Relative permeability at near-critical conditions

We have measured a series of two-phase drainage relative permeability curve s at near-critical conditions by means of the displacement method. As a flu id system we have used the model system methanol/n-hexane that exhibits a c ritical point at ambient conditions. In the measurements we have varied the interfacial tension and the flow rate. Our results show a clear trend from immiscible relative permeability functi ons to miscible relative permeability lines with decreasing interfacial ten sion and increasing superficial velocity. The relative permeability measure ments show that the controlling parameter is the ratio of viscous to capill ary forces on a pore scale, denoted by the capillary number N-c=k\\del Phi\ \/phi sigma. To demonstrate the significance of using the proper relative permeability f unctions, we have calculated the well impairment due to liquid drop-out in a model gas condensate reservoir, for four different rock types showing fou r different relations between relative permeability and the capillary numbe r. The calculations show that near-miscible relative permeability functions come into play in the vicinity of the well bore. This is contrary to what happens if the relative permeability would be a function of interfacial ten sion alone. In addition, the results show that well impairment by condensat e drop-out may be significantly overestimated if the dependence of relative permeability on the capillary number is ignored.