Multiple-Contact Miscibility (MCM) and MMP

PVT Analysis of Reservoir Fluids › 6. Multiple-Contact Miscibility (MCM)

Reviewed by Coretest reservoir-fluids engineers · Updated 21 June 2026

The multiple-contact miscibility test, also called the multiple-contact mixing or multcontact experiment, reproduces in the laboratory the repeated, stage-wise mass exchange that occurs when an injection gas (\(CO_2\), \(CH_4\), \(N_2\), lean or enriched hydrocarbon gas, or a flue-gas blend) advances through reservoir oil during a gas-drive enhanced-oil-recovery (EOR) process. Unlike the single-stage flash, the differential liberation, or the swelling test (each of which contacts the oil with gas only once), the MCM test forces the two phases through a sequence of equilibrations so that the compositional path of the displacement front, and ultimately the development of miscibility, can be observed directly.

Purpose↑ Contents

The MCM test answers the central design question of any gas-injection EOR project: under the prevailing reservoir temperature, at what pressure does the injected gas become miscible with the oil, and by what mechanism (vaporizing, condensing, or combined condensing/vaporizing). Specifically it is used to:

• Determine the minimum miscibility pressure (MMP) at fixed reservoir temperature, or the minimum miscibility enrichment (MME) at fixed pressure when the gas composition is varied.
• Identify the displacement mechanism, that is whether miscibility develops by the gas vaporizing intermediate components out of the oil (vaporizing drive, controlled by the leading edge), by the oil condensing intermediates out of the gas (condensing drive, controlled by the trailing edge), or by a combined condensing/vaporizing drive (the realistic case for most enriched-gas and \(CO_2\) floods).
• Quantify phase compositions, phase volumes, densities, and component K-values along the contact sequence, providing the equilibrium data used to tune an equation-of-state (EOS) model for full-field compositional simulation.
• Evaluate oil swelling, in-situ enrichment/stripping, possible asphaltene or solid (dry-ice for \(CO_2\)) precipitation, and the approach of the two phases toward a common critical (plait-point) composition.

Physical principle↑ Contents

When gas and oil are contacted at fixed pressure \(p\) and temperature \(T\) and allowed to reach equilibrium, each component \(i\) partitions between the equilibrium vapor (mole fraction \(y_i\)) and the equilibrium liquid (mole fraction \(x_i\)) according to its equilibrium ratio (K-value):

$$K_i = \frac{y_i}{x_i}$$

with the closure constraints \(\sum_i x_i = 1\), \(\sum_i y_i = 1\), and component mass balance per contact \(n_{i,\text{feed}} = L\,x_i + V\,y_i\), where \(L\) and \(V\) are the equilibrium liquid and vapor mole fractions of the contacted mixture (\(L + V = 1\)). Thermodynamic equilibrium is the equality of fugacities of every component in both phases:

$$f_i^L(p, T, x) = f_i^V(p, T, y)$$

which, with a cubic EOS (Peng-Robinson or Soave-Redlich-Kwong), is what an EOS tune is fitted to reproduce.

The multiple-contact aspect introduces a marching mass-balance. There are two canonical modes that mimic the two ends of a real displacement front:

• Forward contacts (vaporizing drive, leading edge). The equilibrium gas from contact \(j\) is removed and re-contacted with fresh original oil in contact \(j+1\). The gas is progressively enriched in intermediates (\(C_2\) to \(C_6\)) vaporized from successive oil batches. Symbolically the gas feeding stage \(j+1\) is the vapor product of stage \(j\): \(z^{(j+1)} = \alpha\,y^{(j)} + (1 - \alpha)\,x_{\text{oil,fresh}}\).
• Backward contacts (condensing drive, trailing edge). The equilibrium liquid from contact \(j\) is retained and re-contacted with fresh original injection gas in contact \(j+1\). The oil is progressively enriched by intermediates condensing out of each fresh gas batch: \(z^{(j+1)} = \beta\,x^{(j)} + (1 - \beta)\,y_{\text{gas,fresh}}\).

