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

Pascale Benezeth, Chiara Marieni, Giuseppe Saldi

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Pascale Benezeth, Chiara Marieni, Giuseppe Saldi. Co2 Mineralization in Mafic Rocks: From Laboratory Experiments to Pilot Sites. 16th International Conference on Greenhouse Gas Control Technologies, GHGT-16, Oct 2023, Lyon, France. pp.4274163, 10.2139/ssrn.4274163 . hal-04251938

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Submitted on 7 Nov 2023

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# CO₂ mineralization in mafic rocks: from laboratory experiments to pilot sites

Pascale Bénézeth<sup>a,*</sup>, Chiara Marieni<sup>b</sup>, Giuseppe Saldi<sup>a</sup>

## Abstract

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.

**Keywords:** CO₂ mineralization; mafic rocks; pilot field sites; experiments

## 1. Introduction

Global anthropogenic emissions of carbon dioxide (CO₂) into the atmosphere, largely attributed to the use of fossil fuels, are significantly off-track towards meeting the climate targets set by the Paris Agreement [1]. Solutions are therefore required to promote a substantial reduction in the net flux of CO₂ into the atmosphere, while, at the same time, energy needs are still met. International energy agencies agree that already available climate-mitigation technologies, such as carbon capture utilization and storage (CCUS), must be pushed to their maximum practical limits to meet the Paris climate goals [2, 3]. Among the various CCS solutions considered to reduce CO₂ emissions, mineral sequestration of CO₂ offers a thermodynamically stable and reliable option over very long periods of time [4]. However, the highest carbon mineralization potential is reached within specific environments, in particular mafic (basalts) and ultramafic rocks (mainly peridotites and serpentinites). These lithologies are highly reactive and rich in magnesium, iron and calcium silicates (olivine, pyroxenes, plagioclases), and their efficiency in converting CO₂ into carbonate minerals has been demonstrated by natural occurrences [5, 6]. Such rocks are present in many reservoirs:

<footnote><sup>a</sup>Géosciences Environnement Toulouse-CNRS, IRD, CNES, Univeristy of Toulouse, 14 Avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote><sup>b</sup>Carbfix ohf., Smáratorg 3, 201, Kópavogur, Iceland</footnote>

<footnote>* Corresponding author. Email address: pascale.benezeth@get.omp.eu</footnote>
basaltic (e.g., Iceland, USA, India, Saudi Arabia), ophiolitic (Oman, New Caledonia, Liguria), but also in the form of mine tailings in the case of *ex-situ* sequestration (Canada, Australia, New Caledonia).

Nevertheless, there is a number of scientific and technological hurdles to overcome in order to develop efficient and sustainable CO₂ sequestration methods in these environments worldwide, which have been the focus of much of our research for over a decade, in particular focusing on mineral carbonation in basaltic reservoirs trying to develop the necessary knowledge to make the transition from fundamental laboratory research to industrial applications within our various collaborations. Detailed studies are currently ongoing on (i) abiotic and biotic reaction pathways of weathering and carbonation mechanisms in these rocks (thermodynamics and kinetics), (ii) permeability variations, (iii) measurements of various key parameters by complementary experimental methods, but also (iv) monitoring and analysis of field data within the only pilot injection site in a mafic context in Iceland.

## 2. Mafic reservoirs and pilot sites

As can be seen in Figure 1, there are large basaltic formations in several regions around the world, and both onshore and offshore sites have been considered for storage.

