Impacts of Organic Ligands on Forsterite Reactivity in Supercritical

Mar 25, 2015 - Impacts of Organic Ligands on Forsterite Reactivity in Supercritical. CO2 Fluids. Quin R. S. Miller,*. ,†. John P. Kaszuba,. †,‡...
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Impacts of Organic Ligands on Forsterite Reactivity in Supercritical CO2 Fluids Quin R. S. Miller,*,† John P. Kaszuba,†,‡ Herbert T. Schaef,§ Mark E. Bowden,∥ and Bernard P. McGrail⊥ †

Department of Geology and Geophysics, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, United States ‡ School of Energy Resources, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, United States § Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, United States ∥ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, United States ⊥ Energy and Environment Directorate, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-98, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Subsurface injection of CO2 for enhanced hydrocarbon recovery, hydraulic fracturing of unconventional reservoirs, and geologic carbon sequestration produces a complex geochemical setting in which CO2-dominated fluids containing dissolved water and organic compounds interact with rocks and minerals. The details of these reactions are relatively unknown and benefit from additional experimentally derived data. In this study, we utilized an in situ X-ray diffraction technique to examine the carbonation reactions of forsterite (Mg2SiO4) during exposure to supercritical CO2 (scCO2) that had been equilibrated with aqueous solutions of acetate, oxalate, malonate, or citrate at 50 °C and 90 bar. The organics affected the relative abundances of the crystalline reaction products, nesquehonite (MgCO3·3H2O) and magnesite (MgCO3), likely due to enhanced dehydration of the Mg2+ cations by the organic ligands. These results also indicate that the scCO2 solvated and transported the organic ligands to the forsterite surface. This phenomenon has profound implications for mineral transformations and mass transfer in the upper crust.

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scCO2 solvent,14 and the subsequent impacts to mineral transformations in water-bearing scCO2, which have the potential to alter reservoir properties, have not been previously evaluated or appreciated. In this study, we utilized an in situ X-ray diffraction (XRD) technique to characterize the carbonation steps of synthetic forsterite (Mg2SiO4) during exposure to water-saturated scCO2 fluids that had been equilibrated with organic-bearing aqueous solutions of acetate (C2H3O2−), oxalate (C2O42−), malonate (C3H2O42−), or citrate (C6H5O73−). Although aqueous-based forsterite dissolution has been extensively studied,15 it is also important to investigate the coupled dissolution−precipitation carbonation reactions because forsterite is often in intimate contact with secondary phases in water-poor weathering environments.16−18 Acetate, malonate, and oxalate were chosen as they are the most abundant organic ligands in the subsurface, with concentrations exceeding 20 mM in hydrocarbon-

INTRODUCTION Geologic carbon sequestration (GCS) is a promising technology for reducing atmospheric carbon dioxide (CO2) emissions. Economically favorable options include a carbon utilization component, such as CO2-enhanced oil and gas recovery (EOR and EGR).1,2 Utilization of supercritical CO2 (scCO2) as a fracturing fluid for tight (low permeability) oil and gas formations is also receiving increased attention as an alternative to the conventional approach requiring large volumes of water.3,4 Subsurface injection of scCO2 is predicted to produce dehydration fronts and buoyancy driven mixing that creates regions in which the full spectrum of mutual CO2−H2O solubility can occur.5−8 Importantly, the CO2-dominated (water-bearing or wet scCO2) fluids are highly reactive with regard to the minerals present in the reservoirs via the formation of thin adsorbed water films.6,9−11 The complexities of these multiphase systems are exacerbated in organic-rich environments, as scCO2 is an effective solvent for organic matter.12 Extraction and transport of organic matter by scCO2 has been observed in the field and in experiments with geomaterials (see references compiled in Miller et al.13). These organic additives modify the physicochemical properties of the © 2015 American Chemical Society

Received: Revised: Accepted: Published: 4724

December March 15, March 16, March 25,

13, 2014 2015 2015 2015 DOI: 10.1021/es506065d Environ. Sci. Technol. 2015, 49, 4724−4734

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Table 1. Experimental Parameters Used during the in Situ High-Pressure X-ray Diffraction Study Reacting Water-Saturated (± Organics) scCO2 with Forsterite at 50°C and 90 bar (0.75 g CO2)a experiment ID EXP 1 H2O control EXP 2 Ca−acetate EXP 3 Mn−acetate EXP 4 malonate EXP 5 oxalate EXP 6 citrate

forsterite mass (mg)

salt used to prepare 0.1 m organic solution

4.2

HXRD wt % carbonate pKa of deprotonation reaction(s) at 50 °C

duration (hours)