Here \(\alpha\) and \(\beta\) are the mole fractions of the carried-over phase recombined with the fresh phase at each new contact. The evolution of the liquid composition can be written compactly as

$$x_i^{(\text{new})} = F\!\left(x_i^{(\text{old})}, y_i\right)$$

the explicit functional form being the flash solution of the recombined feed at (\(p\), \(T\)).

Miscibility is reached when the tie line connecting the equilibrium liquid and vapor on the pressure-temperature-composition phase diagram shrinks to zero length, that is \(x_i\) tends to \(y_i\) for all \(i\) and \(K_i\) tends to \(1\). The compositional path of the contacts then converges to a critical (plait) point. The minimum miscibility pressure is the lowest pressure at which this convergence occurs after a finite number of contacts:

$$\text{MMP} = \min\left\{\, p : \lim_{\text{contacts}} (y_i - x_i) = 0 \ \text{for all } i \,\right\}$$

Operationally, at \(p\) below MMP a distinct second phase persists indefinitely no matter how many contacts are run; at or above MMP the two-phase region pinches out and a single phase is produced. The same construction at fixed \(p\), with gas enrichment as the variable, defines the minimum miscibility enrichment (MME).

The swelling that accompanies enrichment is tracked by the swelling factor \(\text{SF} = V_{\text{sat}}(\text{at gas mole fraction } F) / V_{\text{oil,original}}\), where \(V_{\text{sat}}\) is the saturated liquid volume after gas addition and \(V_{\text{oil,original}}\) is the volume of the gas-free oil at the same \(T\). Phase densities (\(\rho_L\), \(\rho_V\)) are recorded because the density difference \(\rho_L - \rho_V\) vanishing is an independent, direct indicator that the system is approaching the critical point and hence miscibility.

Apparatus and conditions↑ Contents

The test is run in a high-pressure, high-temperature (HPHT) PVT cell rated to and above reservoir conditions (commonly to 10,000 to 15,000 psi and up to 150 to 200 deg C), held isothermal at reservoir temperature throughout. A visual (windowed sapphire-tube) cell is strongly preferred so that the number of phases, interface position, phase color, and any solid precipitation can be observed and the vanishing of the meniscus near MMP can be seen directly.

Core equipment:

• Variable-volume cell with a calibrated, motor-driven piston (or, in older mercury-based units, a mercury pump) for setting and reading volumes and for maintaining constant pressure during phase removal.
• Magnetic or rocking stirrer/mixer to accelerate inter-phase contact and equilibration.
• Precision pump(s) and accumulators for charging metered amounts of live oil and injection gas.
• Pressure transducers (typically \(\pm 0.1\) percent of full scale) and a precise temperature bath or air oven.
• Sampling ports with capability to withdraw equilibrium vapor or equilibrium liquid at constant pressure into a downstream flash separator and gas chromatograph (GC) for compositional analysis (\(C_1\) to \(C_{7+}\) plus non-hydrocarbons \(CO_2\), \(N_2\), \(H_2S\)).
• Densitometer (vibrating-tube, see the falling/oscillating-element relation \(\mu = K\,t\,(\rho_b - \rho_f)\) used in coupled viscosity/density measurement) for live-phase densities.

Held constant: temperature (always), and, for a single MCM run, pressure. Varied between runs: pressure (to bracket MMP) or injection-gas composition/enrichment (to bracket MME). Within a run the contact index \(j\) is the marching variable.

