Stable carbon isotope fractionation data used for pH reconstruction for CO2-rich fluids

Anssi Myrttinen1, Veith Becker1, Bernhard Mayer1, Johannes Barth1
1 GeoZentrum Nordbayern, Lehrstuhl Angewandte Geologie, Friedrich-Alexander-Universität Erlangen-Nürnberg

P 11.12 in Wärme-, Energie- und Kohlenstoffspeicherung im Untergrund

Determining the pH value of deep fluids in contact with elevated CO2 pressures (pCO2) is often challenging since such samples are subject to pH shifts due to CO2 degassing at surface prior to actual pH measurements. Examples of environments where fluid samples with elevated CO2 levels may be expected include regions in contact with present or past volcanic activity, or in reservoirs of carbon capture and storage (CCS) projects. Here we propose a proxy-method for reconstructing in situ pH levels using stable carbon isotope fractionation (eDIC-CO2) data between dissolved inorganic carbon (DIC) and CO2. This data is subsequently used to determine the ratio between two co-existing DIC species and hence, provides the necessary data to calculate the pH at defined temperatures.

To verify this approach, a series of experiments were carried out with a pressure-temperature device that contained 50 mL of water and 10 mL of NaOH and was subjected to 55 bar (pCO2). Temperatures ranged between 20 and 100 °C. For stable carbon isotope data analysis, DIC (here a mixture of H2CO3 and HCO3-) and gaseous CO2 were sampled separately in gas bags in order to avoid contamination with atmospheric carbon. Reconstructed pH values fit excellently with theoretically predicted values and ranged between 5.5 and 6.4, with corresponding eDIC-CO2 values of 0.0 and 0.3 ‰, depending on the temperature. IN comparison measured pH values were all higher by an average of 1.4 pH units most plausibly due to CO2 de-gassing. Hence, the described method may be used as a valuable tool to reconstruct in situ pH levels of CO2 charged deep fluids, provided that temperature and carbon isotope data of both the headspace CO2 gas and DIC have been determined accurately. 

Letzte Änderung 28.10.2013