Review articleUnderstanding the importance of iron speciation in oil-field brine pH for CO2 mineral sequestration
Graphical abstract
Introduction
Carbon dioxide (CO2) mineral sequestration using brines is gaining attention for the storage of CO2 into stable mineral carbonates [1]. Brines are commonly found either underground, namely underground brines in saline aquifers, or above-ground, most notably as a by-product of oil and natural gas extraction and known as oil-field brines, and its fate is mainly disposal [2]. However, oil-field brines (above-ground brines) could also be a promising approach to sequester CO2 as they are produced in large volumes [3], [4].
Brines in addition to the dominant Na and Cl ions [5], also have significant concentrations of Ca, Mg and Fe, which react with CO2 to produce CaCO3(s), MgCO3(s), Fe2CO3(s) and other products under favourable conditions. However, most both under and above-ground brines are generally acidic in nature with a typical pH ranging from 3 to 5 [3], so that carbonates will not form in this pH range. The suitable pH range for formation of carbonate is 7.8 or higher [6], where CO32− dominates. Therefore, to boost the precipitation of mineral carbonates by reaction between brine and CO2, the pH of the brine must be modified e.g. by caustic addition before it can be used for the carbonation reaction [3].
In this regard, a factor that should be considered as the first step to boost the precipitation of mineral carbonates by the increase of the pH in above-ground brines is Fe speciation. The hydrolysis of ferrous (Fe2+) and ferric (Fe3+) in aqueous solutions involves pH changes because of the formation of a variety of partially oxidized meta-stable Fe2+-Fe3+ aqueous complexes, which could prevent oil-field brines from using for CO2 mineral carbonation. Fe2+ show high solubility in aqueous solutions, whereas Fe3+ shows a stepwise solubility at pH >1.0 [7]. The Fe2+-hexaquo-complex [Fe (H2O) 6]2+, which is present in acid solutions, hydrolyses by the formation of FeOH+ and precipitates as the hydroxides Fe(OH)2 upon increasing the pH value (Bard et al., 1985). Fe3+ exist only in acidic solutions (pH <2.5) as the hexaquo-complex Fe(H2O) 63+, which increasing pH is subsequently transformed to mono- and the dyhidroxo-complexes. In the pH range ≤5 at least four different Fe3+ ions coexist in aqueous solution: Fe3+; Fe (OH)2+; Fe(OH)+2 and the dimer Fe2(OH)42+ [8]. The spontaneous chemical oxidation of Fe2+ to Fe3+ by O2 is also a complex process involving pH changes with a variety of partially oxidized meta-stable Fe2+-Fe3+ intermediate species, which ultimately transform into a variety of stable Fe-oxide end-products such as hematite (Fe2O3), magnetite (Fe3O4), goethite (α-FeOOH), and lepidocrocite (γ-FeOOH) [9]. These crystalline products form by competing mechanisms and the proportion of each in the final product depends on the relative rates of formation. The master variable governing the rates at which these compounds form is pH. Other important factors are temperature and the presence of additives.
Although the conditions governing Fe chemistry under different environments have been identified by the above mentioned researchers among others, the role of Fe speciation in above-ground brines aimed at mineral sequestration of CO2 and how it can affect brine composition and pH has not been elucidated yet. Druckenmiller and Maroto-Valer [4] reported that brines with low Fe concentration (9 ppm) showed a pH relatively constant after addition of a strong base. By contrast, brines with high Fe concentration (121–476 ppm) had a rapid decline in pH during the first hours, after which the decline seemed to level off. However, it was not ascertained whether Fe2+ or Fe3+ caused the pH drop in their study.
With the advent of making CO2 sequestration through mineral carbonate formation a viable approach by using oil-field brines, the role of Fe speciation in oil-field brines pH needs to be elucidated to reach suitable pH range for further formation of carbonate. Towards this goal, this work i) studies the evolution of the pH in three synthetic brines with different composition prepared as analogues to an oil-field brine; ii) investigates the effect of Fe in the pH of the three above-ground brines when adjusting the pH; and iii) elucidates the role of Fe speciation in brines when adjusting the pH by combining a number of experimental tools and a geochemical model based on acquired experimental data. To reproduce experimental conditions when attempting to use oil-field brine for carbon sequestration, ambient temperature and ambient pressure conditions were chosen for pH studies and modelling calculations.
Section snippets
Brine preparation
Three different synthetic brines, namely B1, B2 and B3, were prepared as an analogue to the natural brine (OH-2) which comes from a natural gas well in Youngstown, Ohio [4]. Owing to the complex composition of natural brines, only major ions were considered to prepare the three synthetic brines, including Na+, K+, Mg2+, Ca2+, Fe3+/Fe2+, Sr2+, Ba2+and Cl−.
Brines with their corresponding duplicates were synthesised by dissolving NaCl, KCl, MgCl2·6H2O, CaCl2·2H2O, FeCl3/FeCl2, SrCl2 and BaCl2·2H2O
pH studies
The pH values of brines and the experimental parameters in the pH stability studies are given in Table 1.
The results of pH studies for B1 over a period of 192 h (8 days) are shown in Fig. 1a. B1 duplicates, namely B1A and B1B, containing Fe3+ show an initial pH of 1.68 and 1.70, respectively (Table 1). The B1A and B1 B pHs were therefore adjusted to ∼ 6.3, an upper pH limit but below the point at which Fe(OH)2 precipitates (∼7.0–9.0.), by the addition of KOH. The evolution of the pH of the B1
Conclusions
The work reported here reveals that Fe speciation in oil-field brines would be a key factor to consider for above-ground mineral carbonation processes that make use of the aforementioned brines. The pH of brines containing Fe is affected when it is adjusted by a strong base whereas the pH of Fe-free brines experiment shows no significant variations.
The pH of brines containing Fe2+ diminishes to very low pH (2.86) by air oxidation of highly alkaline Fe(OH)2 suspension induced by KOH addition by
Acknowledgments
The financial support of the Centre for Innovation in Carbon Capture and Storage (CICCS) through the Engineering and Physical Sciences Research Council, EPSRC (EP/F012098/1 and EP/F012098/2) is gratefully acknowledged.
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