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The backward multi-contact study is performed in the same cell as is used for the
swelling study. A known volume of reservoir fluid is charged to a high-pressure
PVT cell and heated to the reservoir temperature. A partial constant composition
expansion is carried out to just below the bubble point pressure. 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 stabilised in single phase.
The mixture is then restabilised at the selected test pressure. At this pressure
gas and liquid equilibrium phases are formed and the total volume of sample measured.
The equilibrium gas phase composition is determined. A pressure-volume relation
is performed on the fluid remaining in the PVT cell following the displacement of
all the gas. The density of the mixture is calculated from the known weights of
the mixture components and the measured volume of the liquid phase at the test conditions.
The density at other pressures is then calculated using the measured relative volumes
from the pressure-volume relation.
The liquid phase is then repressurised to the working pressure. The above procedure
is repeated for usually 3 further gas steps.
Following the pressure-volume relation of the fluid remaining in the PVT cell after
the final addition of injection gas /removal of equilibrium gas the composition
of this fluid remaining in the PVT cell is determined to C36+.
As an internal quality check the mass and moles of fluid in place are calculated
using the densities, molecular weights and volumes of gas and oil mixed together.
The densities, molecular weights and volumes of gas (stabilised at the test pressure)
are measured at each contact. The density of the fluid remaining from each contact
are calculated from the known weights of mixture components and the volume of fluid
remaining in the PVT cell at the contact pressure and reservoir temperature. These
are then used to calculate the mass and moles and therefore total mass and moles
in place. The values are compared and should be less than 2%.