Geochemistry of High-Ph Waters from Serpentinites of the Gruppo Di Voltri
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Applied Geochemistry 19 (2004) 787–802 www.elsevier.com/locate/apgeochem Geochemistry of high-pH waters from serpentinites of the Gruppo di Voltri (Genova, Italy) and reaction path modeling of CO2 sequestration in serpentinite aquifers Francesco Cipolli, Barbara Gambardella, Luigi Marini*, Giulio Ottonello, Marino Vetuschi Zuccolini Laboratorio di Geochimica, DipTeRis, Universita` di Genova, Corso Europa 26, I-16132 Genova, Italy Received 24 March 2003; accepted 10 October 2003 Editorial handling by H. A´ rmannsson Abstract The large number of geochemical data gathered on the Gruppo di Voltri springs confirm that progressive interaction of meteoric waters with ultramafic rocks variably affected by serpentinization leads initially to the formation of Mg– HCO3 waters when the system is open to CO2, and Na–HCO3 and Ca–OH type water upon further interaction with the 3 rock, under highly reducing closed-system conditions with respect to CO2. As indicated by H data, these high-pH waters have had long residence times underground in deep aquifers hosted by serpentinitic rocks. These waters are the only available evidence of the presence of such deep aquifers. High-pressure injection of CO2 into these deep aquifers was simulated by reaction path modeling. Results indicate that this is a feasible methodology to reduce the inputs of anthropogenic CO2 into the atmosphere. Serpentinitic rocks have a high capacity for CO2 sequestration, mainly through formation of carbonate minerals. Dissolution of serpentinitic rocks and precipitation of magnesite and silica minerals occurs naturally in areas of high terrestrial CO2 fluxes such as in southern Tuscany, corroborating the feasi- bility of this methodology of CO2 sequestration. However, this process causes a progressive decrease in the porosity of the aquifer, at least under closed-system conditions. These side effects must be carefully evaluated by means of further laboratory tests and field activities. # 2003 Elsevier Ltd. All rights reserved. 1. Introduction the last century (Bryant, 1997), has not yet been proven. Nevertheless, humans cannot wait for a definite answer Since the beginning of the industrial revolution, the on this topic. Anthropogenic CO2 inputs to the atmo- CO2 concentration in the atmosphere has been increas- sphere must be drastically reduced, and several strate- ing. A rise from315 ppm in 1958 to370 ppm in 2001 gies have to be undertaken for this purpose. has been documented (Keeling and Whorf, 2002). This Sequestration of CO2 in deep geological reservoirs is global-scale phenomenon has been attributed chiefly to one disposal option. It is currently under way in the the use of fossil fuels and to a lesser extent to concrete Norwegian sector of the North Sea (Korbol and Kad- production (e.g., Vernadsky, 1924; Marland et al., dour, 1995) and its feasibility has been evaluated for 2001). The causative relationship between this CO2 other sites, such as the Alberta sedimentary basin, increase in the atmosphere and global warming, with an Canada (Gunter et al., 1993, 1996, 1997; Bachu et al., approximate temperature increase of 0.4–0.6 C during 1994; Law and Bachu, 1996). According to Hitchon (1996), geological storage of CO2 can be carried out in 3 ways: (1) trapping as gas or * Corresponding author. supercritical fluid below a low-permeability caprock, a E-mail address: [email protected] (L. Marini). process termed hydrodynamic trapping; (2) dissolution 0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2003.10.007 788 F. Cipolli et al. / Applied Geochemistry 19 (2004) 787–802 into deep waters (solubility trapping); (3) precipitation Voltri, variably affected by serpentinization, are covered of carbonate minerals brought about by dissolution of by the comparatively impervious sedimentary rocks of primary silicates upon injection of CO2 into a deep the Piedmont Tertiary Basin potentially acting as an aquifer (mineral sequestration). effective caprock (Bozzo et al. 1986, 1992). Mineral sequestration is especially effective as it These aquifers host waters with unusual chemistry, 2+ À would fix CO2 forever, preventing its return to the dominated by Ca and OH ions, and high pH, typi- atmosphere. It is well known that CO2 is extracted from cally in the 11.3–11.9 range at the surface outlet (Bruni the atmosphere by silicate weathering and consequent et al., 2001, 2002). Other characteristics of these waters deposition of carbonate minerals (Lasaga, 1981). The are the very low concentrations of total dissolved car- 2À balance between this process and the CO2 release to the bonate, mainly present as CO3 ion, and the deposition atmosphere by Earth degassing, controls the CO2 con- of travertines from surface discharges through absorp- centration in the atmosphere on a geological time scale tion of atmospheric CO2 (Bruni et al., 2001, 2002; of 1 million years or more (Walker et al., 1981; Berner et Marini and Ottonello, 2002). al., 1983; Kerrick and Caldeira, 1993). To investigate the feasibility of CO2 