Forward Multiple-Contact Test

A forward multiple-contact test is a laboratory PVT (pressure-volume-temperature) experiment used to determine the phase behavior between a lean injection gas and a reservoir oil under conditions simulating gas-injection enhanced oil recovery — performed by sequentially equilibrating a fixed sample of gas with multiple successive fresh samples of reservoir oil at reservoir temperature and a target injection pressure, allowing the gas to extract intermediate-molecular-weight hydrocarbon components (C2 through C6) from each oil contact and become progressively enriched as it advances through the simulated injection process; in a forward-contact test (corresponding to the gas at the leading edge of an injection front in the reservoir, where fresh oil is being contacted by gas that has already enriched itself by extraction from previously contacted oil), light and intermediate components are stripped from the oil at each contact and accumulate in the gas phase, gradually transforming the lean injection gas into a richer condensate-like fluid; the test continues through 6 to 12 sequential contacts, with the gas composition and phase state recorded after each contact, and the resulting compositional progression is compared against the criterion for first-contact miscibility (FCM) or multi-contact miscibility (MCM) to determine the minimum miscibility pressure (MMP) — the pressure at which the gas becomes miscible with the oil after a finite number of contacts, eliminating the gas-oil interface and allowing 100 percent displacement efficiency in the contacted volume.

Key Takeaways

Vaporizing gas drive mechanism is the type of multiple-contact miscibility achieved in forward-contact tests with lean injection gases (typically methane, natural gas, or nitrogen) — the gas extracts intermediate-molecular-weight components (C2-C6 hydrocarbons) from the oil during each contact, becoming progressively richer in these components and approaching the composition of the original oil; if the injection pressure is high enough that the gas can extract enough intermediates from successive oil contacts to match the oil composition along a thermodynamic path that does not cross the two-phase region, miscibility is achieved (no two-phase boundary exists between gas and oil); if the pressure is below the MMP, the gas-oil composition path passes through the two-phase region and miscibility is not achieved, leaving a residual gas-oil interface and incomplete displacement; the vaporizing gas drive is most effective for light to medium oils (API gravity 30 to 45) where the oil contains substantial intermediate components, and with injection gases that are lean enough to extract intermediates rather than provide them.
Condensing gas drive is the alternative miscibility mechanism that occurs in backward-contact (reverse) tests or with rich injection gases — in this mechanism, intermediate components transfer from the gas into the oil rather than from oil into gas, with the oil becoming progressively richer in light components after each contact and the residual gas becoming progressively leaner; condensing-vaporizing combined drive (the most common situation in real CO2 flooding) involves both mechanisms operating simultaneously, with intermediate components transferring in both directions until the gas and oil compositions converge to a common composition or the two-phase region is bypassed; modern PVT software (Computer Modelling Group WinProp, Schlumberger PVTi, Calsep PVTsim) can simulate both forward and backward contact tests using equation-of-state thermodynamics, allowing the MMP to be predicted from oil and gas compositions before laboratory experiments are conducted, with laboratory tests then used to verify the EOS-predicted MMP.
Slim tube test is the laboratory standard for direct MMP measurement and complements the forward multiple-contact test by simulating the actual displacement process in a long, narrow porous medium where multiple contacts occur naturally as the gas advances through the oil-saturated tube — the slim tube is typically 12 to 40 meters long with 0.5 cm internal diameter, packed with sand or glass beads to provide a porous medium with permeability of 1 to 10 darcies; the tube is saturated with oil at reservoir temperature and target pressure, then injected with gas at constant pressure, and the recovery factor at gas breakthrough (or at 1.2 pore volumes injected) is plotted against pressure; the MMP is identified as the pressure at which recovery factor exceeds 90 to 95 percent, indicating miscible displacement; slim tube tests are more expensive and time-consuming than forward multiple-contact tests (each pressure point requires a separate slim tube run, with full MMP determination requiring 4 to 8 pressure points and several weeks of laboratory time), but they directly simulate the multi-contact miscibility process under flow conditions and provide the gold-standard MMP measurement.
CO2 minimum miscibility pressure determination using forward multiple-contact tests is the most common laboratory application — for typical light to medium reservoir oils (API gravity 30 to 40, with substantial C5-C20 content), CO2 MMPs at typical reservoir temperatures (60 to 100°C) range from 1,500 to 4,000 psi; the MMP increases approximately linearly with reservoir temperature (the Yellig-Metcalfe correlation, MMP = 1833 + 13.4 × (T - 90) + 5500 / (MWC5+) - based on US oil field data) and decreases with increasing reservoir oil API gravity; CO2 miscibility-related EOR is a major commercial application in the Permian Basin, the Williston Basin, and other US oil provinces where natural CO2 sources from McElmo Dome (Colorado) and Bravo Dome (New Mexico) are pipelined to oilfields for injection; understanding the MMP for each target reservoir is essential to designing the CO2 injection program — operating at pressure above MMP gives miscible displacement and high recovery factors, while operating below MMP gives immiscible displacement with much lower recovery factors.
Multiple-contact test versus first-contact miscibility test distinction reflects the geometry of the displacement being simulated — first-contact miscibility (FCM) means that the injection gas and reservoir oil are immediately miscible upon first contact at any composition ratio, requiring much higher pressures than multiple-contact miscibility because the entire oil-gas composition space must be in single-phase region; FCM is typical of CO2 EOR at very high pressures (5,000 to 10,000 psi) or hydrocarbon gas injection at the MMP for the specific oil composition; multi-contact miscibility (MCM) is achieved at lower pressures by relying on compositional changes during multiple contacts to bypass the two-phase region; both FCM and MCM produce miscible displacement and high recovery factors in the contacted reservoir volume, but MCM is achievable at substantially lower pressures and is the basis of most commercial CO2 and hydrocarbon gas injection EOR projects; the forward multiple-contact test is specifically designed to simulate and quantify the MCM process.

