Abstract
The wetting nature of a carbonate reservoir determines the potential of improved oil recovery by water injection, especially if the formation is highly fractured, which often is the case for chalk formations. The initial wetting state is mostly governed by the properties of the crude oil, where the acid number, AN, appeared to be a crucial parameter. The reservoir temperature plays, however, a minor role as a wetting parameter. Improved oil recovery by water injection decreases as the water-wetness decreases due to decrease in spontaneous imbibition. The relative high specific surface area of chalk, about 2-3 m2/g, indicates that compositional properties of injected fluid could influence the wetting properties of the formation somewhat during the production phase. The exceptional good response of seawater injection into the Ekofisk formation is an indication of a special rockwater interaction, which improves the spontaneous displacement of oil. In the present paper, we will summarize the results from ongoing research to improve spontaneous imbibition of water into oil-containing chalk samples at various wetting states and temperatures. The chemical composition of the imbibing fluid is discussed in relation to the composition of initial brine and changes in the rock-fluid (water and oil) equilibrium conditions. Major observations are: (1) The presence of sulfate in seawater improved spontaneous imbibition at all wetting conditions tested. (2) Sulfate is a potential determinig ion towards chalk, which has impact on the wetting state in a positive way. (3) The adsorption of sulfate onto chalk increases as the temperature increases, which implies that the efficiency of sulfate as an active wettability modifier is improved as the temperature increases. (4) The relative increase in sulfate adsorption onto chalk increases at temperatures above 100°C. (5) Injection of produced water with a high Ca2+/SO4 2- ratio is recommended to be used only as pressure support and not as an oil displacing fluid.