<figure><img src="image_3.png" /><figcaption>Figure 1 Basaltic formation around the world (modified from [7])</figcaption></figure>

Among them, geothermal mafic reservoirs are characterized by the presence of fractured basaltic formations and represent the ideal context for the potential injection of CO₂ with the purpose of its sequestration via mineralization. From the carbon storage perspective, the most studied geothermal site, for the feasibility of forming carbonate minerals by injecting CO₂, is in Iceland at the Hellisheiði power plant [8,9], where CO₂ injection started around 2007 with the Carbfix-1 and ANR-CO2FIX projects [10, 11]. These first projects allowed a better quantification of the distribution of secondary mineral phases in fractures and their effects on permeability in many reservoirs. They also provided unique data on the stability and kinetics of carbonate minerals and the reactivity of deep ecosystems associated with basaltic rocks, their role in alteration and carbonate precipitation and the effects on CO₂ injectivity [12, 13]. Indeed, this is the only site in the world where underground CO₂ injections are taking place on an industrial scale and, at the same time, carbon mineralization has proven to be successful [14]. In 2013, a field demonstration occurred into the Columbia River basalt group (USA), the Wallula pilot project that intended to inject ~ 1,000 metric tons of (supercritical) CO₂ at 900 m depth followed by a 2-years post-injection monitoring. A drilling core recovered at the site revealed the presence of various carbonates minerals in clear correlation with the isotopic signature of the injected CO₂ [15].

Building on the success of these basaltic sites, other projects have been developed to study carbon mineralization such as: (i) the Carbfix-2 project, where both CO₂ and H₂S are injected into an altered basaltic reservoir at 260 °C, (ii) the CarbonSAFE Cascadia project, which is conducting a pre-feasibility study of CO₂ injections into an oceanic basalt reservoir offshore Washington State and British Columbia [16]; and (iii) the GECO project, a European consortium to determine the applicability of the CarbFix approach to the geothermal energy industry worldwide, through its application to three new sites in Europe, including the Nesjavellir geothermal storage site in Iceland [17]. In Carbfix-2 integration of a direct air capture technology (DAC) has been implemented in 2017 at the Hellisheiði geothermal
power plant, which has been recently scaled up through the ORCA project capturing around 4,000 tons of CO₂ per year [18, 19].

### 3. Laboratory scale

The GET laboratory has played and continues to play a central role in most of the projects related to the injection of CO₂ into mafic reservoirs (and also ultramafic settings), primarily through conducting diverse and specific experimental research [20, 21], developing thermodynamic [22-27] and kinetic [28-31] databases to reproduce chemical reactions through modeling [14, 15], and compiling a state-of-the-art interdisciplinary and comprehensive synthesis on geological storage of CO₂ [31-33]. Indeed, a number of geological scenarios associated with CCS sites of interest have been reproduced in the laboratory, and results from these experimental studies are helping unravelling the chemical reactions taking place in natural low temperature complex mafic and ultramafic systems, and revealing the limits/potentialities of each setting within the carbon storage strategy. Following the CarbFix approach, where CO₂-charged water injected into reservoir mimics natural reactions, preliminary geochemical models have been performed with the PHREEQC v3 software [34] together with its carbfix.dat v1.1 thermodynamic database [32] to constrain the experimental conditions.

In the MgO-CO₂-H₂O system, the precipitation of magnesite is very sluggish below 50 °C, which is mainly due to the highly hydrated character of the Mg²⁺ cation, as confirmed by studies of magnesite growth kinetics [28, 29]. As a result of its reluctance to dehydrate, magnesium has a strong tendency to form a variety of hydrous magnesium carbonates at ambient temperatures (hydromagnesite, nesquehonite and lansfordite, dypingite) [22]. In an attempt to bridge some of the existing gaps regarding the formation of magnesium-carbonates, we have been thoroughly studying the solubility of these minerals [22-27] and their precipitation rates, including near-equilibrium reactivity [30] and the effect of organic ligands on the solubility and precipitation rates of magnesite [31].

In the CaO-CO₂-H₂O system, the solubility of calcite has been recently revisited up to 160°C and 50 bar [27]. As opposed to magnesite, the precipitation of calcite and/or aragonite is relatively fast and efficient from ambient temperature to 170 °C [14, 33, 35, 36]. Such carbon mineralization efficiency via Ca-carbonate formation was also confirmed by recent calculations on Ca isotopes from field injection fluids reacting with fresh basaltic rocks [37]. Yet, to expand the method applicability from fresh rocks towards commonly altered mafic reservoirs, the reactivity of a suite of secondary minerals (e.g., epidote, amphibole, chlorite, and serpentine) is currently under investigation [38].

