Predicting δ13CDIC dynamics in CCS: A scheme based on a review of inorganic carbon chemistry under elevated pressures and temperatures

https://doi.org/10.1016/j.ijggc.2011.05.001Get rights and content

Abstract

Stable carbon isotopes are important tools to assess potential storage sites for CO2, as they allow the quantification of ionic trapping via isotope mass balances. In deep geological formations high p/T conditions need to be considered, because CO2 dissolution, equilibrium constants and isotope fractionation of dissolved inorganic carbon (DIC) depend on temperature, pressure and solute composition. After reviewing different approaches to account for these dependencies, an expanded scheme is presented for speciation and carbon isotope fractionation of DIC and dissolution of CaCO3 for pCO2 up to 100 bar, pH down to 3 and temperatures of up to 200 °C. The scheme evaluates the influence of respective parameters on isotope ratios during CO2 sequestration. The pCO2 and pH are the dominant controlling factors in the DIC/δ13C/pH system. The fugacity of CO2 has major impact on DIC concentrations at temperatures below 100 °C at high pCO2. Temperature dependency of activities and equilibrium dominates at temperatures above 100 °C. Isotope ratios of DIC are expected to be about 1–2‰ more depleted in 13C compared to the free CO2 at pCO2 values above 10 bar. This depletion is controlled by carbon isotope fractionation between CO2 and H2CO3* which is the dominant species of DIC at the resulting pH below 5.

Highlights

DIC speciation and carbon isotope fractionation in deep formations were investigated ► Relations to describe p, T and TDS dependence of isotope methods have been reviewed ► Schemes determine ambient conditions of fluids depending on pH, DIC and δ13C ► For pCO2 > 10 bar, d13CDIC is dominated by CO2, providing a stable end member ► δ13C is an accurate tracer in CCS, if ambient conditions are considered properly

Introduction

In the context of global warming, subsurface storage of CO2 (Carbon Capture and Storage – CCS) has become an increasingly discussed option to reduce the amount of CO2 released into the atmosphere by industrial point sources (Oelkers and Cole, 2008). The idea of injecting CO2 into geological formations has existed for several decades and originally was mostly associated with enhanced recovery of oil (EOR) or gas (EGR) (Arcaro and Bassiouni, 1987, Jessen et al., 2005, Tontiwachwuthikul et al., 1996). Storage in depleted gas and oil reservoirs provides the security of low permeabilities of the caprock. However, available storage volumes are restricted and more remote locations of reservoirs may increase costs of transport to the storage sites. Consequently more abundant formations such as deep aquifers and sediment formations, that often host brines, are also examined as storage sites for CO2 (Bachu et al., 1994).

Besides physical trapping beneath a sealing caprock, CO2 can be retained by dissolution in formation and deep ground waters as dissolved inorganic carbon (DIC) (Gunter et al., 1997, Gunter et al., 2004, Gilfillan et al., 2009). This is referred to as ionic trapping if the DIC is dominated by HCO3 or solubility trapping if H2CO3* (H2CO3 + CO2(aq)) prevails, which mainly depends on the pH of the formation water. Precipitation of injected CO2 as carbonate minerals is called mineral trapping (IPCC, 2005). Because of its slow kinetics, mineral trapping has been found to be less effective for quantitative storage of CO2 as compared to ionic trapping (Gilfillan et al., 2009). These different trapping mechanisms will have a distinct impact on the sustainability of CO2 storage. Thus, the CO2 and its reactions in the storage reservoir have to be closely monitored to determine the prevailing mechanisms and their development with time.

Monitoring of stable carbon isotope ratios and subsequent determination of isotope mass balances is a method to evaluate the fate of CO2 and the resulting DIC. This has been applied in several fields. For instance, in hydrology and hydrogeology this method is commonly used to determine mixing of waters from different origins (see Barth et al., 2003, Clark and Fritz, 1997). To date EGR, EOR and CCS have only resulted in relatively few studies applying stable isotope techniques (Assayag et al., 2009, Emberley et al., 2005, Kharaka et al., 2006, Kronimus et al., 2008, Raistrick et al., 2006, Van Bergen et al., 2005). One reason for this is the complexity of the effects of high p/T conditions as well as increased salinity of deep geochemical fluid systems that exert significant influence on the carbonate equilibrium as well as on the isotope fractionation of the carbon species. Taking into account the whole system makes the application of extensive modelling inevitable. This, on the other hand, depends on accurate spatial and temporal geochemical data of the investigated system, which often is not fully available in deep formations. Therefore, this study investigates existing geochemical relations for processes in deep formations with respect to their relevance for stable carbon isotope reactions as well as to their convenient applicability for schematic calculations of these reactions.