nesquehonite

n/a, no organics in experiment

n/a

45.0

3.9

Ca(C2H3O2)2·H2O

4.79d

4.2

Mn(C2H3O2)2·4H2O

3.9

b

magnesiteb

total crystalline carbonationc

84

16

100

44.3

76

21

97

4.79d

44.8

63

17

80

Na2C3H2O4·H2O

2.64−5.81e

44.9

52

36

88

4.0

Na2C2O4

1.45−4.40f

43.8

57

41

98

4.1

Na3C6H5O7·2H2O

3.10−4.76−6.48g

44.8

46

40

86

The amount of solution used for all experiments was 10 μl. bUncertainty is ±5 wt %. cUncertainty is ∼±7 wt %. dMesmer et al.116 eKettler et al.117 Kettler et al.118 gBenezeth et al.98

a f

procedure.36,44 The high surface area (26.7 ± 0.1 m2/g) of the forsterite powder contributes to its elevated dissolution kinetics relative to natural samples,32,33,35 making it ideal for laboratory studies with time scales of hours to days. High-Pressure X-ray Diffraction and Ex Situ Characterization. Experiments were conducted at 50 °C and 90 bar for ∼44 h in a high-pressure static reactor semitransparent to Xrays, which has been previously been described in detail.35,46,47 As shown in Figure S2, the forsterite was packed into a sample holder that was not in contact with the solution reservoir to ensure that the forsterite only interacted with water and organic complexes that migrated through the scCO2. This setup ensured that the forsterite was reacted with the water-saturated scCO2 fluid and not an aqueous phase. The reactor was housed in an XRD unit that periodically scanned the sample to collect diffractograms. Quantitative analysis of the collected patterns allowed the evolving mineralogy of the reacting sample to be determined with uncertainties of ±5 wt %. Additional information about the HXRD methods, including sample loading, reactor pressurization, data acquisition, pattern refinement procedures, and determination of uncertainties can be found in the Supporting Information materials. The electron microscopy methods used to characterize the reacted grains are also summarized in the Supporting Information section. Geochemical Calculations. Geochemical calculations were performed using the React module of The Geochemist’s Workbench (GWB) version 9.0.0548 and the b-dot extended form of the Debye−Huckel equation. The most complete GWB thermodynamic database with respect to the relevant organic complexes was thermo_minteq.dat, which was used in conjunction with the Duan et al.49 equation of state to predict the geochemical equilibrium conditions (including organic speciation) that prevailed when the reservoir solutions were exposed to scCO2. Additionally, GWB calculations were used to predict the secondary mineralization and pH that would result when forsterite reacted with water buffered by scCO2 at 50 °C and 90 bar. These calculations, which accounted for the inexhaustible atmosphere of CO2 in the experiment, provided insight into the reaction processes and the initial geochemical behavior of the adsorbed water film environment.

associated waters, in which they can survive in significant concentrations at temperatures up to 200 °C for geologic time scales.19−23 Trivalent citrate ligands, though not naturally prevalent, are often used in forsterite reactivity studies and complement the use of the monovalent acetate and divalent oxalate and malonate ligands.24−28 Given the recent interest in metal complex transport in scCO2,13,14,29,30 calcium and manganese acetate salts were utilized in two experiments to examine the possibility of metal or metal−organic mobilization by scCO2 and incorporation into Mg−carbonate phases. Synthetic forsterite was selected as a model silicate mineral due to its relatively simple chemistry and structure and because there are multiple inorganic studies of scCO2−forsterite interactions to compare to this work.31−36 Additionally, basaltic and ultramafic formations, which are ideal GCS sites for carbon storage via mineralization,37−42 often contain forsterite, whose rapid dissolution rate ensures that it tends to control the composition of natural waters and secondary phases in weathering environments.43 The goals of this study were to clarify how naturally abundant organic ligands modify the reactivity of scCO2 fluids and to determine associated impacts to forsterite carbonation.