All these data are essential to determine not only the mechanisms that control mass transfers, but also to extrapolate the models resulting from the measurements carried out at different scales and to determine the injection and the long-term storage capacity of mafic rocks, as well as to dimension and optimize future industrial projects. As such, a number of geological scenarios associated with CCS sites of interest are reproduced in laboratory.

In the Icelandic settings, temperature ranges from 15 to more than 300 °C, and the mineralogy reflects the hydrothermal alteration associated with the fluid circulation at different thermal conditions. Relatively fresh olivine-tholeitic basalts are present in the shallowest and coldest part of the reservoir, and an increasing occurrence of secondary minerals is observed as a function of depth [39]. Volcanic glass, Ca-plagioclase, pyroxene, Fe-Ti-oxides, and olivine in the host rock breakdown forming (i) smectites and zeolites up to 200°C; (ii) chlorite, albite, and epidote between 200 and 250 °C; and (iii) epidote and actinolite from 250 °C onwards. Consequently, there is the need to understand the carbon storage potential of common alteration phases in mafic (and ultramafic) rocks as part of the CCS portfolio of solutions to reduce CO₂ atmospheric emissions. Examples of carbon mineralization at different scales and geological contexts are presented in this paper with their limitations and optimizations.

#### 3.1 Carbon mineralization potential of alteration minerals

In this study, antigorite, clinochlore, epidote, and tremolite capacity to mineralize CO₂ was investigated via far-from equilibrium experiments at temperature of 150°C, monitoring the fluid composition evolution with time and characterizing the resulting solid products. We report here the results from three experiments: the first one with only epidote (Ep) as solid sample, the second one with a sample composed by 50% epidote and 50% by a mix of antigorite, clinochlore, and tremolite (Ep-ACT), and the third one with a sample of only antigorite (Atg) (see Tables 1 and 2).

##### 3.1.1 Experimental set up, sampling procedure and analyses

Several far-from equilibrium experiments were performed reacting a CO₂-rich solution (from 5 to 20 bar pCO₂) with different solid alteration phases at 150 °C, using a Parr Instrument™ batch reactor. All the components of the system in contact with the fluid, including reactor vessel, tubing, and valves, were made of titanium to avoid any undesired chemical reaction. Each batch-reactor experiment was performed in a 450 mL vessel pre-loaded with ~5 g
of solid powder, and ~350-400 g of ultrapure water or 0.1 M NaCl solution. The solid was maintained in suspension via constant mechanical stirring at approximately 300 rpm. An initial degassing stage allowed to remove the oxygen from within the reactor internal headspace by flushing it with pure CO₂ for ~15 min. Subsequently, the reactor was pressurized to a minimum of ~5 bar pCO₂. Such value of pressure was chosen to avoid any water vaporisation inside the reactor once at 150 °C, and to allow the re-injection of more gaseous CO₂ to reach the desired total pressure. The temperature inside the reactor was controlled by a heating mantle with an uncertainty of ±1 °C. Finally, the desired total pressure was adjusted, when needed, by addition of CO₂ through the gas inlet. Such connection with the CO₂ cylinder was kept open throughout the whole duration of the experiments to ensure a constant pCO₂.

Liquid samples were regularly taken to monitor the fluid chemical changes by various analyses (ICP-OES, UV-visible and alkalinity, pH measurements at 25°C). The fluid in the reactor was sampled through a ~10 µm titanium frit positioned at the bottom of a dip-tube inside the reactor and cooled down to room-T via a coolig sleeve located upstream of the sampling valve. The samples for elemental analyses were subsequently filtered using 0.22 µm syringe filters and acidified with ultrapure concentrated HNO₃ or 0.1 M HCl (for Fe²⁺/Fe³⁺ analyses).

