A known volume of reservoir fluid is charged to a high-pressure PVT cell and heated
to the reservoir temperature. A measured portion of the injection gas is injected
to 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 pressurised and the contents thoroughly mixed and stabilised in single phase
(dependent upon the saturation pressure of the mixture and maximum working pressure
of the PVT cell). The mixture is then re-stabilised at the selected test pressure
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 are 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 then 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. During this process the liquid volume
can also be visually measured as a cross check.
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
and composition of the liquid phase is determined from the cell at a pressure above
the contact pressure. The density at saturation pressure is then calculated using
the measured relative volumes from the above partial pressure-volume relation.
The viscosity on the liquid phase is performed using a high pressure Electro Magnetic
Viscometer at reservoir temperature.
The first contact is between the gas recovered from the original equilibrium contact
mixture (usually the final gas addition of the swelling study) and original reservoir
fluid. A known volume of reservoir fluid is mixed with the known volume of gas in
a second PVT cell and stabilised in single phase. The mixture is then re-stabilised
at the selected test pressure for 24 hours and the volumes, compositions and densities
of each phase and the viscosity of the liquid phase determined as described previously.
This procedure is repeated using the gas recovered from the previous contact and
the original reservoir fluid until the volume of gas formed at the selected test
pressure is insufficient for any analyses. When it is determined that this point
has been reached a final contact is carried out with sufficient reservoir fluid
to produce a mixture which remained in single phase at selected test pressure. The
composition and density of the final mixture are determined.
As an internal quality check for each contact a mass and molar balance is performed.
The mass and moles of fluid in place are calculated using the densities, molecular
weights and volumes of gas and oil contact mixed together. The densities, molecular
weights and volumes of gas and oil phases (at the selected test pressure) are then
used to calculate the mass and moles and therefore total mass and moles in place.
The values are compared and should be within 2%.