Sandstone/Shale-Brine-CO₂ Interactions: Implications for Geological Carbon Sequestration

Peng Lu¹, Qi Fu², William E. Seyfried, Jr.², Craig Griffith, Sheila Hedges³, Yee Soong³, and Chen Zhu^1
1Department of Geological Sciences, Indiana University, Bloomington, IN 47405, [email protected]; [email protected]
²Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455. [email protected]; [email protected].
³U.S. DOE, National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, PA, [email protected]; [email protected].

The injection of CO₂ into deep saline aquifers is presently being considered as an option for greenhouse gas mitigation. However, significant amount of CO₂-water rock interactions brings uncertainties to this potential option because these interactions may either enhance or decrease the potential of the reservoir by dissolution of primary minerals and precipitation of secondary clay. In addition, these reactions will also determine the integrity of the cap rocks: reactions may enhance or compromise the mechanical properties of the seals. Experiments involving Navajo/Mt. Simon sandstones and Eau Clare shale dissolution in CO₂ acidified saline brine were conducted to evaluate the extent of fluid-rock interactions. Fluid-rock reactions took place in the CO₂-brine-sandstone systems in addition to the expected pH decrease and carbonate mineral precipitation. Silicate minerals in the sandstone display textures (dissolution features, secondary mineralization). The formation of carbonate minerals (mineral trapping) is thermodynamically favored and experimentally observed. The chemical reactions likely increase the porosity of the sandstone due to silicate dissolution. However, allophane and illite/smectite cements fill voids of sandstone grains. There is no evidence that suggests the removal of clay coating due to chemical reactions. It is uncertain whether the mechanical forces near in the injection well would mobilize the smectite and allophane and cause pore clogging. In contrast, there are only limited reactions observed in CO₂- brine-shale system. Reactions of the shale tend to make the rock more compact, thereby locally enhancing the integrity of the repository.

AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009