Various Methods to Detemine Relative Permeability in Fractured Reservoirs

Abstract

Abstract The relative permeability curves do not share the same degree of acceptance and credibility. This is particularly serious because the estimation of reserves in natural processes and in EOR projects using immiscible fluids (water, gas, steam) heavily depends on relative permeabilities. Most of the relative permeability in fractured reservoirs measurements are performed through the unsteady state method, by which the reported end points correspond to steady state flow conditions (homogeneous saturation and single phase flow), while the intermediate values are calculated during a transient stage in which sample saturation is not homogeneous. Because of this, the end point saturations should have greater validity than the remaining points in the curve, since the shape of the curves in heterogeneous systems depends - among other things - on the mobility ratio, while it is generally accepted that the end points - if properly extrapolated at the end of the test - are independent from it. It is generally believed that residual oil saturation, Sor, is obtained only after injecting "infinite" poral volumes of water. In many cases, the production curve, the residual oil saturation and the permeability to water - at such saturation - notably depend on the flow rate used during displacement. Although laboratories take different steps to correct, or not, the measurements performed, they mostly fail to inform what flow rates were used in the lab or the range of application of the reported data. The relative permeability curves presented by the labs are usually continuous along the entire saturation range between Siw and Sor. Based on the above considerations, it would seem reasonable to conclude that, in order to properly characterize the reservoir, the different displacement mechanisms and flow directions should be studied to analyze their effect on the production curves used to determine the system relative permeabilities.