Low salinity waterflooding (LSF) provides an opportunity for improved oil recovery. However the complexity of the process makes both the fundamental understanding of the underlying mechanism(s) and the interpretation of laboratory experiments difficult. Therefore we use a model system for sandstone which consists of clay minerals deposited on a glass substrate and covered with crude oil droplets in order to study the kinetics of oil detachment when exposed to low salinity brine. The system is equilibrated first under high saline brine and then exposed to brines of varying (lower) salinity while the shape of the oil droplets is continuously monitored at high resolution allowing for a detailed analysis of the contact angle and the contact area as a function of time.
We observe that the contact angle and contact area of oil with the substrate reach a stable equilibrium at high saline brine and show a clear response to the low salinity brine towards less oil wetting conditions and ultimately detachment from the clay (Na-montmorillonite) substrate. This behavior is characterized by the motion of the 3-phase (oil-water-solid) contact line which is initially pinned by clay particles at high salinity conditions and that pinning decreases upon exposure to low salinity brine leading to a decrease in contact area and contact angle which indicates wettability alteration towards a more water-wet state. When the contact angle reaches a critical value around 40-50°, oil drops start to detach from the clay. During detachment most of the oil is released but in some cases a small amount of oil residue is left behind on the clay substrate.
Our results for different salinity levels indicate that the kinetics of this wettability change correlates with a simple buoyancy over adhesion force balance and has a time constant of hours to days; i.e., it takes longer than commonly assumed.
The unexpectedly long time constant, i.e. longer than expected by diffusion alone, is compatible with an electrokinetic model. It is an important finding which provides new insights into the pore-scale mechanism of LSF and also has implications for the execution of low salinity coreflooding experiments, i.e. provides how long it takes to reach equilibrium and at which time scale a response to LSF can be expected.