Scale-dependent dynamics of CO2–brine interfaces in mixed-wetting porous media: From sub-grain to grain levels

Abstract

The mixed wettability characteristics of porous media significantly influence CO2-brine two-phase transport processes. This multi-scale wettability heterogeneity manifests not only across core-to-field scales but also occurs at sub-grain to grain scales due to variations in mineral composition, surface roughness, and pore structure of rock surfaces. This study uses finite element numerical simulations to investigate the mechanisms governing mixed wettability effects on fluid migration at both sub-grain and grain scales. Through two-dimensional pore-scale porous media modeling, we simulate brine displacement of CO2 under four distinct wettability distribution patterns and conduct parametric studies across varying capillary numbers. The research findings demonstrate that at the grain scale, brine flow pathways preferentially propagate through regions with stronger wettability clustering. Under sub-grain-scale mixed wettability conditions, quadruple-directional interfacial propagation occurs, achieving brine saturations of 78.2% and 87.6% under capillary numbers of 10−5 and 10−4, respectively. These results nearly exceed all grain-scale wettability scenarios. At lower capillary numbers, pore filling is primarily characterized by burst and overlap events, while at higher capillary numbers, touch and overlap events dominate. Burst events exhibit a nearly 15-fold acceleration in velocity accompanied by significant pressure gradients spanning over seven pores. At the grain scale, CO2 is trapped in three distinct forms: singlet, ganglia, and clusters, with the spatial distribution of wettability governing the trapped area. Under random-order conditions, CO2 clusters exceeding 4 mm2 are captured. In contrast, at the sub-grain scale, adhesion-dominated trapping mechanisms prevail, resulting in a captured area that remains below 0.3 mm2.

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