At the end of each experiment the reactor was cooled down to ~40°C while the agitation system was stopped to minimize retrograde water/rock interaction. After opening the reactor, all its content was placed in a vacuum filtration system with a 0.45 µm membrane filter to decant all the liquid and retain the solid phases for successive analyses. The solids were rinsed with ultrapure water and ethanol and dried in an oven at 50°C. Various analyses were performed on the solid phases before and after the experiments (specific surface area by BET; SEM-EDX, XRF, Electron Microprobe and TEM). Calculations of the pH at temperature as well as saturation indices were computed with PHREEQC.

### 3.1.2 Results

In Tables 1 and 2 below some details are given about the three experiments performed with Ep, Ep-ACT and Atg at 150°C and pCO₂ from 5 to 20 bar.

<table><caption>Table 1: details of the three batch experiments performed at 150°C.</caption><thead><tr><th>ID</th><th>pH/T</th><th>P<sub>total</sub><br>(bar)</th><th>Solid</th><th>Solid mass<br>(g)</th><th>pCO<sub>2</sub><br>(bar)</th><th>Fluid composition</th><th>CO<sub>2</sub><br>(mol/kg)</th><th>Duration<br>(days)</th></tr></thead><tbody><tr><td>Ep-5CO<sub>2</sub></td><td>4.02<sup>a</sup></td><td>10</td><td>Epidote</td><td>5.00</td><td>5</td><td>Pure water</td><td>0.045<sup>a</sup></td><td>30</td></tr><tr><td rowspan="2">Ep-ACT-20CO<sub>2</sub></td><td rowspan="2">3.74<sup>a</sup></td><td rowspan="2">25</td><td>Ep</td><td>2.63</td><td rowspan="2">20</td><td rowspan="2">Pure water</td><td rowspan="2">0.166<sup>a</sup></td><td rowspan="2">30</td></tr><tr><td>ACT</td><td>2.39</td></tr><tr><td>Atg-20CO<sub>2</sub></td><td>3.69</td><td>25</td><td>Atg</td><td>5.11</td><td>20</td><td>NaCl 0.1 M</td><td>0.173</td><td>31</td></tr></tbody></table>

<table><caption>Table 2: Composition of the solid phases used in the experiments with their specific surface area (BET)</caption><thead><tr><th>ID</th><th>Phases</th><th>vol%</th><th>Chemical formula</th><th>BET<br>(m²/g)</th></tr></thead><tbody><tr><td>Ep</td><td>Epidote</td><td>100</td><td>Ca<sub>2.07</sub>Al<sub>2.00</sub>(Al<sub>0.08</sub>Fe<sup>+3</sup><sub>1.02</sub>Mn<sub>0.01</sub>K<sub>0.01</sub>S<sub>0.01</sub>Ti<sub>0.01</sub>)Si<sub>3.00</sub>O<sub>12</sub>OH</td><td>0.0863<sup>a</sup></td></tr><tr><td rowspan="3">ACT</td><td>Antigorite</td><td>45</td><td>Mg<sub>2.49</sub>Fe<sub>0.29</sub>Cr<sub>0.01</sub>(Al<sub>0.10</sub>Si<sub>2.02</sub>O<sub>5</sub>)(OH, F)<sub>4</sub></td><td>0.4627</td></tr><tr><td>Clinochlore</td><td>5</td><td>Mg<sub>9.06</sub>Fe<sub>1.05</sub>Cr<sub>0.03</sub>Na<sub>0.01</sub>Ca<sub>0.01</sub>Mn<sub>0.01</sub>Ni<sub>0.01</sub>Si<sub>6.44</sub>Al<sub>3.26</sub>O<sub>20</sub>)(OH, F)<sub>16</sub></td><td></td></tr><tr><td>Tremolite</td><td>50</td><td>Ca<sub>1.70</sub>Na<sub>0.43</sub>Mg<sub>4.57</sub>Fe<sub>0.35</sub>K<sub>0.11</sub>Mn<sub>0.01</sub>Cr<sub>0.01</sub>(Al<sub>0.06</sub>Si<sub>7.98</sub>°<sub>22</sub>)(OH, F)<sub>2</sub></td><td></td></tr><tr><td>Atg</td><td>Antigorite</td><td>100</td><td>Mg<sub>2.76</sub>Fe<sub>0.03</sub>Si<sub>2.13</sub>O<sub>5</sub>(OH)<sub>4</sub></td><td>0.8011</td></tr></tbody></table>

