-Nickel Oxide buffer), and their results confirm the important control of oxygen fugacity (fO2) on S abundance in mafic magmas and on S partitioning between fluid and melt phases (DSfluid/melt). The observed DSfluid/melt value increases from 51 ± 4 to 146 ± 6 when fO2 decreases from NNO + 1.7 ± 0.5 to NNO + 0.3. Based on the calculated DSfluid/melt and a careful selection of previously published data, an empirical model is proposed for basaltic magmas in order to predict the variation of DSfluid/melt values upon variations in P (25–300 MPa), T (1030–1200 °C) and fO2 (between NNO-0.8 and NNO + 2.4). Olivine-hosted melt inclusions (Fo89-91) from tephra of the prehistoric (4 ka BP) sub-plinian picritic eruption, named FS (“Fall Stratified”), have been investigated for their major element compositions, volatile contents and iron speciation (expressed as Fe3+/ƩFe ratio). These primitive MIs present S content from 235 ± 77 to 3445 ± 168 ppm, while oxygen fugacity values, estimated from Fe3+/ƩFe ratios, range from NNO + 0.7 ± 0.2 to NNO + 1.6 ± 0.2. Iron speciation has also been investigated in more evolved and volatile-poorer Etnean MIs. The only primitive melt inclusion from Mt. Spagnolo eruption (4–15 ka BP) presents a S content of 1515 ± 49 ppm and an estimated fO2 of NNO + 1.4 ± 0.1. The more evolved MIs (from 2002/2003, 2006, 2008/2009 and 2013 eruptions) have S content lower than 500 ppm, and their Fe3+/ΣFe ratios result in fO2 between NNO-0.9 ± 0.1 and NNO + 0.4 ± 0.1. Redox conditions and S behaviour in Etnean magmas during degassing and fractional crystallization were modelled coupling MELTS code with our empirical DSfluid/melt model. Starting from a FS-type magma composition and upon decrease of T and P, fractional crystallization of olivine, clinopyroxene, spinel and plagioclase causes a significant fO2 decrease. The fO2 reduction, in turn, causes a decrease in sulphur solubility and an increase in DSfluid/melt, promoting S exsolution during magma ascent, which further enhances the reduction of fO2. For the evolved melt inclusions of 2002–2013 eruptions, magma differentiation may therefore have played a crucial role in decreasing redox conditions and favouring efficient S degassing. Differently, during the unusual FS eruption, only limited melt evolution is observed and S exsolution seems to have been triggered by a major pressure decrease accompanied by H2O and CO2 exsolution during fast magmatic ascent.

Characterization of CO2 self-release during Heletz Residual Trapping Experiment I (RTE I) using a coupled wellbore-reservoir simulator

In order to quantify CO2 residual trapping in situ, two dedicated single-well push-pull experiments have been carried out at the Heletz, Israel pilot CO2 injection site. Field data from some parts of these experiments suggests the important effect of the hydrodynamic behavior in the injection-withdrawal well. In the present work a model capturing the CO2 transport and trapping behavior during Heletz Residual Trapping Experiment I is developed, with a special focus on coupled wellbore-reservoir flow. The simulation is carried out with the numerical simulator T2Well/ECO2N (Pan et al. 2011) which considers the wellbore-reservoir coupling. Of particular interest is to accurately model the period when the well is open to the atmosphere and self-producing CO2 and water in a geyser-like manner. It is also of interest to identify what conditions are causing the oscillating pressure-temperature behavior and the associated periodic gas-liquid releases, as well as to determine the amount of gas lost from the reservoir during this period. The results suggest that the behavior is due to cyclical CO2 exsolution from the aqueous phase along with a reduction of mobility of both CO2 and brine in the near wellbore-reservoir area, the latter being due to a zone of dispersed CO2 bubbles near the wellbore. This behavior could be successfully captured with a new set of relative permeability functions developed earlier for CO2 exsolution in laboratory experiments (Zuo et al., 2013).

Fluid Infiltration and Mass Transfer along a Lamprophyre Dyke–Marble Contact: An Example from the South-Western Korean Peninsula

In this contribution, we report the metasomatic characteristics of a lamprophyre dyke–marble contact zone from the Hongseong–Imjingang belt along the western Gyeonggi Massif, South Korea. The lamprophyre dyke intruded into the dolomitic marble, forming a serpentinized contact zone. The zone consists of olivine, serpentine, calcite, dolomite, biotite, spinel, and hematite. Minor F and Cl contents in the serpentine and biotite indicate the composition of the infiltrating H2O-CO2 fluid. SiO2 (12.42 wt %), FeO (1.83 wt %), K2O (0.03 wt %), Sr (89 ppm), U (0.7 ppm), Th (1.44 ppm), and rare earth elements (REEs) are highly mobile, while Zr, Cr, and Ba are moderately mobile in the fluid. Phase equilibria modelling suggests that the olivine, spinel, biotite, and calcite assemblage might be formed by the dissolution of dolomite at ~700 °C, 130 MPa. Such modelling requires stable diopside in the observed conditions in the presence of silica-saturated fluid. The lack of diopside in the metasomatized region is due to the high K activity of the fluid. Our log activity K2O (aK2O)–temperature pseudosection shows that at aK2O~−40, the olivine, spinel, biotite, and calcite assemblage is stable without diopside. Subsequently, at ~450 °C, 130 MPa, serpentine is formed due to the infiltration of H2O during the cooling of the lamprophyre dyke. This suggests that hot H2O-CO2 fluids with dissolved major and trace elements infiltrated through fractures, grain boundaries, and micron-scale porosity, which dissolved dolomite in the marble and precipitated the observed olivine-bearing peak metasomatic assemblage. During cooling, exsolved CO2 could increase the water activity to stabilize the serpentine. Our example implies that dissolution-reprecipitation is an important process, locally and regionally, that could impart important textural and geochemical variations in metasomatized rocks.

Pore-scale investigation of CO2/oil exsolution in CO2 huff-n-puff for enhanced oil recovery

A pore-scale high-pressure visualization experimental system is used to investigate CO2 exsolution during the CO2 huff-n-puff process for enhanced oil recovery and geological CO2 storage. Eighteen different experimental cases are examined to investigate the mechanisms by which depressurization-induced CO2 exsolution is affected by a near-miscible vs an immiscible CO2/oil initial state, by the depressurization rate, and by the presence of a water phase under different wettability conditions. CO2 exsolution is divided into three processes: nucleation, growth and coalescence, and migration. Visual observations and statistical results indicate that a near-miscible CO2/oil initial state causes intense and instantaneous CO2 nucleation. The presence of water effectively hinders the coalescence and migration of CO2 ganglia, reducing the generation and rapid departure of large CO2 ganglia in both water-wet and oil-wet cases. The amount of residual CO2 ganglia increases significantly in the presence of water, and the volumes of most of the increased residual CO2 ganglia are small. Hindrance by the presence of water is predominantly due to contact angle hysteresis rather than the Jamin effect, since it is found that the triple-phase contact lines do not move under most conditions. The residual CO2 saturation increases substantially in the presence of water, regardless of the wettability (oil vs water), with the improvement reaching 95%.