Capillary Trapping Following Imbibition in Porous Media: Microfluidic Quantification of the Impact of Pore‐Scale Surface Roughness

Abstract

Due to diagenesis, pores in subsurface rocks such as sandstones exhibit varying degrees of surface roughness in the forms of authigenic cement coatings and mineral dissolution. Previous work describing capillary trapping in porous media has primarily focused on pore‐space geometry, wettability, and fluid viscosity contrast, while acknowledging, but not quantifying, the potential impact of surface roughness. We introduce a method to implement surface roughness with controlled variation of hillock density and heights into glass microfluidic chips and investigate surface roughness impacts on gas trapping following imbibition of water into air. We demonstrate that surface roughness with hillock height‐to‐pore‐depth ratios (herein called Ω) less than a media‐dependent threshold (Ω = 6%–10% in the micromodels) does not promote nonwetting phase (gas) trapping. By contrast, rougher micromodels with Ω values larger than the aforementioned roughness threshold show a dramatic increase in the saturation of trapped gas (gas saturation values up to 64%) due to an observed change in imbibition dynamics from binary filling to pendular‐ring formation within pore throats as well as capillary pinning within pore bodies. Furthermore, when the micromodel intermediate capillary number results are compared to Land's model, only the roughest microfluidics chips (Ω > 10%) fall within the literature‐described values of the characteristic trapping constant, C, implying that surface roughness is also a key gas trapping control, independent of or in addition to pore‐space geometry and wettability. An a priori menisci stability criterion and a heuristic explanation based on local contact angle variations are proposed to explain surface roughness‐induced trapping.

Subpore‐Scale Trapping Mechanisms Following Imbibition: A Microfluidics Investigation of Surface Roughness Effects

“…(2019b). Previous experiments show that the effect of surface roughness on two‐phase displacement becomes significant when Ω exceeds 0.1 (Lenormand & Zarcone, 1984; Mehmani et al., 2019a, 2019b). Geistlinger et al.…”

“…Given the above mechanisms of surface roughness affecting fluid behavior, however, effects of surface roughness on multiphase displacement are relatively less studied and understood, especially in the context of micromodel experiments. This is partly due to difficulties (a) in performing direct measurements of roughness, (b) in resolving surface roughness features with CT-scanning, and (c) in controlling the implementation of surface roughness in micromodels (Kibbey, 2013;Mehmani et al, 2019b;Rücker et al, 2020). Consequently, our current understanding of the role played by surface roughness in pore models and the development of reliable numerical simulation methods is limited.…”

“…Consequently, surface topology and chemical heterogeneity impact the wetting alteration process (Rücker et al, 2020). Mehmani et al (2019b) developed a method to include surface roughness in glass micromodels where no clay particle was involved. For fluid transport through fractures and pores, not just the roughness, but the ratio of roughness, R a , to pore size/fracture aperture, h, is the relevant parameter, that is,…”

“…Geistlinger et al (2015) showed that residual gas saturation in natural sand packs after imbibition is 3 times higher than in packs of glass beads that are similar to the sands but smoother. Mehmani et al (2019b) showed that trapped gas saturation can be up to 64% during imbibition when Ω exceeds a threshold value (around 0.1).…”

“…The purpose of this paper is to investigate subpore-scale phenomena dominated by surface roughness and evaluate their effect on capillary trapping during imbibition via microfluidics experiments. We implement the micromodel fabrication method developed by Mehmani et al (2019b) and apply advanced visualization techniques, including fluorescence microscopy, micromolding in capillaries (MIMIC, Kim & Whitesides, 1997), as well as scanning electron microscope (SEM) to quantify surface roughness effects. Specifically, we include pore-wall roughness in straight channels with corners and other classical geometries such as pore constrictions and doublets.…”