From the swelling study a plot of saturation pressure versus gas injected is generated.
Pressures are selected at varying injection ratios where the fluid exists as two
phases at those conditions.
A known volume of reservoir fluid is charged to a high-pressure PVT cell and heated
to reservoir temperature. A measured portion of the injection gas is injected into
the cell. The quantity of injection gas is expressed in terms of mole fraction of
injection gas as a fraction of total moles in place at the start. The cell contents
are then pressurised and the contents thoroughly mixed and stabilised in single
phase. The mixture is then stabilised at the selected contact pressure and temperature
for 24 hours. At this pressure gas and liquid equilibrium phases are formed and
the total volume of sample measured.
The equilibrium contact gas phase composition and density is determined.
The composition of the equilibrium contact gas phase is used in conjunction with
the correlations of Lee, Gonzales and Eakin to calculate the equilibrium gas phase
viscosity at the selected pressure at reservoir temperature.
The remaining gas is displaced from the PVT cell. The total volume of gas is now
known from the total volume pumped from the cell. The liquid volume is equal to
the total volume minus the gas volume.
A partial pressure-volume relation is performed on the liquid phase to confirm the
saturation pressure used for the equilibrium contact phase separation. The density
of the liquid phase is determined. The density at saturation pressure is then calculated
using the measured relative volumes from the above partial pressure-volume relation.
The liquid phase is then re-pressurised to the working pressure. The equilibrium
contact liquid phase composition is determined.
The viscosity on the liquid phase is performed using a high pressure Electro Magnetic
Viscometer at reservoir temperature.
For each equilibrium contact the density and volumes of each phase are used to calculate
a weight to weight ratio. This ratio was used along with the measured compositions
of the gas and liquid phases are used to derive a calculated recombined fluid composition.
The calculated compositions are compared with the calculated fluid compositions
determined during the swelling study as an internal quality check.