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EXPERIMENTAL SECTION Materials. Reagent-grade calcium acetate monohydrate, manganese acetate tetrahydrate, sodium oxalate, sodium malonate dibasic monohydrate, and sodium citrate dihydrate were purchased from Sigma-Aldrich or Arcos. These salts of acetate (C2H3O2−), malonate (C2H3O22−), oxalate (C2O42−), and citrate (C6H5O73−) were used as received to prepare 0.1 m (with respect to the organic ligand) solutions with deionized distilled (DDI) water. Nanocrystalline synthetic forsterite, previously described by Thompson et al.32 was prepared using the procedure of Saberi et al.44 XRD characterization of the sample, shown in Figure S1 (Supporting Information), indicated a virtually pure forsterite that matches the International Centre for Diffraction Data powder diffraction file (PDF) no. 034-0189 and the patterns obtained by Saberi et al.44,45 A trace contribution to the pattern at ∼43°2θ (degrees two theta) was detected and corresponds to a negligible ( oxalate > acetate.91,94 It is notable that the magnesite/nesquehonite ratios for EXP 4 and EXP 5 deviate slightly from the overall trend. The ratios are unexpectedly similar, considering the greater logKassoc for the Mg2+−oxalate reaction relative to that of the Mg2+− malonate reaction (Table 2). We speculate that this is due to kinetic factors. The water exchange rate for Mg2+−oxalate at 20 °C is over 2 orders of magnitude lower than the rate for Mg2+− malonate complexes at 20 °C.95 Assuming the relative differences in exchange rates are similar at 50 °C, it is kinetically favorable for the Mg2+−malonate complexes to be dehydrated and contribute to magnesite precipitation, which is reflected in the similar mineral compositions of EXP 4 and EXP 5 (Table 2). Additionally, it appears that the oxalic acid transfer rate to the forsterite through the scCO2 could have been slightly limited due to the low amount of oxalic acid initially present in the solution reservoir at experimental conditions (see Table 2). Interestingly, the thin water film environment also likely enhanced the ligand-promoted Mg2+ dehydration. As emphasized by Thompson et al.34 and Hellmann et al.,96 confined water films on mineral surfaces are highly ordered and therefore can have dielectric constants up to an order of magnitude less than those of a bulk water phases.65,97 The organic ligand−H2O exchange reactions are enhanced when the dielectric constant of the solution is low.92 Additionally, the probable thin water film pH range of 5.0−6.3 ensures that the majority of organic ligands are not fully protonated (see pKa values in Table 1), free to complex and dehydrate Mg2+. For that pH range, approximately 60−81% of the acetate and >99% of the three other organic ligands would not be completely protonated. In addition, although Table 1 lists the citrate pKa values at 50 °C as 3.10−4.76−6.48,98 these values may be as low as 2.17− 4.00−5.49,99 and this discrepancy has apparently not been noted or otherwise addressed in the literature. Whichever the case, it is clear that unprotonated acetate, malonate, oxalate, and citrate organic species are likely present in impactful concentrations in the interfacial water films on the forsterite. Exact proportions cannot be calculated due to unknown Mg and organic concentrations in the thin water film. The ionic strengths of the initial organic solutions were not controlled and varied from ∼0 (EXP 1) to ∼0.47 (EXP 6) at 50 °C due to different amounts of metals contained in the organic salts. Experimental investigations of the CO2−H2O−salt system indicate that the solubility of water in scCO2 decreases with increasing salt concentration.30,100,101 Saline solutions are able to control the equilibrium water concentrations in scCO2 fluids. However, the calculated water solubility decreases only ∼4% for scCO2 buffered by a solution with an ionic strength of ∼1.44 (0.85 m CaCl2 at 50 °C) compared to scCO2 in contact

thickness, it is apparent that only small amounts of organic ligands need to diffuse through the scCO2 to create large effective concentrations in the thin water film environment. For instance, a 0.1 M organic solution was possible if only ∼2% of the total organic components migrated to the interfacial water film. This phenomenon has profound implications for mineral transformations and mass transfer, as scCO2, with its high diffusivity and low viscosity,84 has the potential to infiltrate lithologic pathways that may be inaccessible to aqueous fluids. The solubilities in CO2 and H2O−CO2 partitioning coefficients for naturally abundant organic complexes are poorly constrained. However, organic transport by CO2 fluids may be important for fluxes and time scales relevant to both anthropogenic (subsurface CO2 injection) and geological processes. Organic Impacts to Carbonation. As shown in Figure 2a, overall forsterite dissolution trends were negligibly affected by the presence of organic ligands. Some studies indicate that there are minimal impacts to forsterite dissolution in organic ligand-rich solutions, while others report measurable enhancement of forsterite dissolution rates. The inconsistencies are due to the fact that the effects of organic ligands are sensitive to pH, ligand type and concentration, surface charge, temperature, and the presence of other solutes.24,25,28,43,85,86 These investigations evaluated the effects of organics on forsterite dissolution in bulk aqueous solutions and not adsorbed water films, so the comparisons with this study are imperfect. Other aqueous studies have demonstrated that organic acids can inhibit carbonate precipitation,87,88 but this phenomenon was not observed, as shown in Figure 2a, which emphasizes that the total carbonation was also negligibly affected by the presence of organic ligands. Likewise, Bonfils et al.27 did not observe impacts to olivine carbonation with organic ligands, unless Mg−organic minerals (such as glushinskite [Mg(C2O4)· 2H2O]) precipitated and monopolized the Mg, which drastically inhibited carbonate precipitation. Analysis of HXRD data did not result in the identification of any organic−mineral precipitates, which was expected due to the similar forsterite dissolution and carbonate precipitation trends observed in all of the experiments. The organic ligands also did not appear to affect the shapes of the carbonates. Recent work has demonstrated deviations from typical rhombohedral morphology when magnesite precipitated in a 0.087 M Na− citrate solution.26 This departure from the typical precipitate morphology was not observed in samples reacted in our organic-containing experiments. Precipitate morphology (examples in Figure 3) was consistent in all of the experiments. Also, the possible quantification of carbonate-incorporated organic ligands and analysis of subsequent crystal defects was outside the scope of this study.89,90 Although the carbonate shapes and overall extent of carbonation were unaffected, the type of carbonation was impacted by the presence of organic ligands. As shown in Figure 1, Figure 2b,c, and Table 1, the relative proportions of magnesite increased from EXP 2 and EXP 3 to EXP 6. It is likely that the organic ligands facilitated the partial dehydration of Mg2+, which produced higher proportions of less strongly hydrated Mg2+ and helped overcome the kinetic limitations to anhydrous magnesite precipitation. This organic ligand−Mg2+ complexing and subsequent displacement of solvating waters has been investigated experimentally and computationally91−93 and is the probable mechanism that favors enhanced magnesite precipitation in the thin water film environments. The data in 4730