sequestration The appeal of permanent CO2 fixation in carbonates through injection into deep aquifers hosted by the in deep geological formations (Seifritz, 1990; Bachu et ultramafic-serpentinitic rocks of the Gruppo di Voltri it al., 1994) has stimulated the experimental investigation was decided to gather additional geochemical data on of mineral trapping. Pearce et al. (1996) and Rochelle et local springs, especially those of high-pH and to carry al. (1996) have injected supercritical CO2, at pressures of out reaction path modeling of CO2 sequestration. The 90 and 200 bar, into a sandstone reservoir at tempera- results achieved so far are presented below. tures of 105 and 80 C, for periods of 1 and 8 months. Significant alteration of pre-existing calcite and dolo- mite was observed, as well as dissolution of anhydrite 2. The high-pH waters accompanied by precipitation of calcite and possible corrosion of detrital feldspars with precipitation of Na- 2.1. Geological background smectite, although the evidence supporting this last process was not conclusive. Gunter et al. (1997) performed The Gruppo di Voltri includes (Chiesa et al., 1975): 1-month-long experimental tests at 105 C and 90 bar of (1) antigoritic serpentinites, less frequent eclogitic meta- PCO2 on glauconitic sandstones from the Alberta sedimen- gabbros, and infrequent metarodingites, produced tary basin. Under these conditions, only a small amount through metamorphism of original associations of of CO2 was trapped through reactions with Al-silicate ultramafic rocks, prevailingly peridotites, and gabbros; minerals, whereas dissolution of mineral carbonates (2) prasinites and calc-schists, derived by metamorphism took place. Geochemical modeling suggests that a per- of original basalts and associated oceanic sediments. iod of much longer than one month, probably 6–40 a, is High-pH, Ca–OH springs are situated in zones where needed in this experimental system to attain chemical serpentinites and related rocks crop out, generally equilibrium. Completion of CO2 fixation in the natural along faults and fractures. The physical and chemical system would require even longer periods of time, of the characteristics of these waters suggest that they come order of hundreds of years. However, these periods are from relatively deep aquifers, which are mainly hosted shorter than the residence times of fluids in deep aquifers. in serpentinites and related rocks (Bruni et al., 2001, From these considerations, it is clear that geochemical 2002). modeling is an appropriate tool in projects aimed at evaluating CO2 sequestration through carbonate 2.2. Sampling and analyses mineral deposition, at least during the pre-feasibility stage, because of the slow kinetics of alteration reactions A total of 25 samples were collected from the 15 of silicate and Al-silicate minerals and the difficulties springs of high pH identified in the Gruppo di Voltri and cost of field tests. area. Field characteristics, including location, are As pointed out by Lackner et al. (1995), disposal of described by Marini and Ottonello (2002). In the field, large amounts of CO2 through mineral carbonation outlet temperature, pH, Eh, sulfide (by the methylene requires involvement of silicate rocks rich in Mg and blue colorimetric method), and total alkalinity (by Ca. Ultramafic rocks, such as peridotites and serpenti- acidimetric titration) were determined. Spring water was nites, are especially suitable for this purpose as they filtered through 0.45 mm cellulose acetate membranes. A contain 40–50 weight% of MgO on average. filtered portion was stored as such, for Ion Chromato- A very promising geological setting for CO2 seques- graphy (IC) analysis, whereas concentrated HCl was tration through injection into deep aquifers hosted by added before storage to a second filtered portion, for ultramafic-serpentinitic rocks is the southern Piedmont, Atomic Absorption Spectrophotometry (AAS) and SiO2 Italy, where the ultramafic rocks of the Gruppo di analysis. New polyethylene bottles were used. F. Cipolli et al. / Applied Geochemistry 19 (2004) 787–802 789 A separate aliquot was collected into an evacuated In the d18O vs. dD correlation plot (Fig. 1), the 184 glass bottle, containing concentrated HCl in excess with spring waters from the Gruppo di Voltri area are dis- respect to total alkalinity to determine total dissolved tributed, independent of their compositions, into a inorganic C, which is also called total dissolved carbon- unique alignment, which is conveniently described by ate or total dissolved CO2 in the geochemical literature. the linear regression line (R=0.986): It consists of the sum of the molal concentrations of 18 À 2À D ¼ 7:64 Â O þ 12:4: ð3Þ CO2, HCO3 ,CO3 and related aqueous complexes and is here indicated with the acronym TDIC. For a given 18O/16O ratio, the dD values of spring All the high-pH waters were analyzed for: (1) Na, K, waters are 3–5 % units less negative than those of the Mg, Ca, by AAS; (2) Cl, SO4,NO3 by IC; (3) molyb- meteoric waters from the Genova-Sestri Ponente IAEA- date-reactive SiO2 by the heteropoly blue colorimetric WMO station.