Abstract
Wettability is an important factor which controls the displacement of immiscible fluids in permeable media, with far reaching implications for storage of CO2 in deep saline aquifers, fuel cells, oil recovery, and for the remediation of oil contaminated soils. Considering the paradigmatic case of random piles of spherical beads, fluid front morphologies emerging during slow immiscible displacement are investigated in real time by X-ray micro–tomography and quantitatively compared with model predictions. Controlled by the wettability of the bead matrix two distinct displacement patterns are found. A compact front morphology emerges if the invading fluid wets the beads while a fingered morphology is found for non–wetting invading fluids, causing the residual amount of defending fluid to differ by one order of magnitude. The corresponding crossover between these two regimes in terms of the advancing contact angle is governed by an interplay of wettability and pore geometry and can be predicted on the basis of a purely quasi–static consideration of local instabilities that control the progression of the invading interface.
Highlights
A similar cross–over has been found in a numerical model by Cieplak and Robbins[28, 29], who considered a two-dimensional ‘fluid’ front invading an arrangement of circular disks
We show by in situ X–ray micro-tomography that the interface of a non-wetting fluid invading a random bead pile prefers to penetrate through larger throats while it develops a ramified morphology
In the case of large contact angle, we find an asymptotic value of a∞ ≈ 3.3a0 of the surface–to–volume ratio for the percolated oil phase after injecting several pore volumes of the non–wetting invading phase (PV 5)
Summary
A similar cross–over has been found in a numerical model by Cieplak and Robbins[28, 29], who considered a two-dimensional ‘fluid’ front invading an arrangement of circular disks. Fluid dynamics simulations that allow to study partial wetting conditions are computationally expensive because they demand a high spatial resolution close to moving three phase contact lines It is not clear whether the pronounced difference of front morphologies that were found in a regular two–dimensional lattice of circular discs by Cieplak & Robbins[28, 29] exist in three dimensions. A wetting fluid invading a random bead pile obeys no such preference in its flow path and the emerging front morphology remains compact These observations strongly support the assumption that the invasion process is quasi– static and dominated by non–cooperative interfacial instabilities for high contact angles[5, 6]. A strong correlation between the experimentally determined residual saturation and the relative frequency of Haines jumps corroborates once more that pore–scale processes during slow invasion are fully captured in the quasi–static picture put forward in this article
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