Measurements and Simulation of Inertial and High Capillary Number Flow Phenomena in Gas-Condensate Relative Permeability

Abstract

Abstract The productivity of most gas condensate wells is reduced significantly by condensate banking when the bottom hole pressure falls below the dew point. The most important parameter for determining condensate well productivity is the effective gas permeability in the near well region, where very high velocities can occur. An understanding of the characteristics of high-velocity gas-condensate flow is necessary for accurate forecasts of well productivity. A number of laboratory experiments have demonstrated that gas condensate relative permeabilities increase at high velocity, reducing the negative impact of condensate banking on well productivity. On the other hand, inertial (non-Darcy) flow effects can reduce the effective gas permeability and lead to lower productivity. This paper presents results of relative permeability measurements on a low permeability sandstone core, using a 5-component gas-condensate fluid. The experiments used a pseudo-steady-state technique at high pressure and high velocity, measuring relative permeability under conditions similar to the near-well region of a gas-condensate reservoir. By carrying out measurements at a range of interfacial tensions and velocities, the results can be used to distinguish between high capillary number and inertial flow effects, and to quantify the impact of these two conflicting phenomena. The experiments suggest that the inertial flow coefficient in a 3-phase gas-condensate-water system is about 50% higher then in the equivalent 2-phase gas-water system. The results of these experiments have been modelled through a correlation of relative permeability versus capillary number, together with an inertial flow correction to the gas permeability. The paper discusses these models and demonstrates how they can be used to calculate gas-condensate well performance in full-field reservoir simulation. Introduction Well productivity is an important issue in the development of many gas-condensate reservoirs. Accurate predictions of well productivity are needed to select the best development plan, to optimise the number of wells and to set gas sales contracts. When the well bottom hole flowing pressure falls below the dew point, a condensate bank forms near the well, impairing the flow of gas and reducing productivity. Condensate saturations as high as 50 or 60 per cent can be attained in the near-well region, reducing gas productivity by up to an order of magnitude. The most important parameter for determining the impact of condensate blockage is the effective gas permeability in the near-well region. Most of the pressure drop from condensate blockage occurs within a few feet of the well bore, where the gas phase will be flowing at a high velocity. An understanding of the characteristics of high-velocity gas-condensate flow is necessary for accurate forecasts of well productivity. In the flow of gas-condensate fluids through porous media at high velocities, there appear to be two competing phenomena which may cause the effective gas permeability to be rate-dependent.An increase in relative permeability with velocity, which has been demonstrated in numerous laboratory core flood experiments1,2,3,4,5,6,7,8. This effect is sometimes termed ‘velocity stripping’ or ‘positive coupling’.Inertial (non-Darcy) flow effects, which reduce the effective gas permeability at high velocity. These two high-velocity phenomena act in opposite directions. Velocity-dependent relative permeability has the effect of improving well productivity, while inertial flow reduces the effective gas permeability and leads to lower productivity. At the flow rates in typical gas-condensate wells, it appears that the change in relative permeability has a much larger impact, so that the overall effect of these high-velocity phenomena is an improvement in productivity.