Backward Multiple Contact Test: PVT, Miscibility, and EOR

Key Takeaways

• Vaporizing-drive mechanism and backward contact geometry: In a vaporizing-drive gas flood, the injection gas (lean natural gas, CO2, nitrogen, or a mixture) is lighter than the reservoir oil and enters the pore space at the injection well, where it contacts oil that has not been previously altered by the injected gas. The gas immediately strips intermediate components (propane through hexane) from the oil into the gas phase (vaporization), making the gas progressively richer in intermediates as it moves toward the production well. The oil left behind by each gas contact is leaner in intermediates and heavier in composition. At the production end, the gas that has been enriched by multiple contacts with oil can eventually develop a composition so close to the oil that the interfacial tension between them approaches zero, reaching miscibility. The backward MCT replicates this process in a sample cell: oil is contacted with a fresh gas sample, equilibrated, and the equilibrium gas phase is removed and discarded; the altered (leaner) oil is then contacted with another fresh gas sample; this cycle repeats until either the oil changes so much that it becomes a single phase with the gas (miscibility achieved) or reaches a stable composition that no longer changes (miscibility not achievable at the test pressure). The pressure at which the backward MCT achieves miscibility in the smallest number of contacts is the MMP.
• Backward MCT versus forward MCT (condensing-drive): The forward multiple contact test simulates the condensing-drive gas injection mechanism, which is relevant when the injection gas is rich in intermediates (for example, a high C2-C5 enriched gas or a gas plant liquids-enriched stream). In the forward MCT, a fixed sample of oil is contacted repeatedly with fresh charges of gas, and the equilibrium gas phase is retained after each contact while fresh gas is mixed with the retained gas of the previous step; the gas gradually becomes enriched in intermediates from the oil until it achieves miscibility with the original oil. The forward MCT is appropriate for enriched natural gas injection projects where the injection gas is intentionally blended with propane, butane, or natural gas liquids to achieve MME at the reservoir pressure. The backward MCT is appropriate for lean gas injection or CO2 injection where the mechanism is vaporizing: the injection gas is initially lean, becomes enriched by vaporizing oil components, and achieves miscibility through progressive enrichment of the gas phase by multiple contacts with the oil. In practice, most real gas floods involve both condensing and vaporizing mechanisms simultaneously, and modern PVT simulators use equation-of-state compositional modelling to compute the coupled equilibrium through multiple contact stages rather than relying solely on the backward or forward MCT measurement.
• Experimental procedure and PVT cell requirements: The backward MCT is conducted in a high-pressure visual PVT cell capable of maintaining reservoir pressure (typically 15 to 60 MPa) and reservoir temperature (typically 50 to 130 degrees C) while allowing the fluid phases inside the cell to be observed, sampled, and transferred without pressure loss. The cell is charged with a precisely weighed sample of reservoir crude oil (typically 50 to 100 mL, recombined from separator oil and gas samples to the correct GOR and composition) at the desired test pressure and temperature. A precisely metered volume of injection gas is introduced into the cell, and the contents are mixed by a magnetically driven stirrer until equilibrium is reached (typically 1 to 4 hours). The equilibrium phases are allowed to settle, the gas phase is sampled for compositional analysis by gas chromatography, and the gas phase is then removed from the cell. The cell now contains only the altered oil phase, which is the starting material for the next contact cycle. Fresh injection gas is introduced, equilibrated, sampled, and removed again, and the cycle repeats typically 8 to 20 times until the oil phase achieves miscibility (disappears into the gas phase as a single phase) or the test is terminated because the oil composition has stabilised without achieving miscibility. Full compositional analysis of each contact's oil and gas phases (C1 through C20+ by GC, with physical properties including density, viscosity, and Z-factor measured or calculated) provides the data needed to calibrate the equation-of-state model used in compositional reservoir simulation of the gas flood.
• Determining MMP from the backward MCT and slim-tube experiments: The backward MCT alone does not directly provide the MMP, because the test is conducted at a single fixed pressure and reveals only whether miscibility is achieved at that pressure. The MMP is determined by conducting a series of backward MCTs at different pressures (typically 5 to 8 pressures spanning the expected MMP range) and identifying the minimum pressure at which miscibility is achieved within a specified number of contacts (typically 20 contacts). The slim-tube test is the complementary experimental method that directly measures MMP as an alternative to the multiple-contact test series: a slim-tube apparatus is a coiled stainless steel tube (typically 12 to 25 mm internal diameter, 8 to 20 m long) packed with sand or glass beads and saturated with reservoir oil; injection gas is injected at one end at a constant pressure and temperature, and the oil recovery factor (fraction of original oil in the tube recovered at injector-breakthrough) is measured. A plot of recovery factor versus injection pressure shows a sharp inflection point (typically at 80 to 95 percent recovery) that marks the MMP. The slim-tube method has a semi-empirical character that makes it a less precise physical simulator than the multiple contact test sequence but is widely used as an independent MMP confirmation and regulatory validation tool for EOR project approvals.
• MMP and MME in WCSB CO2 EOR applications: CO2 miscible flooding in western Canada targets primarily the Cardium Formation (Pembina field and Cardium pool in central Alberta), the Viking Formation (Provost and other east-central Alberta pools), and the Weyburn Midale carbonate in southeast Saskatchewan. CO2 achieves first-contact or multiple-contact miscibility with most light to medium crude oils (API 30 to 45 degrees) at pressures of 12 to 22 MPa, which coincides with the reservoir pressures in the Cardium and Viking at initial development depths of 1,200 to 1,800 m. For the Pembina Cardium (average reservoir pressure 12 to 16 MPa at typical development depths of 1,600 m), CO2 MMP determined from backward MCT studies conducted on Pembina crude oil (API 38 to 42 degrees, C6+ content 28 to 34 mole percent) typically falls in the range of 14 to 17 MPa, indicating that miscibility is achievable at most Pembina Cardium reservoir pressures above the early-depletion range. The enrichment requirement (MME) for lean Pembina field natural gas to achieve miscibility is approximately 22 to 26 mole percent C2 through C5 in the injection gas, which requires adding approximately 0.3 to 0.4 litres of propane-butane liquids per cubic metre of lean gas to bring the lean gas composition (typically 95 percent methane) above the MME threshold.