Modelling of gas-condensate flow in reservoir at near wellbore conditions
The behaviour of gas condensate flow in the porous media is distinctly different from that of gas-oil flow. The differences are attributed to the difference in fluid properties, phase behaviour, and condensation and vaporisation phenomena that distinguishes gas condensate fluids from the aforementioned fluid types. These differences manifest themselves into an important flow parameter that is typically known as relative permeability. Relative permeability is known to be related to the phase saturation, and the interfacial tension (EFT). Also, at high phase velocities, its reduction with increasing velocity, known as Forchheimer (turbulence) or inertia effect, is well documented. An unconventional behaviour of gas condensate fluids has been experimentally proven in Heriot-Watt laboratory and confirmed by other experimental studies performed elsewhere. These tests have shown that at intermediate velocities, before the inertia becomes significant, the gas and the condensate relative permeabilities are significantly improved by increase in velocity. This phenomenon is referred to as the rate-effect. None of the conventional relative permeability models include this experimentally proven favourable rate effect. In this work the flow of gas condensate fluids in porous media is modelled with emphasis on near wellbore conditions. Theoretical, empirical as well as simulational investigations are used to improve the present technology on the treatment of the flow of gas condensate in reservoirs. The use of X-ray or y-ray devices to monitor saturation profile during displacement experiments is investigated and the appropriate test conditions leading to reliable measured relative permeability data are determined. The regimes of the gas condensate flow at the core level, where the rate effect is evident, are investigated using the concept of Reynolds number. Then a mechanistic flow model, where the flow of gas condensate fluids is assumed to follow an annularmist flow criterion, is presented to capture the essence of the rate effect in perforations. The favourable EFT and rate effects are incorporated into the modelling of gas condensate relative permeability by correlating it with capillary number (I\Ic). Two forms of the correlation are presented. The impact of EFT and Ne together with the Forchheimer (inertia) on well deliverability is thoroughly investigated using the above correlation. The gas condensate relative permeability correlation is combined with the Forchheimer effect and used in a specially modified version of a commercial simulator, Eclipse 300V 98a development, to investigate the impact of IFT, Nc, and inertia on well productivity. The impact is found to accelerate production from gas condensate reservoirs. At practical production rates, the significance of the impact on phase recoveries cannot be ignored regardless of reservoir fluid richness or absolute permeability.