The precision of geochemical models is limited by the accuracy of the available sampling data. This is of special importance for geochemical calculations of active or depleted oil and gas reservoirs because influence on the quality and frequency of sampling data usually is determined by the available infrastructure on-site. Because of the above-mentioned influence of pressure and temperature on the geochemistry of the samples, different sampling techniques will have essential influence on the quality of the collected samples. The volume of down-hole samplers, their ability to preserve reservoir pressure and the cooling time of the sampler influence the geochemistry of the sample. Another important question is if the complete sample including gaseous and aqueous phase is preserved or if only a sub-sample is available. In contrast to down-hole sampling, some sites also use preinstalled sampling tubes, whose exact sampling position is site specific (Emberley et al., 2005). The influence of different sampling techniques also renders data comparison between different sites and projects difficult, as long as the individual influence of sampling on the data is not quantified. This is especially relevant for the monitoring of isotope ratios as they may equilibrate at slower rates than the governing chemical reactions and the δ13CDIC is also very sensitive to contamination with atmospheric CO2. This topic is further discussed in Myrttinen et al. (2010a).

The calculations of this study are intended as a tool to efficiently simplify geochemical analysis based especially on CO2 injection monitoring data and thus taking into account the uncertainties of sampling data. Additionally this study establishes an overview of the fundamental pressure and temperature dependencies to be considered when stable carbon isotope methods are transferred from close-to-surface conditions to deeper realms. In doing so, this study aims to further increase application of stable isotope methods in assessment of potential CO2 storage sites.

The ambient conditions of deep formations require an extended approach for CO2 isotope investigations. For convenient evaluation of the ambient conditions of water samples gained from injection sites, a DIC/δ13CDIC scheme for typical conditions of carbonatic CO2 storage sites has been developed. It covers large ranges of partial pressures and pH values found in deep formation storage of CO2 and enhances the considerations at normal conditions for surface waters and shallow ground waters presented by Clark and Fritz (1997). Reservoir pressure, pH and temperatures investigated here reach up to 200 bar, down to 3 and up to 200 °C, respectively, for partial pressures of CO2 (pCO2) up to 100 bar.

Section snippets

Theory

The 13C/12C isotope ratio is measured on a mass spectrometer and its natural abundance changes are usually expressed as a permille deviation from the Vienna Pee Dee Belemnite (VPDB) standard. It is expressed asδCVPDB13=C13/Csample12C13/Cstandard1121000

The 13C/12C ratio of the VPDB standard is 0.011796 (Coplen et al., 2002).

Deviations towards more positive values indicate more 13C in the carbon pool to be measured and values with more negative numbers indicate more 12C in the carbon pool. This

Methodology

To derive the scheme of DIC speciation and its stable carbon isotope ratios, the speciation of DIC in equilibrium with pCO2 between 10−3.5 and 102 was calculated based on Eq. (5). As carbonate is negligible at pH values below 10, its influence in the pH calculations was neglected. This allows straightforward calculation of the speciation in spreadsheet applications. For the calculation of chemical equilibria, the constants K1* and K2* of Millero et al. (2007) were applied, that include activity

Results

The presented scheme shows evolution of DIC and δ13C at various pCO2 values within a range of temperatures, pressures and brine salinity from 0 to 200 °C, 1 to 300 bar and 0 to 6 mol kg−1, respectively. Fig. 1 shows the relationship between DIC concentration and pH for scenario 1 with partial pressures of CO2 from 10−3.5 to 102 bar at conditions to be expected for shallower storage sites with a 3 mol kg−1 brine at 50 °C and 100 bar. Fig. 2 shows scenario 3 for 200 °C, 200 bar, 6 mol kg−1. For each partial

Discussion

Variation of pressure, temperature and solute concentration have noticeable impacts on the solubility of CO2 in the formation water. DIC concentrations in the less extreme scenario (Fig. 1) are around half an order of magnitude lower than at more extreme conditions (Fig. 2). In contrast to this observation, solubility of CO2 is known to generally decrease with temperature (Appelo and Postma, 2005). The lower DIC concentrations in the first scenario are caused by the fugacity of the CO2. The

