Swelling / Mixing Test (Gas Injection)

PVT Analysis of Reservoir Fluids › 5. Swelling / Mixing Test (Gas Injection into Reservoir Oil)

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

Purpose↑ Contents

The swelling/mixing test answers the central question of any gas-injection enhanced oil recovery (EOR) project: how does a given injection gas (\(\text{CO}_2\), \(\text{CH}_4\), \(\text{N}_2\), separator gas, or an engineered blend) interact with the reservoir oil, and at what pressure does it become miscible with that oil at reservoir temperature? Concretely, the test quantifies:

• How much injection gas the oil will dissolve at successive pressures (gas solubility, expressed as added gas-oil ratio).
• How much the oil expands (swells) as it takes up that gas, which governs the residual oil saturation that can be displaced and the in-situ volume the displaced oil will occupy.
• How the saturation pressure of the oil-plus-gas mixture moves as more gas is added, which is the diagnostic for the appearance of a second equilibrium phase.
• The minimum miscibility pressure (MMP) or, for a fixed gas, the minimum miscibility enrichment (MME), the threshold above which the gas and oil mix in all proportions with no interfacial tension.
• Whether a near-critical or asphaltene-precipitation hazard exists at the planned injection composition.

These results feed directly into compositional reservoir simulation (equation-of-state tuning), injectant selection, and the choice between immiscible, multi-contact-miscible, and first-contact-miscible flood design.

Physical principle↑ Contents

When injection gas contacts undersaturated or saturated oil at reservoir temperature \(T\), components partition between the liquid and any coexisting vapor according to phase equilibrium. At equilibrium the fugacity of each component \(i\) is equal in both phases, \(f_i^L = f_i^V\), which is equivalent to the K-value relation

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

where \(y_i\) is the mole fraction of component \(i\) in the vapor (gas-rich) phase and \(x_i\) is its mole fraction in the liquid (oil-rich) phase. As pressure increases at fixed composition, the lighter injection components dissolve preferentially into the oil; their dissolution lowers the molar volume penalty of the mixture relative to a free gas phase, and the liquid expands. The fractional expansion is captured by the swelling factor

$$SF = \frac{V_{oil}(gas)}{V_{oil}(initial)}$$

where \(V_{oil}(gas)\) is the saturated liquid volume of the oil after it has absorbed the added gas at the test pressure and temperature, and \(V_{oil}(initial)\) is the volume of the original (gas-free or original-GOR) oil at the same temperature, usually referenced at its own bubble point or at a stated datum pressure. \(SF\) is dimensionless and is typically greater than 1 for \(\text{CO}_2\) and rich hydrocarbon gases (strong swelling) and closer to 1, or even less than 1 after shrinkage of light ends stripping, for lean \(\text{N}_2\).

The incremental dissolved gas is reported as an added solution gas-oil ratio,

$$R_{s,mix} = \frac{V_{gas,dissolved}}{V_{oil}}$$

with \(V_{gas,dissolved}\) measured at standard conditions (sc), so \(R_{s,mix}\) carries units of \(\text{sm}^3/\text{sm}^3\) or \(\text{scf}/\text{STB}\). The density of the swollen, saturated liquid follows from mass conservation,

$$\rho_{mix} = \frac{m}{V_{oil}(gas)}$$

where \(m\) is the total mass of oil plus dissolved gas in the cell and \(V_{oil}(gas)\) is the corresponding saturated liquid volume; \(\rho_{mix}\) is reported in \(\text{kg/m}^3\) or \(\text{g/cm}^3\).

The thermodynamic state of the mixture is described by the gas deviation (compressibility) factor of any free gas phase,

$$Z = \frac{pV}{nRT}$$

with \(p\) the absolute pressure, \(V\) the gas-phase volume, \(n\) the moles of gas, \(R\) the universal gas constant (\(8.314\ \text{J/mol·K}\)), and \(T\) the absolute temperature; \(Z\) is dimensionless and is needed to convert measured vapor volumes to standard-condition gas volumes.

