Modelling of gas-condensate flow in reservoir at near wellbore conditions.
Abstract
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.