Simultaneous determination of gas–water relative permeability and capillary pressure from steady-state corefloods

Abstract

For traditional calculations of relative phase permeability (Kr) from coreflood Steady-State Test (SST), the capillary pressure (Pc) is required. Usually, Pc is determined from a separate test, using a centrifuge or porous-plate methods. However, during SSTs, water cut and pressure drop are measured during the transition period between two sequential fractional-flow steps. We developed a novel method for simultaneous determination of Kr and Pc from SST by using both steady-state and transient data. In the proposed method, the transition data on the pressure drop across the core are used instead of the traditionally utilised Pc-curve. The main idea is that the stabilisation period during each fractional-flow step is inversely proportional to the non-linear saturation-diffusion coefficient in the capillary term of the Rapoport-Leas equation, which in turn is proportional to the derivative of capillary pressure.Interpretation of the pressure-drop transient data depends on their variation during the stabilisation period. The curves for “stabilisation period versus water cut” have a U-shape for water-wet rocks, and the pressure drop variation during the stabilisation period has an S-shape; the corresponding laboratory data approximation regularises the inverse problem. The stability of the algorithm has been checked in wide intervals of the accuracy of measured data. The algorithm was applied to three gas–water laboratory data sets from the literature where X-ray-based saturation profiles have been measured along with the steady-state water cut and pressure drop curves. The close match of the laboratory data validates the method developed.