Conclusions

The work presented here shows how stable carbon isotopes are useful tools for tracing dissolution of CO2 and distribution of DIC in deep formations. They also enable the reconstruction of ambient conditions during equilibration of DIC when the complete geochemistry of the system cannot be constantly monitored. Detailed knowledge of the changing isotope fractionation between the species of DIC and CO2 under changing p/T conditions is crucial for accurate definition of end members for mass

Acknowledgements

This work was financed by the German Federal Ministry of Education and Research (BMBF) under the grant number 03G0704X. Many thanks to Ian Clark from the Ottawa-Carleton Geoscience Center at the University of Ottawa (Canada) for the supply of principles and calculations of isotope mixing in ground waters, which served as a starting point for this work.

References (55)

  • S. Halas et al.

    Experimental determination of carbon isotope equilibrium fractionation between dissolved carbonate and carbon dioxide

    Geochimica et Cosmochimica Acta

    (1997)
  • H. Inoue et al.

    Carbon isotopic fractionation during the CO2 exchange process between air and sea water under equilibrium and kinetic conditions

    Geochimica et Cosmochimica Acta

    (1985)
  • K. Jessen et al.

    Increasing CO2 storage in oil recovery

    Energy Conversion and Management

    (2005)
  • J.W. Johnson et al.

    SUPCRT92: a software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000 °C

    Computers & Geosciences

    (1992)
  • Y.K. Kharaka et al.

    Potential environmental issues of CO2 storage in deep saline aquifers: geochemical results from the Frio-I Brine Pilot test, Texas, USA

    Applied Geochemistry

    (2009)
  • A. Kronimus et al.

    A preliminary evaluation of the CO2 storage potential in unminable coal seams of the Munster Cretaceous Basin, Germany

    International Journal of Greenhouse Gas Control

    (2008)
  • P.M. Lesniak et al.

    Determination of carbon fractionation factor between aqueous carbonate and CO2(g) in two-direction isotope equilibration

    Chemical Geology

    (2006)
  • F. Millero et al.

    The dissociation of carbonic acid in NaCl solutions as a function of concentration and temperature

    Geochimica et Cosmochimica Acta

    (2007)
  • C. Monnin

    The influence of pressure on the activity coefficients of the solutes and on the solubility of minerals in the system Na–Ca–Cl–SO4–H2O to 200 °C and 1 kbar and to high NaCl concentration

    Geochimica et Cosmochimica Acta

    (1990)
  • W.G. Mook et al.

    Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide

    Earth and Planetary Science Letters

    (1974)
  • A. Myrttinen et al.

    Carbon and oxygen isotope indications for CO2 behaviour after injection: first results from the Ketzin Site (Germany)

    International Journal of Greenhouse Gas Control

    (2010)
  • V.B. Polyakov et al.

    Effect of pressure on equilibrium isotopic fractionation

    Geochimica et Cosmochimica Acta

    (1994)
  • F. Schilling et al.

    Status report on the first European on-shore CO2 storage site at Ketzin (Germany)

    Energy Procedia

    (2009)
  • J. Szaran

    Achievement of carbon isotope equilibrium in the system HCO3(solution)–CO2(gas)

    Chemical Geology

    (1997)
  • P. Tontiwachwuthikul et al.

    Carbon dioxide production from coal-fired power plants for enhanced oil recovery: a feasibility study in Western Canada

    Energy

    (1996)
  • J.V. Turner

    Kinetic fractionation of carbon-13 during calcium carbonate precipitation

    Geochimica et Cosmochimica Acta

    (1982)
  • I. Wendt

    Carbon and oxygen isotope exchange between HCO3 in saline solution and solid CaCO3

    Earth and Planetary Science Letters

    (1971)
  • Cited by (28)

    • Assessing geochemical reactions during CO<inf>2</inf> injection into an oil-bearing reef in the Northern Michigan basin

      2019, Applied Geochemistry
      Citation Excerpt :

      However, a comparison between a SrCO3 precipitated sample and DIC from a fluid sample indicate that these changes would be less than 4‰. A fractionation effect occurs during the dissolution of CO2 gas into a fluid that is dependent on temperature and speciation (Becker et al., 2011, 2015; Clark and Fritz, 1997; Mayer et al., 2015; Myrttinen et al., 2012a,b). Geochemical analysis of the brine samples indicate that the DIC is predominately bicarbonate (HCO3−) (Table 2).

    View all citing articles on Scopus
    View full text