Wettability controls the distribution of fluid phases and flow properties in oil reservoirs. Wettability characterization can be accomplished using standard techniques such as Amott-Harvey and USBM. Nevertheless, these experiments are time consuming and limited numbers are carried out for each oil reservoir. The objective is to evaluate the possibility to use the flotation technique combined with geochemical simulations for fast wettability characterization.

The flotation technique relies on the affinity of the minerals to either the brine or the oil, and was used to characterize the wettability of minerals. The amounts of oil-wet particles is determined for the mineral-brine-oil mixtures after aging the mineral in brine and oil respectively. Two formation water compositions and two stock tank oils were selected for the flotation experiments. As an introduction to this study, the wettability of six (6) minerals found in sandstone reservoir rocks were investigated by flotation test. The mineral-brine interactions such as solubility and surface complexation of minerals were modelled with the geochemical simulator PHREEQ-C, and the results were compared with their experimental counterpart. The flotation tests showed that the crude oils altered the wettability of some of the water-wet minerals to oil-wet. It was inferred that the clay minerals were less water-wet. Calcite with cationic surfaces, became more oil-wet by aging with crude oil, and this indicated direct adsorption of carboxylic acids.

Surface Complexation Modelling (SCM) results reveal that the surface charges of both quartz-brines and STOs-brine are mostly negatively charged and hence electrostatic repulsion exist between the two interfaces leading to lack of oil adhesion. Unlike quartz, the calcite-brine and the STOs-brine interfaces were positively and negatively charged respectively. Hence, direct adhesion of the polar oil components onto the calcite surface is the reason for the high oil-wet nature of calcite. This was also consistent with the total bond product which expresses the tendency of oil adhesion onto minerals surfaces. The total bond product for calcite (0.95 – 1.06) was greater than quartz (0.01 – 0.07) and hence confirming that more oil was adsorbed on the calcite surface unlike quartz. Both the SCM and the flotation test results reaveal that the calcite is strongly oil-wet while quartz is strongly water-wet.

The flotation technique combined with geochemical simulation is a promising and cheap approach of characterizing the wettability. In the flotation tests only small rock samples are required. This approach has the potential to provide fast estimation of the wettability of reservoir rocks.


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