Iron and Sulfur Geochemistry in Class H Wellbore Cements Exposed to CO2 and H2S
Abstract
The effects of CO2 & co-constituents (such as H2S) sequestration on cement seal integrity are not well understood in the context of wellbore integrity for CO2 storage. This study evaluates the redox effects that co-contaminants such as H2S present to the CO2-cement reaction system via synchrotron X-ray XANES mapping specific to the iron and sulfur edge energies. Portland Class H cement was exposed to various proportions of H2S:CO2 in 1% NaCl saturated brine (1%, 21 mol%, and 40% H2S) under supercritical conditions (50°C and 15MPa). The reaction of cement with H2S-CO2 results in the formation of pyrite associated with the Fe-rich cement clinker phase, brownmillerite (also termed ferrite). Ferrite has not reacted in previous exposures to CO2 alone, which was confirmed by synchrotron spectroscopic analysis of cement exposed to supercritical CO2 alone. Thus it is hypothesized that the reaction is a result of redox conditions introduced by H2S. The synchrotron X-ray microprobe fluorescence (μXRF) imaging and spectroscopy capabilities at beamlines 2-3 and 14-3 at SSRL were used to collect multiple energy (ME) maps for both Fe and S in order to evaluate reaction fronts in the cement matrix and to monitor the chemical changes in the cement associated with exposure to CO2 (and H2S) at sequestration conditions. The use of this micro-spectroscopy technique allows for in-situ identification of any reaction intermediates (including amorphous materials) for the Fe and S phases in the cement. The coupled μXANES and μXRF data were used to generate iron and S speciation maps of the cement cores. Synchrotron microprobe capabilities at 2-3 were used to collected ME maps of Fe, and show differences in Fe oxidation between the rims (Fe2+) and cores (Fe3+) of the cement thin section. Analysis of Fe XANES indicates that there are potentially 4+ distinct coordination environments for the Fe in the cement cores studied: pyrite, ferrihydrite, brownmillerite (Fe3+, and/or Fe2+) and potentially amorphous Fe-S. In a similar manner, S was evaluated by collecting multiple energy maps through the S-edge using beamline 14-3 at SSRL. The S ME maps revealed much broader reaction fronts as revealed by the iron mapping alone. Analysis of S XANES spectra is ongoing, but preliminary results indicate that possibly 6 different binding environments are likely in the cements depending on the H2S concentration: FeS, FeS2, S, a sulfite phase, and potentially two different sulfates (gypsum and ettringite). The results indicate that S oxidation state likely grades from S+6 in the core of the cement to S-1 in the rim of the cement, with S oxidation states ranging from S4+ to S0 in zones in between. This work will serve to better understand the geochemical reactions in the cement upon addition of S co-constituents in order to better assess potential impacts on long-term cement integrity.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V31D..06L
- Keywords:
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- 1099 GEOCHEMISTRY General or miscellaneous;
- 1858 HYDROLOGY Rocks: chemical properties;
- 1042 GEOCHEMISTRY Mineral and crystal chemistry