Miscibility is identified through the saturation-pressure locus. As gas is added in successive batches, the mixture saturation pressure \(p_{sat}\) first rises (bubble-point branch) and, for \(\text{CO}_2\) and rich gases, eventually passes through a maximum and turns over (dew-point branch) as the mixture moves toward and around the critical point of the oil-gas system. The minimum miscibility pressure is the pressure above which a single phase exists for the relevant contact path. For the slim-tube or interfacial-tension definition, MMP is the pressure at which interfacial tension between the contacting phases vanishes; in the swelling test it is bracketed by the pressure at which the second phase disappears at the critical mixing composition. For interfacial-tension confirmation by pendant or rising drop, the apparent density used in tension reduction relates to the geometry through

$$\mu = K \cdot t \cdot (\rho_b - \rho_f)$$

where here the grouping \((\rho_b - \rho_f)\) is the density contrast between bulk and drop fluid and \(K\), \(t\) are the apparatus calibration constant and a shape/time factor; in IFT practice the working relation is \(\sigma = \dfrac{(\rho_b - \rho_f)\, g\, R_0^2}{\beta}\), where \(\sigma\) is interfacial tension (\(\text{mN/m}\)), \(g\) is gravitational acceleration, \(R_0\) is the drop radius of curvature at the apex, and \(\beta\) is the dimensionless Bond shape factor. MMP is the pressure at which \(\sigma\) extrapolates to zero.

Apparatus and conditions↑ Contents

The test is run in a high-pressure, high-temperature visual PVT cell, almost always a mercury-free, variable-volume cell with a moving piston driven by a positive-displacement pump and an isolating fluid (hydraulic oil) behind the piston. A sapphire window or full visual barrel allows direct observation of meniscus position, phase boundaries, opalescence near the critical point, and any solid (asphaltene/wax) deposition. The cell is held in an air bath or jacket at the fixed reservoir temperature \(T\) (held constant for the entire test). A magnetic stirrer or rocking mechanism accelerates mass transfer between gas and oil. Pressure is generated and measured by the servo pump with a calibrated quartz or strain-gauge transducer; cell volume is read from the calibrated piston displacement. Auxiliary equipment includes an injection-gas accumulator with precise volumetric metering, a gasometer or wet-test meter for flashed-gas volumes, a densitometer (vibrating-tube, DMA-type) for liquid and gas density, and a gas chromatograph for compositional analysis of injectant and equilibrium phases.

In the test, temperature is held constant; injection-gas cumulative composition is the primary variable (stepped up batch by batch); and at each composition the pressure is varied to locate the mixture saturation pressure. Thus each gas-addition level produces one point on the saturation-pressure-versus-composition swelling curve.

Procedure↑ Contents

1. Charge the cell with a known mass and volume of representative, recombined live oil at reservoir temperature, and establish its original bubble point as the baseline; record \(V_{oil}(initial)\) and the baseline density.
2. Measure and record the exact composition of the injection gas by GC, and load it into a calibrated, temperature-controlled accumulator at known \(p\), \(V\), \(T\) so that injected moles can be computed.
3. Inject the first metered batch of gas into the oil cell, recording the moles (or standard volume) added and the new overall mixture composition.
4. Pressurize the cell above the expected mixture saturation pressure to force the gas fully into solution (single liquid phase), then stir or rock until pressure and meniscus position stabilize, confirming thermodynamic equilibrium (no further pressure decay at fixed volume over a hold period).
5. Perform a constant-composition (pressure-depletion) expansion on this mixture: step the pressure down in increments, recording cell volume at each step, until a second phase first appears. The pressure at first phase appearance is the mixture saturation pressure \(p_{sat}\) for this composition.
6. At \(p_{sat}\) record the saturated liquid volume \(V_{oil}(gas)\), compute the swelling factor, and sample/measure the saturated liquid density.
7. If a vapor phase is present, equilibrate, then measure equilibrium gas and liquid volumes and sample each phase for GC to obtain \(x_i\) and \(y_i\) and hence \(K_i\).
8. Repeat steps 3 to 7 for several successive gas-addition levels, increasing the cumulative dissolved gas (and thus moving along the composition axis).
9. Track the saturation-pressure locus as gas is added: note where the branch changes from bubble-point to dew-point behavior and where the curve passes through its maximum near the critical mixing composition.
10. For each level near the inferred critical composition, observe the cell for critical opalescence, vanishing density contrast, and disappearance of the meniscus to bracket the miscibility condition.
11. Throughout, inspect the cell for asphaltene flocculation or wax onset that could compromise injectivity; record the pressure/composition at which any solids appear.
12. On completion, flash a final sample to standard conditions to confirm overall material balance against the metered gas additions.

Measured parameters↑ Contents

At each gas-addition level and pressure step the following are recorded directly:

• Cumulative moles or standard volume of injection gas added.
• Cell pressure \(p\) at every step, and the mixture saturation pressure \(p_{sat}\) at first phase appearance.
• Cell (liquid) volume \(V\) at each pressure, and the saturated liquid volume \(V_{oil}(gas)\) at \(p_{sat}\).
• Equilibrium liquid and vapor volumes when two phases coexist.
• Liquid density \(\rho_{mix}\) (and gas density if applicable) by vibrating-tube densitometer.
• Compositions \(x_i\) and \(y_i\) of equilibrium liquid and vapor by GC.
• Visual observations: meniscus position, phase boundary, critical opalescence, solids onset.
• Cell temperature (held constant; logged for QC).

