A method for determining a relation between an initial saturation and a residual saturation in a first fluid in a porous sample, comprising the following steps saturating a porous sample with a second fluid; measuring a local volume of the second fluid in the porous sample; establishing a steady state profile of a saturation in the first fluid in the porous sample; generating a rise of a capillary ascension flow of the second fluid through the porous sample; during the capillary ascension flow, simultaneously measuring a local volume of the second fluid; and determining the relation between the initial saturation and the residual saturation based on the measured local volume.