The Relative Significance of Positive Coupling and Inertial Effects on Gas Condensate Relative Permeabilities at High Velocity
Authors: Henderson G.D. , Danesh A. , Tehrani D.H. , Al-Kharusi B. 
Published: December 2007

Abstract
The authors were the first to report that gas-condensate relative permeability will increase with increasing velocity. This positive rate effect, which was later confirmed by other investigators, was attributed to the coupling of the flow of the two phases and was referred to as the "positive coupling" effect. The observation was made in tests conducted at velocities where the effect of "inertia" was not significant. The objective of the latest study was to investigate the competition between the two effects of "negative inertia" and "positive coupling" on gas-condensate relative permeability at velocities up to one order of magnitude above the velocity boundary with significant inertia. The maximum tested velocity was 700 m/day, which was representative of the flow regime within fractions of a meter from the wellbore of a typical producer. The tests were conducted on different cores at various interfacial tension (IFT) values.
The results have shown that "inertia" was dominant in cores saturated with 100% gas at the tested conditions. However, as the condensate saturation increased, an improvement in relative permeability due to "positive coupling" was observed over the entire range of velocities at all values of IFT tested. This resulted in the generation of unique relative permeability curves, showing decreasing relative permeability with increasing velocity at low condensate saturations, and increasing relative permeability with increasing velocity at high condensate saturations. This trend was observed mainly for the gas phase. Previously published data had indicated that inertia reduced the gas relative permeability at high velocity.
The data has been used to develop empirical correlations, which relate the change of gas-condensate relative permeability to variations in fluid saturation, velocity and IFT.

Introduction
In June 1994, the authors reported for the first time that the relative permeability of the gas phase in particular, and to a lesser extent the condensate phase, increased with increasing velocity when measuring relative permeability using condensing fluids in long cores[1]. This new phenomenon was referred to as the "positive coupling effect", and was attributed to the coupling of the flow of the gas and condensate phases, based on the results of studies which had highlighted that gas and condensate can easily flow together in the same pore space[2,3]. Since this initial finding, interest in gas condensate relative permeability by other researchers has increased significantly[4,5,6,7,8,9,10,11,12], but few have reported the positive coupling effect using condensing fluids.
Over the period from 1994 to the present, the authors have published a series of papers reporting the data generated when using long cores containing condensing fluids to generate steady-state relative permeability curves[1,3,13,14,15,16,17]. The measurements were made over a range of IFT (interfacial tension) values covering 0.015 to 0.7mNm-1, and at different velocities. Initial tests[1,3], were conducted using the technique of flashing the gas condensate fluid from above the dew-point pressure to the core pressure, to establish steady-state flow and measure relative permeability, a technique since adopted by others[11,12]. Subsequent measurements by the authors using the steady-state technique to measure the relative permeability confirmed the initial results[13,14,15,16,17]. A summary of the reported findings is given below;The relative permeability of condensing fluids increased with increasing velocity, at conditions where the inertia was not significant for the dry gas (zero condensate saturation).The positive coupling effect was not limited to one core type, but was observed for different lithologies and over a range of permeabilities, from 10 to 550 md.Positive coupling was also observed when connate water was present in the cores. The gas relative permeability was observed not only to be a function of its own saturation, but that of the condensate phase too.When the relative permeability was measured using unsteady-state procedures at the same test conditions as the steady-state tests, the observed positive coupling effect was minimal.