Relative Permeability Measurements of Composite Cores, an Experimental Approach

Some experimental tests require floods to be carried out on longer cores, typically 1-3 feet long. When whole cores are not available, side-wall cores each measuring 3-6 inches long are put together to make a composite core. It is the prevailing practice in the industry for composite core floods to order cores in an ascending permeability order, as this is thought to lower capillary forces for high flow rates and thus lessen the capillary end-effect. Huppler (1969), through a numerical study and neglecting capillary end-effects, proposed the criteria that cores should be ordered in such a way that the harmonic average between sections should be as close as possible to the overall average of the composite. Also, the core with permeability closest to the average is placed at the outlet position. Recently, with certain assumptions, Langaas et al. (1998) have demonstrated through a theoretical study that a new criteria should be followed, i.e. ordering cores in a descending order. They showed that this would produce residual oil saturation closest to the value for the composite, if it were considered as a single core with average properties. In this work, we present results of an experimental composite core flooding study that was designed to test how the properties of the individual cores in a composite core-stack influence the measured residual oil saturation and relative permeabilities for an oil-water system typical of a water flood. The study was conducted for carbonate cores, predominant in the lower Arabian Gulf region, and involved composite cores stacked in an ascending, descending, and random order (according to the Huppler criteria). Results of the experimental runs in this study show a significant effect of ordering on relative permeability evaluation, with values for K_rw and K_ro for composite cores in a descending order significantly different from the values for both random ordering and ascending ordering. Also, the recovery factor was highest for the composite core ordered in a descending order, followed by ordering according to Huppler's criteria, and then ascending order. These findings support Langaas et al findings, i. e. the best ordering criteria is after decreasing permeability along the flow direction such that the core with the highest permeability is placed at the inlet.