Abstract
Contrary to sandstone reservoirs, 90% of carbonate oil reservoirs are characterized as neutral to preferential oil-wet. It is documented that the water wetting nature increases as the temperature increases, which is opposite to the behavior of sandstone reservoirs. The content of carboxylic acid groups in the crude oil, i.e. the acid number, AN, appeared to be a crucial wetting parameter, which very often dictates the wetting nature of the reservoir: the water wetness decreases as the AN increases. It is, however, well known from organic geochemistry that the AN of a crude oil is linked to the reservoir temperature because decarboxylation takes place as the temperature increases. Therefore, the temperature and the AN are not independent wetting parameters for carbonates. In the present experimental investigation, we have decoupled the connection between temperature and AN and studied the individual effects of these two wetting parameters towards chalk. The porous media used are outcrop chalk from Stevns Klint with a very high porosity, about 45-50%, and low permeability, about 2 mD. A number of tests were performed using oils with different AN (0.17-2.07 mgKOH/g). The cores, which contained initial brine saturation of about 25-39%, were aged for 30 days in the corresponding crude oils at different temperatures (40, 80 and 120 C). A recently developed new wettability test method was used to quantify the wetting index. This is a very important technique, because the well known Amott test is not very sensitive close to neutral conditions. The conclusion from the work was that the AN was the most important wetting parameter, and the aging temperature only played a very minor role in determining the wetting conditions. Also a correlation between the new wetting index and Amott water index was established.
