Pore-by-pore wettability characterization in sandstone and carbonate rocks

Abstract:

Wettability is a key control of multiphase flow phenomena in porous rocks and substantially contributes to determine the efficiency of geological CO2 sequestration, hydrocarbon production and contaminant remediation. Due to the multi-scale heterogeneous nature of sedimentary deposits and the complexity of the interaction between crude oil and rock surfaces, characterizing the wettability of a reservoir is a challenging task. In Garfi et al. (2020) a theoretical and practical approach to rock wettability characterization based on the analysis of fractional fluid surface coverage of rock mineral surfaces was proposed and demonstrated. In this work, we build on and modify the approach to investigate fluid surface coverage on a pore-by-pore basis, therefore aiming at improving the spatial description of wettability.

Two carbonates and two sandstone rocks are imaged with an X-ray micro CT scanner during quasi-static fractional flow experiments. For each lithology, after the drainage process (performed by centrifugation), one of the two samples was treated with crude oil at 80°C for 30 days to mimic the wettability alteration processes often encountered in hydrocarbon deposits. The segmented image of pore space of the rocks is then separated into individual pore regions employing a network extraction algorithm. The registration of the separated pore regions image with those acquired during the imbibition fractional flow steps allows for the identification of the fluid-fluid and fluid-solid interfaces within each pore.

The analysis of the fluid-solid interfaces in a way analogous to that introduced in Garfi et al. (2020) reveals that the wettability alteration protocol has induced changes in the solid surfaces wetting preference. In the sandstone rocks, a transition from water-wetting to intermediate-wetting is observed. On the other hand, the carbonate rock exhibits a more heterogeneous wetting state, with the simultaneous presence of oil-wet and water-wet pores. Having derived a model for fluid surface coverage as a function of wettability, we are able to provide 3D wetting maps that may serve to inform pore network models of mixed-wet porous media.

Garfi et al. (2020). Fluid surface coverage showing the controls of rock mineralogy on the wetting state. Geophysical Research Letters, 47, e2019GL086380.