Fast Facts

The forward multiple-contact test methodology was developed in the 1960s and 1970s as gas injection EOR matured into a commercial practice, with key technical contributions from Stalkup, Holm, Moses, and others at major oil company research laboratories. The slim tube test methodology was developed in parallel and provides the validation standard for the multiple-contact test approach. Modern PVT laboratories (Core Laboratories, Weatherford, Stratum Reservoir, Schlumberger Reservoir Lab) routinely perform multiple-contact tests as part of comprehensive PVT analysis for any reservoir being considered for gas injection EOR, with typical test programs costing $50,000 to $200,000 per reservoir oil sample depending on the number of pressure points evaluated and the gas compositions tested. The MMP determined by forward multiple-contact tests is one of the primary inputs to the reservoir simulation models that predict the performance of CO2 EOR or other gas injection projects, and accurate MMP values are essential for project economic evaluation.

What Is a Forward Multiple-Contact Test?

When a lean gas (such as methane, natural gas, nitrogen, or CO2) is injected into a reservoir containing crude oil, the gas and oil interact through component exchange — light components from the oil vaporize into the gas phase and intermediates from the gas (if rich enough) condense into the oil phase. As the gas advances through the reservoir, it contacts oil at multiple sequential locations, and at each contact the composition exchange continues. If the pressure is high enough, the gas eventually becomes compositionally similar to the oil through this multiple-contact extraction process, and the gas-oil interface disappears (multi-contact miscibility). If the pressure is too low, the gas extraction stops at some intermediate composition that retains a two-phase boundary with the oil, and miscibility is not achieved.

The forward multiple-contact test simulates this process in a PVT cell at controlled conditions. A fixed sample of gas is held in a high-pressure cell and brought into contact with reservoir oil at the test pressure and temperature. The gas and oil are mixed and allowed to equilibrate, then the equilibrated gas is removed and analyzed, the remaining oil is discarded, and the same gas (now slightly richer in extracted components) is contacted with a fresh sample of reservoir oil. This sequence repeats for 6 to 12 contacts, with the gas composition tracking through the experiment showing how it evolves toward miscibility (or fails to converge if the test pressure is below MMP). The compositional progression through the test, plotted on a ternary phase diagram, reveals whether multi-contact miscibility was achieved and at what number of contacts.

Forward Multiple-Contact Test Design and Execution

A standard forward multiple-contact test program for a target reservoir oil typically tests 4 to 8 pressures spanning the expected MMP, with each pressure requiring a complete sequence of 6 to 12 sequential gas-oil contacts. The PVT cell used is a high-pressure visual cell capable of pressures to 15,000 psi and temperatures to 200°C, with provisions for thoroughly mixing the gas and oil during the equilibration step. After each contact, the cell is allowed to settle for several hours to achieve complete phase equilibrium, and the gas and oil phases are sampled separately for compositional analysis by gas chromatography (typically extended to C36+ for the oil and C20+ for the gas). The compositional data are entered into PVT modeling software with an equation of state (typically Peng-Robinson or Soave-Redlich-Kwong) tuned to match the experimental data, allowing the simulation of additional contacts beyond what was experimentally measured. The MMP is identified as the lowest pressure at which the gas composition trajectory converges toward the oil composition without the trajectory crossing into the two-phase region — equivalent to multi-contact miscibility under the simulated injection conditions. The MMP determined by forward multiple-contact testing is then validated against slim tube test results when both are available, with typical agreement of ±200 psi between the two methods.

Forward Multiple-Contact Tests Across International CO2-EOR Operations

United States (API / EIA): The US CO2 EOR industry, concentrated in the Permian Basin and other mature oil provinces, requires forward multiple-contact testing for every target reservoir as part of the screening and design process for CO2 injection projects; major operators (Occidental Petroleum, Kinder Morgan CO2 Company, Denbury Resources/now part of ExxonMobil) maintain in-house PVT laboratory capability supplemented by commercial laboratories for routine MMP determination; the EIA's CO2 EOR resource assessments use MMP data from forward multiple-contact tests to determine which Permian Basin reservoirs are technically suitable for CO2 injection at currently achievable injection pressures, with reservoirs requiring MMP greater than 4,000 psi at typical depths being economically marginal due to compression costs.

Canada (AER / WCSB): Canadian CO2 EOR projects in the Weyburn-Midale field (Saskatchewan, where SaskPower CO2 from the Boundary Dam CCS facility is used for EOR) rely on forward multiple-contact testing to determine the MMP for the Mississippian carbonate reservoir; AER and Saskatchewan provincial regulatory frameworks require submission of PVT analysis including MMP determination for CO2 injection schemes; the Weyburn project, in operation since 2000, has produced more than 250 million barrels of incremental oil through CO2 miscible flooding, with the MMP determined by extensive PVT work conducted at the Petroleum Technology Research Centre and equivalent laboratories.