Mineral Grain Spatial Patterns and Reaction Rate Up-Scaling

Reactive transport models that describe mineral reactions in porous media rely on laboratory measurements of rate parameters that may fail to represent reactions defined at larger averaging scales. Laboratory experiments usually use crushed minerals in well-mixed systems that are designed to eliminate mass transport limitations. The resulting data provide valuable information on mineral reactivities but have questionable value for describing reaction rates in porous media in which mass transport may be rate-limiting. In recently completed work, we used pore-scale network models to investigate the effects of heterogeneities in pore structure and mineral distribution on geochemical reaction rates in porous media. Our findings revealed significant scaling effects from variations in reactive mineral distribution, especially for weathering reactions driven by highly acidic conditions such as those encountered in deep subsurface geological sequestration of carbon dioxide. In ongoing research, we are using electron microprobe analysis, a form of X- ray emission spectroscopy, to analyze spatial patterns of mineral grains in sedimentary rocks. Samples include sandstones and shales from the Viking formation in the Alberta basin in western Canada, at depths ranging from 2000 to 4000 m. Viking sandstones are calcareous litharentites, with porosities (mostly primary) ranging from 1% to 24% and permeabilities ranging from 0.01 to 1000 md. This presentation will include the findings quantifying mineral content and mapping mineral grain distributions of these rocks, and the results will be presented in the context of the pore-scale network modeling work and the broader question of up- scaling of geochemical reactions.