In Figure 2 the measured elemental concentrations and recalculated in situ pH (symbols) are compared with the results of the simulation obtained by PHREEQC as a function of time for the batch experiment with only epidote (Ep), at 5 bar of pCO₂. In this experiment, an Al-OH layer (boehmite with some Fe³⁺) formed on the surface of the mineral grains, as evidenced by SEM observations and TEM analyses of a FIB ultrathin section extracted from the final solid (Fig. 3). The boehmite precipitation is at the origin of the surface passivating effect, which strongly inhibited the dissolution reaction (Fig. 2) preventing the aqueous solution from reaching the equilibrium with respect to calcite. At these conditions (150 °C and pH 4) the solubility of boehmite is at its minimum [40] and the precipitation of this phase was large. To limit the precipitation of boehmite and enhance the carbonation reaction we reproduced the same
experiment with a mixture of epidote (Ep) and ACT (Table 1). In this case, only a limited amount of secondary Al-phases was observed and the reaction led to the formation of abundant Fe-rich magnesite crystals of ~ 1 µm in size (Fig. 3).

<figure><img src="image_4.png" /><figcaption>Figure 2. Experimental data from the batch experiment conducted with epidote at 150 °C and pCO₂ = 5 bar (symbols) and results from corresponding PHREEQC calculations (continuous lines).</figcaption></figure>

At this pCO₂ Fe-Mg carbonates are favoured over Ca-carbonates and ICP-OES analyses showed that both Fe and Ca abundances in solution grew over time, as opposed to the experiment with only epidote where dissolution was hampered by the protective action of precipitated boehmite. Fe concentrations of the liquid samples – as measured by UV-VIS spectrophotometry following [21] – indicate that almost all the dissolved Fe is in a reduced state (Fe²⁺). The presence of some sulfide minerals in the Ep-ACT mixture (Fig. 3) may have kept the environment in a reducing state, and driven the Fe redox reaction in water (from Fe⁺³ to Fe⁺²). Note that SEM observations did not bring any evidence of hematite that could have precipitated during the experiment. Al was consumed by secondary minerals such as boehmite and Ca-aluminosilicates, whereas ankerite was at equilibrium with the aqueous fluid.

<figure><img src="image_5.png" /><figcaption>Figure 3. FIB section cut across an epidote grain surface observed via TEM and analysed by EDX. (Red: O; Green: Al; Blue: Fe) after reaction at 150 °C and pCO₂ = 5 bar (left) and SEM microphotograph of the Ep-ACT mixture reacted at 150 °C and pCO₂ = 20 bar (right).</figcaption></figure>

Another experiment was conducted with a natural antigorite sample at the same pCO₂ as the Ep-ACT experiment but using a NaCl 0.1 M solution instead of pure water. The fluid attained the thermodynamic equilibrium relative to magnesite after ~ 5 days of reaction and kept moderately supersaturated with respect to this phase till the end of the experiment. However, SEM analyses of the post-reaction solid did not allow identifying any precipitated Mg-Fe-carbonate. The slowest rates of magnesite precipitation and the relatively weak degree of supersaturation reached by
the fluid during the course of the reaction (SI<sub>Mgs</sub> < 0.2) were not sufficient to promote the formation of this carbonate phase. In addition the aqueous solution approached the equilibrium with talc and likely other secondary Mg-Fe-silicates, which could inhibit the conversion of antigorite to ferroan magnesite [41].