Procedure↑ Contents

1. Characterize the feeds. Recombine or restore the reservoir oil to its bubble point and verify \(P_{\text{sat}}\), GOR and composition; analyze the injection gas by GC. Record original-oil density and volume at reservoir \(T\).
2. Charge the cell. Load a known mole/volume of live oil and a known mole/volume of injection gas at the target \(T\), fixing the first-contact overall composition \(z^{(1)}\) and the cell pressure \(p\).
3. Equilibrate contact 1. Stir until pressure stabilizes at constant volume (or hold \(p\) constant by piston while two phases settle); allow full gravity separation of the equilibrium vapor and liquid. Record equilibration time.
4. Measure phase volumes and density. Read the equilibrium liquid volume \(V_L\) and vapor volume \(V_V\) (interface position), cell pressure, and, where instrumented, \(\rho_L\) and \(\rho_V\).
5. Sample and analyze both phases. At constant pressure, displace a measured aliquot of the equilibrium vapor and of the equilibrium liquid through the separator to the GC; obtain \(y_i\) and \(x_i\) for contact 1.
6. Set up the next contact. For a forward (vaporizing) sequence, transfer the equilibrium vapor to a fresh charge of original oil; for a backward (condensing) sequence, retain the equilibrium liquid and add a fresh charge of original injection gas. Record the carried-over moles and the fresh charge.
7. Repeat equilibration and measurement. Re-equilibrate the recombined mixture (steps 3 to 5) and record \(p\), \(V_L\), \(V_V\), densities, \(y_i\), \(x_i\) for the new contact.
8. Iterate the contacts. Continue for a sequence of contacts, typically 5 to 15, until either the compositions stop changing (the path has reached a stable equilibrium-gas or equilibrium-oil composition away from criticality) or the two phases converge (tie line collapses, single phase appears).
9. Track the tie-line length and density difference. After each contact compute \(K_i\) and the tie-line length in composition space and the density difference \(\rho_L - \rho_V\); watch for monotonic approach to zero.
10. Determine miscibility for this pressure. Classify the run as immiscible (second phase persists) or miscible (phases merge) at the test pressure.
11. Repeat at other pressures (or enrichments). Re-run the full contact sequence at several pressures bracketing the expected MMP (or several gas enrichments bracketing MME).
12. Locate MMP/MME. From the series, interpolate the pressure (or enrichment) at which the two-phase region just disappears.

Measured parameters↑ Contents

Recorded at every contact \(j\):

• Equilibrium pressure \(p\) and temperature \(T\) (\(T\) constant).
• Equilibrium liquid volume \(V_L\) and vapor volume \(V_V\), hence total volume and the vapor/liquid split.
• Equilibrium vapor composition \(y_i\) and equilibrium liquid composition \(x_i\) for all components (GC), including \(CO_2\), \(N_2\), \(H_2S\), \(C_1\) through \(C_6\), and \(C_{7+}\) fractions/molar mass.
• Phase densities \(\rho_L\) and \(\rho_V\) (where instrumented), and phase color/clarity and number of phases by visual observation.
• Any solid/asphaltene or precipitate onset, and the saturated liquid volume after each gas addition (for swelling).
• Carried-over phase quantity and fresh-charge quantity used to build the next contact (for the mass balance).

Calculated and derived parameters↑ Contents

• Component equilibrium ratios: \(K_i = \dfrac{y_i}{x_i}\), reported per contact and per component.
• Per-contact phase split from flash mass balance: \(L\) and \(V\) from \(\sum_i \dfrac{z_i\,(1 - K_i)}{1 + V\,(K_i - 1)} = 0\) (Rachford-Rice), with \(z_i\) the recombined overall composition.
• Updated liquid composition along the path: \(x_i^{(\text{new})} = F\!\left(x_i^{(\text{old})}, y_i\right)\), the flash result of the recombined feed.
• Swelling factor: \(\text{SF} = \dfrac{V_{\text{sat}}(F)}{V_{\text{oil,original}}}\), where \(F\) is the cumulative injected-gas mole fraction and \(V_{\text{oil,original}}\) is the gas-free oil volume at \(T\).
• Tie-line length: \(L_{\text{tie}} = \sqrt{\sum_i (y_i - x_i)^2}\), tracked toward zero.
• Density difference: \(\Delta\rho = \rho_L - \rho_V\), an independent criticality indicator (\(\Delta\rho\) tends to \(0\) at miscibility).
• Minimum miscibility pressure: \(\text{MMP} = \min\{\, p : L_{\text{tie}} \text{ tends to } 0 \ (\text{equivalently } K_i \text{ tends to } 1) \text{ over the contact sequence} \,\}\). Minimum miscibility enrichment MME defined analogously at fixed \(p\).
• Optional EOS-derived check: critical point of the converged compositional path, used to confirm whether the displacement is vaporizing, condensing, or combined condensing/vaporizing.

