Adsorption III. Nonequilibrium adsorption of surfactants onto reservoir cores from the North Sea. The effects of oil and clay minerals

Abstract

Nonequilibrium adsorption of ethoxylated surfactants, mixtures of sulfonates and nonionic alcohols, onto two North Sea reservoir sandstone cores has been studied at reservoir temperature. One of the cores was cleaned prior to the adsorption studies, and the other core contained residual oil saturation, thus maintaining the reservoir wettability. The adsorption was followed by circulating the surfactant solution through the cores for several months (1700 PV), using synthetic seawater. A fast surfactant adsorption was observed, followed by a slow diffusion-controllecd adsorption process. In between the two adsorption regimes there was a temporary delay in the adsorption. The diffusion-controlled adsorption process showed an extraordinary behavior, which was independent of both the surfactant concentration and the surface area available for adsorption. Furthermore, the slow adsorption amounted to about 50 and 25% of the total adsorption for the cleaned and the noncleaned core, respectively. The diffusion-controlled adsorption mechanism has been discussed in relation to the mineralogical and petrophysical properties of the cores, which contained about 20 wt% of clay minerals. The differences in the adsorption rate and quantities for the two cores are discussed in relation to the residual oil and reservoir wettabilities. The relative adsorption of the anionic sulfonate and the nonionic alcohol during the adsorption process is discussed, and the observations support the proposed adsorption mechanisms.