<figure><img src="image_6.png" /><figcaption>Figure 4. Chemical evolution of the aqueous fluid interacting with antigorite at 150 °C and pCO₂ = 20 bar (left) and SEM picture of the reaction products of Oman peridotite reacted under the similar conditions and for the same experimental duration.</figcaption></figure>

To better understand the role of various secondary minerals and quantify the sequestration potential of different mafic and ultramafic reservoirs a parallel study on the conditions and processes that control the extent of mineral carbonation in harzburgites and partially serpentinized peridotites has also been conducted in the laboratory as part of the ANR-LISZT project. This work has showed that, under comparable conditions, the interaction of a fluid at pCO₂ = 15 bar with a partially serpentinized peridotite (~ 10 % of serpentine) from Oman led to the precipitation of a significant amount of ferroan magnesite crystals (Fig.4, right) with carbonation yields that reach 30 %. These higher rates of carbonate formation can be attributed to the presence of olivine (~55 % wt), the dissolution rates of which are known to be faster than those of serpentine minerals, although the role of pyroxenes and other alteration minerals in the carbonation process has yet to be determined.

## 4. Conclusion

The high Ca-carbonate mineralization potential of epidote and other Ca-Al-silicates may be limited by the precipitation of Al-minerals. At the same time, the formation of Al-Si complexes in solution can also accelerate the carbonation process by increasing the solubility of dissolving silicates, such as olivine [42], while reducing the inhibiting effect of secondary coatings. The presence of other secondary minerals, such as actinolite and chlorite, along with Fe-sulfides, may provide the conditions required to suppress the inhibiting effect on mineral dissolution and promote the formation of Fe-Mg carbonates in mafic rocks under relatively high pCO₂. On the contrary, the dissolution of poorly reactive secondary Mg-silicates with low Mg/Si ratios, such as serpentine polymorphs, can hinder the carbonation process as it would also favor the transition to different secondary silicate phases.

The current work on the complex mineralogy of mafic/ultramafic reservoirs indicates that the effects of secondary phases on the CO₂ mineralization reactions are ambiguous, depending on the chemical composition and mineralogical identity of these phases as well as on their relative abundances. In the light of such variability, further work is therefore needed to establish the role of various physico-chemical parameters and to better understand the influence of relevant reactions on the CO₂ sequestration capacity of targeted mafic (and ultramafic) reservoirs.

In addition to direct applications for in- and ex-situ CO₂ storage, this work will help to better quantify the impact of the reactions between natural fluids and basic/ultrabasic rocks on the global CO₂ budget, especially in relation to the current anthropogenic emissions.
## Acknowledgements

This work has received funding from the EU under grant agreement No. 818169 for the GECO project, No. 283148 for the CarbFix project and from the French ANR-LISZT and ISIFoR C-Store projects.

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[24] Gautier Q, Bénézeth P, Mavromatis V, Schott J. Hydromagnesite solubility product and growth kinetics in aqueous solution from 25 to 75°C. Geochim Cosmochim Acta 2014; 138: 1-20.

[25] Lindner M, Saldi GD, Carrocci S, Bénézeth P, Jordan G, Schott J. On the growth of anhydrous Mg-bearing carbonates – Implications from norsethite growth kinetics. Geochim Cosmochim Acta 2018; 238:424-437.

[26] Harrison A L, Mavromatis V, Oelkers EH, Bénézeth P. Solubility of the hydrated Mg-carbonates nesquehonite and dypingite from 5 to 35°C: Implications for CO₂ storage and the relative stability of Mg-carbonates. Chem Geol, 2019; 504:123-135.
[27] Bychkov AYu, Bénézeth P, Pokrovsky OS, Shvarov Yu V, Castillo A, Schott J. Experimental determination of calcite solubility and the stability of aqueous Ca- and Na-carbonate and -bicarbonate complexes at 100-160°C and 1-50 bar pCO2 using in-situ pH measurements Geochim Cosmochim Acta 2020; 290: 352-365.

[28] Saldi GD, Jordan G, Schott J, Oelkers EH. Magnesite growth rates as a function of temperature and saturation state. Geochim Cosmochim Acta 2009; 73: 5646-5657.

