Enhanced Oil Recovery – EOR
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 pressurized and stabilized in single phase. The mixture is then restabilized 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 repressurized 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 (stabilized 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%.