Dynamics of fluid displacement in mixed-wet porous media

Abstract

We identify a distinct two-phase flow invasion pattern in a mixed-wet porous medium. Time-resolved high-resolution synchrotron X-ray imaging is used to study the invasion of water through a small rock sample filled with oil, characterized by a wide non-uniform distribution of local contact angles both above and below 90 ° . The water advances in a connected front, but throats are not invaded in decreasing order of size, as predicted by invasion percolation theory for uniformly hydrophobic systems. Instead, we observe pinning of the three-phase contact between the fluids and the solid, manifested as contact angle hysteresis, which prevents snap-off and interface retraction. In the absence of viscous dissipation, we use an energy balance to find an effective, thermodynamic, contact angle for displacement and show that this angle increases during the displacement. Displacement occurs when the local contact angles overcome the advancing contact angles at a pinned interface: it is wettability which controls the filling sequence. The product of the principal interfacial curvatures, the Gaussian curvature, is negative, implying well-connected phases which is consistent with pinning at the contact line while providing a topological explanation for the high displacement efficiencies in mixed-wet media.

Dynamics of water injection in an oil-wet reservoir rock at subsurface conditions: Invasion patterns and pore-filling events

“…III B 1. The average geometric contact angle distribution in the reservoir rock is similar to that obtained on a Ketton limestone sample, 109 • ± 19 • , where the same wettability-alteration protocol was employed [36] [ Fig. 3(a)].…”

“…In the previous study on Ketton, the thermodynamic angle increased from 110 • to 130 • , shown in red in Fig. 3(c), throughout the displacement; however, it was still lower than the thermodynamic angle for the reservoir rock [36]. Hence, we observe that the thermodynamic angle provides a better discrimination between the two cases than the geometric angle which displayed a similar distribution.…”

“…In this study, we use synchrotron x-ray microtomography to visualize the pore-scale dynamics during water injection in a reservoir rock with altered wettability. A previous study investigated the dynamics of water injection into a quarry limestone (Ketton) which had been in contact with crude oil [36]. The experimental conditions, mineralogy (mainly calcite), fluids, and wettability alteration protocol of the study conducted in Ketton are almost identical to our study in a reservoir rock.…”

“…The distribution of in situ geometric contact angles θ g between oil and water was measured in a subvolume of size 1.75 × 1.75 × 1.75 mm 3 , located in the center of the field of view, during water injection using the automated method developed by AlRatrout et al [53]. Figure 3 shows the distribution of the geometric oil-water contact angles in our reservoir rock compared to that measured on Ketton limestone by Scanziani et al [36]. The mean of the geometric contact angle distribution in the reservoir rock is 110 • with a standard deviation of ±20 • .…”

“…As we show later, although the geometric contact angles measured in situ are similar in the two cases, the macroscopic manifestation in terms of displacement sequence, energy balance, and capillary pressure are different, emphasizing the importance of the interaction of pore geometry and wettability on displacement. In the Ketton rock, the behavior revealed mixed-wet conditions, defined by the simultaneous filling of both small and large pores during water injection, indicating that the displacement was controlled by both pore geometry and wettability (local contact angles); water invasion was not a drainagelike process [36]. However, reservoir rocks are likely to undergo a more significant wettability alteration when in contact with crude oil [37,38], rendering the surfaces largely oil wet (water repellent or hydrophobic).…”