[29] Saldi GD, Schott J, Pokrovsky OS, Gautier Q, Oelkers EH. An experimental study of magnesite precipitation rates at neutral to alkaline conditions and 100 to 200 °C as a function of pH, aqueous solution composition and chemical affinity. Geochim Cosmochim Acta 2012; 83: 93-109.

[30] Schott J, Oelkers EH, Bénézeth P, Godderis Y, François L. Can accurate kinetic laws be created to describe chemical weathering? CR Geoscience 2012; 344: 568-585.

[31] Gautier Q, Bénézeth P, Schott J. Magnesite growth inhibition by organic ligands: An experimental study at 100, 120 and 146°C. Geochim Cosmochim Acta 2016; 181: 101-125.

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[34] Parkhurst DL, Appelo CAJ. Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one dimensional transport, and inverse geochemical calculations. 2013 No. 6-A43. *US Geological Survey*.

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[36] Ellis, AJ. The solubility of calcite in sodium chloride solutions at high temperatures. Am. J. Sci. 1963; 261:259-267.

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[41] Saldi GD, Daval D, Morvan G., Knauss KG. The role of Fe and redox conditions in olivine carbonation rates: an experimental study of the rate limiting reactions at 90 and 150 °C in open and closed systems. Geochim Cosmochim Acta 2013; 118: 157-183.

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# CO₂ MINERALIZATION IN MAFIC ROCKS: FROM LABORATORY EXPERIMENTS TO PILOT SITES

Pascale Bénézethª, Chiara Marieniᵇ, Giuseppe Saldiª

## INTRODUCTION

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Magnesite hydrothermal veins, California</figcaption></figure>

Mineral trapping represents a safe, sustainable and complementary alternative to geological storage in sedimentary basins

<figure><img src="image_16.png" /><figcaption>Basaltes & péridotites</figcaption></figure>

<figure><img src="image_17.png" /></figure>

Among the various CCS solutions considered to reduce CO₂ emissions, mineral sequestration of CO₂ offers a thermodynamically stable and reliable option over very long periods of time. However, the highest carbon mineralization potential is reached within specific environments, in particular mafic (basalts) and ultramafic rocks (mainly peridotites and serpentinites). These lithologies are highly reactive and rich in magnesium, iron and calcium silicates (olivine, pyroxenes, plagioclases), and their efficiency in converting CO₂ into carbonate minerals has been demonstrated by natural occurrences(1-3). Such rocks are present in many reservoirs: basaltic (e.g., Iceland, USA, India, Saudi Arabia), ophiolitic (Oman, New Caledonia, Liguria), but also in the form of mine tailings in the case of ex-situ sequestration (Canada, Australia, New Caledonia).

time since injection stops (years)

## PILOT SITE

The study of the feasibility of forming carbonate minerals by injecting CO₂ into basic rocks began in 2006-2008 with the first pilot site at Hellisheiði in Iceland (CARBFIX and ANR-CO2FIX projects). These first projects allowed a better quantification of the distribution of secondary mineral phases in fractures and their effects on permeability in many reservoirs. They also provided unique data on the stability and kinetics of carbonate minerals and the reactivity of deep ecosystems associated with basaltic rocks.

Building on the success of these basaltic sites, other projects have been developed to study carbon mineralization: Carbfix-2, CarbonSAFE Cascadia (Washington State and British Columbia)(5) and H2020-GECO(6). In Carbfix-2 integration of a direct air capture technology (DAC) has been implemented in 2017 at the Hellisheiði geothermal power plant, which has been recently scaled up through the ORCA (Climeworks) project capturing around 4,000 tons of CO₂ per year(7,8)

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>Direct air capture</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Hellisheiði geothermal power plant</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>CARBON DIOXIDE (CO₂) IN ATMOSPHERE</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>GEOLOGICAL RESERVOIRS</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>CO₂ STORED AS MINERAL</figcaption></figure>