Reporting and typical curves↑ Contents

The deliverables are both tabular and graphical:

• Tables of \(x_i\), \(y_i\), \(K_i\), \(V_L\), \(V_V\), \(\rho_L\), \(\rho_V\) per contact, for each test pressure (or enrichment).
• K-value plots: \(\log K_i\) versus contact number for each component; convergence of all curves toward \(K_i = 1\) signals approaching miscibility, and the convergence pressure on a \(\log K_i\) versus \(1/p\) style plot supports the MMP estimate.
• Compositional path on a pseudo-ternary diagram (light/intermediate/heavy pseudo-components) at reservoir \(T\) and \(p\), showing the contact path approaching the two-phase boundary and, at MMP, passing through or tangent to the critical (plait) point. The shape distinguishes a leading-edge (vaporizing) from a trailing-edge (condensing) controlled path.
• Tie-line length and density difference versus contact number, both extrapolated to zero to confirm miscibility.
• Swelling-factor and saturation-pressure versus injected-gas mole fraction (read jointly with the swelling test).
• The defining MMP plot: a measured indicator (residual second-phase volume, tie-line length at the limiting contact, or recovery proxy) versus pressure; the break point where it goes to zero is read as the MMP. For MME, the same indicator versus gas enrichment.

A reader interprets these by looking for the pressure (or enrichment) at which the K-values collapse to unity, the tie line and density difference vanish, and the visual second phase disappears, all of which must be mutually consistent before an MMP is reported.

Practical notes and pitfalls↑ Contents

• MCM gives a thermodynamic, dispersion-free MMP. Because it neglects viscous fingering, gravity override, and dispersion, the MCM/EOS MMP is best cross-checked against a slim-tube test and, for \(CO_2\), against rising-bubble or vanishing-interfacial-tension methods; report the basis used.
• Equilibration time is the most common error source. Insufficient stirring or settling leaves the phases off equilibrium, biasing \(y_i\), \(x_i\) and \(K_i\). Confirm stable pressure and unchanging GC composition on repeat sampling before recording a contact.
• Constant-pressure sampling is essential. Phases must be withdrawn while the piston holds pressure; a pressure drop during sampling re-flashes the sample and corrupts the composition. Withdraw small aliquots and account for the removed moles in the mass balance.
• Representative, fully restored live fluids. Use properly recombined oil at the correct GOR/\(P_{\text{sat}}\) and a correctly metered gas; errors in feed composition propagate through every contact.
• Mercury-free, dead-volume control. Modern variable-volume cells avoid mercury for safety and to prevent amalgam/contamination artifacts; whatever the design, calibrate and subtract all dead volumes (lines, valves, separator) so phase-volume readings are accurate.
• Watch for solids. \(CO_2\) floods can precipitate asphaltenes (and, near the \(CO_2\) triple point at low \(T\), dry ice); these plug ports, foul windows, and invalidate volume readings. A visual cell and asphaltene-onset screening are advised.
• Number of contacts. Too few contacts can miss late convergence (especially in condensing/vaporizing drives where convergence is slow); run enough contacts (often toward the 15 end) to confirm the trend rather than extrapolating prematurely.
• Carry-over bookkeeping. The recombination ratio (\(\alpha\) or \(\beta\)) and the carried-over moles must be logged exactly; an unrecorded volume of carried-over phase breaks the marching mass balance and makes the K-value path uninterpretable.
• Use the MCM data primarily to tune the EOS, then let the tuned EOS interpolate MMP/MME and feed compositional simulation; the lab path and the simulator must reproduce the same measured tie lines.