Reactive transport processes play a key role in subsurface CO2 storage. The rates and extents of these processes are governed by pore scale chemical and physical phenomena that are now observable laboratory micron scale X-ray imaging. We report observations and analysis of a number of key pore scale processes governing CO2 movement and trapping in rocks: The pore scale distribution of mineral surface area can be estimated from X-ray imagery. The average surface area calculated from the imagery correlates strongly to measurements using nitrogen BET. Quantitative differences in the correlations are dependent on the presence of microporous regions or clays with significant sub-resolution surface area. Mineral groupings can be distinguished, e.g., quartz can from kaolinite, on the basis of grey scale segmentation enhanced by dual-energy scanning. We use the ratio of local mineral group surface area to pore volume to construct statistical descriptions of the pore space heterogeneity relevant to reactive transport modeling. The wetting state of mineral surfaces controls flow and trapping in subsurface rocks. Observations of the distribution of CO2 in rocks characteristic of saline aquifers are water wet which leads to significant trapping. Altering the rock to the mixed-wet state characteristic of oil fields results in a change in the morphology of the CO2 to more rarefied ganglia that can inhabit small pores. This allows for desaturation of the CO2 and leads to decreased trapping. The mass transfer of CO2 across the CO2-brine interface is a key process for stabilizing CO2 storage. Solute transport is key in driving this process as well as fluid-solid chemical reactions. Observations of the distribution of fluid-fluid interfaces at the pore scale in a Bentheimer sandstone reveals a dynamic connectivity which has a significant implications for mass transfer across the interfaces. Solute transport observed at the centimeter scale in rocks with varying degrees of heterogeneity reveals a range of rock types, some which are well described using the conventional advection dispersion equation and others where non-Fickian spreading and mixing dominate.

Characterising pore scale mineral heterogeneity and solute transport to model reactive transport for CO2 storage

253rd National Meeting (2017)

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