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with pure water at 50 °C and 91 bar.100 Previous mineral reactivity studies have shown that the amount of H2O dissolved in the scCO2 affects the adsorbed water film thickness and the extent of carbonation.32−35,47,102−104 However, these and other wet scCO2 studies6,8−11,31,46,105−107 controlled the water concentration in the scCO2 by varying the mass of dilute water (no saline solutions) added to the system. To the best of our knowledge, no studies have so far linked salinity-controlled water concentrations to interfacial water film thicknesses and subsequent variations in the types of precipitated minerals, though this could potentially be a fruitful area of future research. Therefore, given the lack of established connections between solution ionic strength and mineralization in the thin water films, the dominant control on magnesite/nesquehonite precipitation was probably the organic−Mg2+ complexing in the water film environment and not the differences in the starting compositions of the organic solutions. Future and ongoing work in which organic-rich solutions of varying concentrations are deposited on mineral surfaces prior to CO2 exposure will help clarify and confirm the mechanisms. Recent geochemical simulations of carbon storage via mineralization stress the utility of experiments that identify the types of minerals precipitated under different conditions.108 Knowledge of probable carbonate assemblages can be used to make these predictive simulations considerably more robust. Environmental Implications. This study has demonstrated that water-saturated scCO2 fluids modified by solvated organics are agents of geochemical change that can impact silicate carbonation products. It highlights how an understanding of the mineral−fluid interfaces in, and the organic content of, a system is critically important for predicting how CO2-dominated phases will react in different settings. In GCS and CO2-enhanced hydrocarbon recovery environments scCO2 fluids can redistribute organics, which in turn can affect the reactivity of minerals in low-water environments in pores, pore throats, and fractures. The physicochemical transformations of dissolution and reprecipitation have the potential to affect the porosity and permeability of the reservoirs and sealing rocks. Mineral precipitates may also affect the CO2 wetting angle, which could then alter the effectiveness of the sealing rock.109 The transport of organics is also important in light of concerns about CO2 leakage from storage or utilization reservoirs and potential impacts to overlying potable aquifers.110−112 These concerns are especially important considering the increases observed in metal and organic concentrations after CO2 injection into hydrocarbon reservoirs.113,114 Transport and reactivity phenomenon must be considered when CO2 leakage risk assessment models are made. Lastly, there are also natural settings where CO2- and H2O-rich phases are interacting with rocks, organics, and each other, such as metamorphic aureoles.14,115 Application of knowledge concerning CO2− H2O−rock−organic interactions will help clarify mass-transfer processes and make predictive models of the fate and transport of CO2 in the crust more robust.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (209) 920-7846. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Fossil Energy. Development of the HXRD instrumentation was funded through Pacific Northwest National Laboratory’s Carbon Sequestration Initiative, which was part of a Laboratory Directed Research and Development Program. Part of this work was performed at EMSL, a national scientific user facility at PNNL that is managed by the DOE’s Office of Biological and Environmental Research. Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Q.R.S.M. also acknowledges support from as University of Wyoming (UW) Energy Graduate Fellowship. J.P.K.’s work was also supported by the UW School of Energy Resources. We thank Toni Owen and Paul Martin for assistance at PNNL, and Susan Swapp and Norbert SwobodaColberg for assistance in the UW Materials Characterization Laboratory. We especially wish to thank John Loring for generously providing the forsterite used in this study. Furthermore, we would like to thank Dr. Giammar for his constructive comments and editorial handling of this paper, and the five anonymous reviewers for their thorough and thoughtful reviews, all of which helped improve the manuscript.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of the methods and Figures S1−S3. This material is available free of charge via the Internet at http:// pubs.acs.org. 4731

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