Water and oil capillary absorption in rocks studied by quantitative magnetic resonance imaging

The knowledge of capillary properties of rocks is of great importance for understanding the fluid transport in a reservoir. Magnetic Resonance Imaging (MRI) can be successfully applied to study the structure and fluid transport properties in rocks in a non-destructive and non-invasive manner: MRI makes it possible, by repeated measurements on the same sample over time, to follow the kinetics of the phenomena being studied. Usually, the saturating fluid is water and the signal from 1H nuclei is detected, but other fluids, such as hydrocarbons, can be imaged and nuclei other than 1H can be detected. In this paper, the height reached by the capillary front in the central section of three samples of Lecce Stone, as a function of time, has been determined in different capillary absorption experiments. Two fluids have been used, distilled water and soltrol 170 isoparaffin, an oil particularly useful in laboratory tests, in order to evaluate different capillary properties for variation of wettability conditions. Several experiments have been performed, each time having cleaned the sample by the use of toluene as solvent by a Soxhlet extractor. The positions of the imbibition fronts in the course of time, obtained from acquired images by the use of an in-house software, which is also able to determine the extension of the front itself, were then fitted to the Washburn model for capillary rise in order to evaluate the sorptivity, a parameter which expresses the affinity of a porous medium for the capillary uptake of a liquid and which is related to the capillary properties of the rock and the fluid used. The images clearly show that the saturation behind the front is not complete and that the interface is not abrupt; nevertheless the model gave very good fits to the data, giving reproducible values of the sorptivity for water and for oil.