To date, the visualisation of flow through porous media assembled in microfluidic chips was confined to mineralogically homogenous systems. Here we present a key evolution in the method that permits the investigation of mineralogically realistic rock analogues.

End point relative permeabilities were measured in three limestones with permeabilities ranging from 0.6 to 220 mD under five wettability states established by adding different organic acids, of similar molecular structure but different alkyl chain length, to the oil phase. The altered wettability corresponding to each oil/brine pair is characterized by their dynamic contact angle on a polished calcite substrate, θw, which varied between 50° and 150°. Saturation‐normalized relative permeability to oil exceeds one at θw<140° in all rock considered. The equivalent slip length, defined by modeling the porous medium as a capillary tube with the defending phase distributed as an annular film on the tube wall, was below 200 nm in all experiments. The results indicate that commonly used models of relative permeability, which assume that the maximum permeability is the single‐phase permeability, underestimate oil displacement for a much wider range of contact angles than previously documented.

Optical microscopy was used to measure depth-averaged oil distribution in a quasi-monolayer of crushed marble packed in a microfluidic channel as it was displaced by water. By calibrating the transmitted light intensity to oil thickness, we account for depth variation in the fluid distribution. Experiments reveal that oil saturation at water breakthrough decreases with increasing Darcy velocity, U w , between capillary numbers Ca = w U w ∕ = 9 × 10 −7 and 9 × 10 −6 , where w is the dynamic viscosity of water and is the oil/water interfacial tension, under the conditions considered presently. In contrast, end-point (long-time) remaining oil saturation depends only weakly on U w . This transient dependence on velocity is attributed to the competition between precursor film flow, which controls early time invasion dynamics but is inefficient at displacing oil, and piston-like displacement, which controls ultimate oil recovery. These results demonstrate that microfluidic experiments using translucent grains and fluids are a convenient tool for quantitative investigation of sub-resolution liquid/liquid displacement in porous media.

Remaining oil saturation established by waterflooding from maximum initial oil saturation was measured in Indiana limestone under four mixed-wet conditions established using different organic acids. The altered wettability is characterized by the advancing contact angle of the aqueous phase on a calcite substrate submerged in the oil phase, which ranged from θ o = 50°to 150°. Remaining oil saturation decayed as a power of time for up to 55 pore volumes of water injected and then reached a constant value. The duration of oil production increased linearly with θ o . In contrast, remaining oil saturation decreased and then increased with increasing θ o within the range of experimental conditions, with optimal wettability for recovery shifting from θ o = 110°to 135°as waterflood progressed.

We use a Darcy-scale simulator to extract residual oil saturation, forced imbibition capillary pressure, and relative permeability by history matching to measured pressure drop and cumulative oil production during multi-speed centrifuge experiments and constant-rate waterfloods in Indiana limestone cores under four different wettability states established by adding different naphthenic acids to the oil phase. Residual oil saturation decreases monotonically as advancing bulk contact angle increases from θ a = 110 • to 150 • , in sharp contrast to the nonmonotonic dependence displayed by the core-averaged oil saturation which are often mis-interpreted to be representative of true residual saturation. The magnitude of the capillary pressure required to establish a particular water saturation increases with θ a . Saturation-normalized water relative permeability exceeds one at θ a ≥ 125 • , with equivalent slip lengths of up to O(200) nm. The simulations indicate that capillary end effects may be significant during displacement experiments under typical laboratory conditions, even in mixed-wet media of relatively low permeability, and highlight the importance of using numerical simulation to interpret displacement experiments under capillary-dominated conditions.