<footnote>ªGéosciences Environnement Toulouse-CNRS-IRD, CNES University of Toulouse, 14 Abvenue Edouard Belin 31400 Toulouse, France,</footnote>

<footnote>ᵇCarbfix ohf., Smáratorg 3, 201, Kópavogur, Iceland</footnote>
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

## LABORATORY SCALE

The GET laboratory plays a central role in most of the projects related to the injection of CO₂ into mafic reservoirs (and also ultramafic settings), primarily through conducting diverse and specific experimental research from the pore scale to the injection site. The results are helping unravelling the chemical reactions taking place in natural low temperature complex mafic and ultramafic systems, and revealing the limits/potentialities of each setting within the carbon storage strategy.

<figure><img src="image_26.png" /><figcaption>CO₂-minéral interaction at the pore scale
(Credits C. Noiriel)</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>Solubility and thermodynamic properties of carbonate minerals (credits P. Bénézeth)</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>Experimental mineralization of CO₂ at the laboratory scale
(Crédits C. Marieni)</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>reservoir</figcaption></figure>

CO₂ Injection site
Carbfix (Iceland)

## Carbon mineralization potential of alteration minerals

In this study, antigorite, clinochlore, epidote, and tremolite capacity to mineralize CO₂ was investigated via far-from equilibrium experiments at temperature of 150 °C, monitoring the fluid composition evolution with time and characterizing the resulting solid products. We report here the results from three experiments: **a)** with only epidote (Ep) as solid sample, **b)** with a sample composed by 50% epidote and 50% by a mix of antigorite, clinochlore, and tremolite (Ep-ACT), and **c)** with a sample of only antigorite (Atg).

a)

<figure><img src="image_30.png" /><figcaption>Epidote at 150°C, 5 bars pCO2 (lines: PHREEQC calculations)</figcaption></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

Al-OH layer (boehmite with some Fe³⁺) formed on the surface of epidote mineral grains as observed by TEM anlysis of a FIB foil cut across the epidote surface. EDX chemical map colours: Red: O; Green: Al; Blue: Fe. Experimental conditions: T = 150 °C and ρCO₂ = 5 bar.

b)

<figure><img src="image_33.png" /><figcaption>Formation of Fe-rich magnesite crystals and Fe-Ni sulfides within a mixture Ep-ACT at 150 °C and 20 bars pCO2.</figcaption></figure>

c)

The slowest rates of magnesite precipitation and the relatively weak degree of supersaturation reached by the fluid during the course of the reaction with antigorite ($Si_{Mgs} < 0.2$) were not sufficient to promote the formation of magnesite. In addition the aqueous solution approached the equilibrium with talc and likely other secondary Mg-Fe-silicates, which could inhibit the conversion of antigorite to ferroan magnesite like in the Ep-ACT experiments.

<figure><img src="image_34.png" /></figure>

Under comparable conditions, the interaction of a fluid at pCO₂ = 15 bar with a partially serpentinized peridotite (~ 10 % of serpentine) from Oman (ANR LISZT project, Godard et al., GHGT16, 2022) led to the precipitation of a significant amount of ferroan magnesite crystals.

<figure><img src="image_35.png" /></figure>

<footnote>(1) Kelemen P, Matter J PNAS 2008; 105: 17295-17300.</footnote>

<footnote>(2) Kelemen P, et al. Energy Procedia 2018; 146: 92-102.</footnote>

<footnote>(3) Noel J, et al., Lithos 2018, 323: 218-237.</footnote>

<footnote>(4) Kelemen et al. Front. Clim. 2019 1: 1-20.</footnote>

<footnote>(5) Dolberg D et al. Energy Procedia 2018; 146: 158-165.</footnote>

<footnote>(6) Galeczka IM et al. Int J Greenh Gas Control 2022; 115: 103610.</footnote>

<footnote>(7) Clark DE. et al. Geochim Cosmochim Acta 2020; 279: 45-66.</footnote>

<footnote>(8) https://climeworks.com/roadmap/orca.</footnote>