Calculated and derived parameters↑ Contents

• Swelling factor: \(SF = \dfrac{V_{oil}(gas)}{V_{oil}(initial)}\), dimensionless.
• Added/total solution gas-oil ratio: \(R_{s,mix} = \dfrac{V_{gas,dissolved}}{V_{oil}}\), in \(\text{sm}^3/\text{sm}^3\) or \(\text{scf}/\text{STB}\); cumulative across additions.
• Swollen liquid density: \(\rho_{mix} = \dfrac{m}{V_{oil}(gas)}\), in \(\text{kg/m}^3\), with \(m\) from material balance of oil plus all added gas.
• Equilibrium K-values: \(K_i = \dfrac{y_i}{x_i}\) for each component, dimensionless, used to tune the EOS.
• Gas-phase Z-factor: \(Z = \dfrac{pV}{nRT}\), to convert measured vapor volumes to standard volumes for the GOR and material balance.
• Mixture critical composition and saturation-pressure maximum: read from the \(p_{sat}\)-versus-composition curve.
• Minimum miscibility pressure (MMP) or minimum miscibility enrichment (MME): inferred from the pressure/composition at which the second phase vanishes at the critical mixing composition, and confirmed where available against interfacial-tension extrapolation \(\sigma\) to \(0\) or a companion slim-tube test.
• Isothermal compressibility of the single-phase swollen oil above \(p_{sat}\), \(c_o = -\dfrac{1}{V}\left(\dfrac{\partial V}{\partial p}\right)_T\), from the constant-composition expansion slope.

Reporting and typical curves↑ Contents

The principal deliverables are:

• Swelling-factor curve: \(SF\) plotted against either saturation pressure or mole fraction of injected gas in the mixture. A monotonically rising curve indicates progressive dissolution and expansion; for \(\text{CO}_2\) it can reach values well above unity. The slope shows how aggressively the gas swells the oil.
• Saturation-pressure curve: \(p_{sat}\) versus overall injected-gas mole fraction. For \(\text{CO}_2\) and rich gases this curve rises, peaks, and turns over; the peak abscissa is the critical mixing composition and the peak/turnover region brackets miscibility. For lean \(\text{N}_2\) it tends to rise steeply and monotonically (poor miscibility, high MMP).
• Solution-GOR curve: \(R_{s,mix}\) versus pressure, quantifying gas uptake.
• Density curve: \(\rho_{mix}\) versus pressure or composition, showing whether the oil-gas system is approaching equal phase densities (a near-critical/miscibility indicator).
• K-value tables and plots versus pressure for EOS regression.
• An MMP/MME determination with supporting visual and, where run, IFT data.

Engineers read these plots together: a high swelling factor with a saturation-pressure peak occurring below reservoir pressure signals achievable multi-contact miscibility; a saturation curve still climbing at reservoir pressure signals an immiscible or only partially miscible displacement and a need for gas enrichment or higher pressure.

Practical notes and pitfalls↑ Contents

• Use a genuinely representative live oil at the correct original GOR; using stock-tank or under-recombined oil shifts the saturation locus and corrupts MMP.
• Meter the injection gas with high accuracy and verify its composition by GC before each campaign; small errors in injected moles propagate directly into \(SF\), \(R_{s,mix}\), and the composition axis of every curve.
• Allow full equilibration. Gas-into-oil mass transfer is slow; rely on stirring/rocking and confirm by a stable pressure plateau at fixed volume rather than a fixed clock time. Premature readings overstate \(p_{sat}\) and understate solubility.
• Prefer mercury-free, fully visual cells. Mercury cells are being phased out for safety and because they hide solids deposition; direct observation is essential for catching critical opalescence and asphaltene onset.
• Account for dead volume in lines, valves, and the densitometer loop in every material balance; uncorrected dead volume biases \(SF\) and density.
• Near the critical mixing point, phase densities and the meniscus become indistinct; do not over-interpret a single ambiguous reading, bracket the miscibility condition with closely spaced compositions and confirm with IFT or slim-tube where the project warrants.
• Watch for asphaltene precipitation, especially with \(\text{CO}_2\), which can occur inside the miscibility window and is invisible in a non-visual cell; record onset pressure/composition because it can dominate field injectivity decisions.
• Maintain strict isothermal control. Because every saturation point is read at fixed \(T\), even small temperature drift moves \(p_{sat}\) and distorts the swelling and saturation-pressure curves.
• Cross-check the final flash material balance against cumulative metered gas; a closing error flags a leak, an unequilibrated step, or a dead-volume miscount before the data are passed to EOS tuning.