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type     June, 2005

Vol 2 Chapter 7: Geophysical and Geochemical Effects of Supercritical CO2 on Sandstones

Hartmut Schutt, Marcus Wigand and Erik Spangenberg

Abstract: The overall objective of this laboratory study was to investigate the geophysical and geochemical effects of CO2 storage in deep saline formations. We used a triaxial cell and autoclaves to reproduce reservoir pressure and temperature conditions that are representative of depths down to 2000 m. The CO2 is in the supercritical state (CO2,scr) at depths greater than approximately 800 m. We measured a number of geophysical parameters, such as seismic wave speeds and attenuation, and collected liquid samples that had been in contact with the rock. Geochemical reactions were studied in detail in autoclaves that are charged with either milled rock or mineral separates. We used three sandstone samples as reservoir rock, and 1 M NaCl solution in doubly deionized water as brine. The geophysical data showed that some effects were qualitatively predictable by standard models. The Gassmann model predicted the dependence of the saturating fluid on the bulk modulus, but underestimated the measured results by approximately 10%. This discrepancy may be due to the modulus dispersion between the low-frequency range of the Gassmann model and the ultrasonic laboratory frequency. The Voigt model reproduced the saturation dependence of vp: Some experiments, however, indicated the existence of fluid front instabilities by reaching only 50% saturation. This corroborated the results of numerical modeling qualitatively. Unexpected was the increase of the compressional wave attenuation for CO2,scr saturation. Scattering can be excluded as a cause, and a local fluid flow model failed to predict the observed effect. Also unexpected and not predicted by the Gassmann equations was the dependence on the saturating fluid of the shear modulus, which is a few percent smaller for CO2,scr saturation than for brine saturation. This may be caused by fluid–mineral interactions. Mineralogical analysis of the rock before and after CO2 flooding indicated that the concentration of major and trace elements decreased, whereas the Si content increased. The mobilization and removal of these elements was caused by the alteration of rock-forming minerals, e.g. biotite, plagioclase, alkali feldspar. Furthermore, we observed the mobilization of heavy metal cations. Precipitation of mineral phases (e.g. dawsonite) was not observed in the short-term experiments. We are still lacking a thorough understanding of the correlation between geophysical and geochemical data. Longterm experiments (duration of several weeks) and careful analysis on a smaller length-scale (individual rains and grain contacts) may help to address this issue.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project Geologic Storage of Carbon Dioxide with Monitoring and Verification - Volume 2
Edited by:
Sally M. Benson, Lawrence Berkeley Laboratory, Berkeley, CA, USA

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