Samenvatting

Laboratory research was done on flat glass substrates to study the effect that brine salinity has on adhesion mechanisms between oil and clay particles. This is done by evaporating a small clay suspension droplet (6 ul) on a glass slide that leaves a thin layer of clay upon which oil can be attached. When this glass slide is flooded under water, the oil that is attached to clay will form a droplet that can be investigated. It is expected that these droplets will detach when the salinity of the water is lowered. But the mechanism that is responsible for this detachment is still heavily debated and herefore not very well understood. To gain a microscopic perspective of the low salinity effect, the oil droplets have first been flooded in high saline brine (26,000 g/L totally dissolved solids, TDS) for two days so that the adhesion force and bouncy force get enough time to equilibrate. When the forces on the droplets equilibrated, the salinity of the brine was lowered to see how the droplets would
respond to the low salinity water. This response is qualitatively measured by plotting the contact angle and contact area change of the droplets as a function of time. But before experiments were done in a
controlled environment, the optimum experimental protocols were established first.

In a systematic study on the clay concentration it was found that the amount of clay in the suspension was a crucial factor whether the oil drops would respond to the low salinity. If the amount of clay in the
suspension was too high (>250 mg/l for Montmorillonite), than the oil drops adhered too strongly to the clays and ultimately no sign of the low salinity effect would be observed. However, when the amount of clay in the suspension was too low (<100 mg/l for Montmorillonite), than the droplets would detach during the high salinity so the experiment had to be aborted.

The data from the new experimental methodology showed experimentally how exposure to low salinity water causes the oil to decrease its adhesion to the clay, and ultimately even detachment. The oil droplets presented in this study show a significant change in contact angle when they become exposed to low saline water, even when the uncertainty of 5 degrees is taken into account. Caused by the experimental protocol, oil droplets varied in size (volume), initial contact area and initial contact angle. This variation gave dditional experimental variables from the following observed general trends:
(1) Droplets with larger volumes require less time to detach. That is indicative for the force balance acting on the contact area of the droplet: larger droplet volumes means larger buoyancy forces responsible for the detachment, i.e. larger buoyancy forces means shorter time to detachment.
(2) An initially high contact angle (at start of LS), indicates stronger adhesion and as a consequence the detachment time is also relatively long.
(3) When the initial contact area between the oil and clays, Acontact area, is high, then the detachment time of oil is long.

The third trend is interpreted as diffusion controlled equilibration of salt ions in the clay layer between the oil and glass substrate, which therefore critically influences the detachment kinetics of oil. The diffusion time scales were of the same order of magnitude as the experimentally observed detachment times. That supports the view that diffusion plays an important role in the detachment kinetics of oil.

Next, the detachment time was normalized by the diffusion time in order to account for contact area variation. The dimensionless time is defined as such that when it is plotted as function of salinity, it is inversely analogical to the oil production statistics found in the previous work. In the previous study the most oil production was found in the "controlled formation damage" section. Whereas in this study the lowest dimensionless time was observed in the "formation damage" section, meaning that formation damage (i.e. clay swelling) does not influence the oil detachment for the new methodology.