Reactive transport experiments of coupled carbonation and serpentinization in a natural serpentinite. Implication for hydrogen production and carbon geological storage
Introduction
In the context of global warming and climate change, the development and implementation of Carbon Capture and Storage (CCS) technologies appears as an unavoidable mid-term solution. In particular, the sequestration of CO2 in solid form through carbon mineralization combines the advantages of deep geological storage with an increased safety compared with classical deep saline aquifer storage. Indeed, even in the presence of an open pathway to the surface, no leakage is possible since the carbon dioxide is completely mineralized (Seifritz et al., 1990, Kelemen and Matter, 2008). Among the different options, mafic to ultramafic formations, rich in divalent cations (Mg, Fe, Ca) are probably the most attractive (Oelkers et al., 2008) as basalts, peridotites and serpentinites are known to quickly react with carbon-rich fluids (Gadikota et al., 2020), precipitating carbonates, storing the CO2 safely over geological times (Mcgrail et al., 2006, Kelemen et al., 2018). Interestingly, these mineralization reactions are also naturally occurring in a variety of settings – e.g. as surface and near surface carbonation of ophiolites (Boschi et al., 2009, Kelemen et al., 2011, Beinlich et al., 2012), or as higher temperature alteration as evidenced by the carbonate chimneys at hydrothermal vents or by the formation liswanite (i.e. quartz + carbonates) originating from the complete carbonation of ultramafic lithologies (Hansen et al., 2005, Ulrich et al., 2014, Tominaga et al., 2017). Target formations are for example the ophiolitic complex in Oman which could, according to Kelemen and Matter (2008), store up to 77 trillion tons of CO2, or basaltic formations in Iceland which have been the target for the first pilot scale injection of carbon dioxide with great success by the CarbFix project team (Gíslason et al., 2018).
Additionally, natural carbonation of mafic and ultramafic rocks is always tightly linked to serpentinization processes themselves (Klein et al., 2013, McCollom et al., 2020), a specific set of reactions transforming the minerals of anhydrous mantle peridotites into hydrated and less dense phyllosilicates – serpentine, talc or clay minerals (sepiolite, saponite…) – through the influence of percolating seawater (Cannat et al., 2010). One of the byproducts of these reactions is molecular hydrogen H2(McCollom and Bach, 2009). Engineered carbonation of mafic and ultramafic rocks could then also potentially be coupled with hydrogen H2 production, an economically inescapable component in the energy transition, but whose production means are currently tightly linked to fossil fuel usages (International Energy Agency, 2019). The concomitant industrial production of H2 and carbon dioxide sequestration in ultramafic settings is a young field of investigation (e.g. (Wang et al., 2019)) but whose results could be particularly interesting both for the energy transition but also for the global understanding the deep geochemical cycles (Escartín et al., 1997, Gaillardet et al., 1999, Früh-Green et al., 2004, Paulick et al., 2006).
While batch experiments and reaction path studies of both carbonation and serpentinization of ultramafic rocks are well described in the literature (Neubeck et al., 2011, Klein and Garrido, 2011, Malvoisin et al., 2012, Grozeva et al., 2017, Miller et al., 2017, McCollom et al., 2020), percolation studies featuring solid cores, in which a reactive solution is being injected at a constant flow rate or constant pressure drop, are more scarce. This lack of experiments is regrettable because, while batch experiments help delve into the complex geochemical interactions during the reaction, only reactive percolation experiments allow the observation of the coupling between hydrodynamics and chemistry, and its potential relationship with reaction-induced fracturing (Jamtveit et al., 2000, Ulven et al., 2014, Rudge et al., 2010, Evans et al., 2018). This so-called THMC behavior (Thermo-Hydro-Mechano-Chemical) is what actually controls the final assemblage of the altered lithology and not only the stability fields of the considered minerals.
In most of the reactive percolation experiments – (Peuble et al., 2015a, Peuble et al., 2015b, Peuble et al., 2018, Peuble et al., 2019) for carbonation and (Godard et al., 2013, Escario et al., 2018) for serpentinization on sintered olivine; Luhmann et al. (2017b) with the serpentinization of intact dunite at 150 °C and 200 °C; Tutolo et al. (2018) on the silicification of brucite; or Farough et al. (2016) on the serpentinization of fractured ultramafic cores at 260 °C – the permeability is presenting a sharp drop during the experiment with usually a complete clogging after several days of injection. This loss of permeability is generally associated with the precipitation of neoformed minerals in the percolation paths and is tightly linked to the coupling between hydrodynamics and chemical kinetics.
In this study, we present two original experiments on a natural partially-serpentinized peridotite subjected to the injection of a carbonated fluid at 160 °C and 280 °C. Compared to the current literature on the topic, these experiments present two original novelties. First, the starting material is a natural partially serpentinized peridotite containing olivine, clino- and orthopyroxenes as well as serpentine and aragonite, contrary to the ideal sintered olivine or even dunite used in the literature. This type of rock is representative of most deep ocean ultramafic formations (e.g. at ultraslow spreading ridges (Dick, 1989)) and is thus better suited for in situ carbonation and hydrogen production studies. Secondly, the second experiment was performed at 280 °C, a common temperature of this kind of alterations (Menzel et al., 2018), but which had not been investigated before. The goal of these experiments was to track the evolution of the petrophysical and geochemical characteristics of the cores and assess their potential for carbon dioxide sequestration and hydrogen generation, as well as analyze the complex reactive transport couplings of these reactions.
Section snippets
Petrology
Three different thin sections were extracted from the initial serpentinite and analyzed with optical microscopy, SEM imaging coupled with EDS analyses, Electron Microprobe, Raman spectroscopy, as well as optical cathodoluminescence and micro-X-rays fluorescence (XRF). Modal composition was obtained from point counting (0.5 mm 1 mm steps), including the 5 main identified phases: olivine, ortho- and clinopyroxenes, serpentine, aragonite and spinel. Magnetite was not included in the point
Petrography
The starting material is a partially serpentinized lherzolite containing olivine, ortho- and clinopyroxenes and serpentine as major minerals plus several minor phases (aragonite, magnetite and spinel). The sample shows classic features of serpentinization on which carbonation events are superimposed (carbonates are identified as aragonite on XRD diffractograms and Raman spectra). It comprises two parts, both rich in olivine, which alternate at the cm scale and are best distinguished by their
General evolution
Despite very similar experimental conditions, except for the experimental temperature, HTE and LTE present fundamentally different mineralogical and petrophysical behaviors. Indeed, the LTE presents little evolution from the starting material, with relicts olivine and pyroxenes embedded in a serpentine matrix through which run veins and veinlets of aragonite. Yet, despite the absence of obvious modifications from the starting material mineralogy, the fast decrease in permeability of LTE is
Conclusion
We presented in this article two experiments of reactive percolation of a NaHCO3-rich brine at 160 °C and 280 °C in natural serpentinite cores in order to study the dynamic competition between serpentinization and carbonation of ultramafic formations. During the 10 to 14 days of the experiments, the permeability of the cores dropped by several orders of magnitude, while the final yields at 280 °C were 5.6% for CO2 sequestration (quantity of CO2 stored over quantity injected) and 0.8% for H2
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank P. Penhoud for XRD and cathodoluminescence data. This research was supported by the LABEX Voltaire (ANR-10-LABX-100–01) and EQUIPEX PLANET (ANR-11-EQPX-0036). We would also like to thank B. Tutolo, P. Kelemen, G. Pokrovski, and an anonymous reviewer for their comments and suggestions on the manuscript.
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