slim-tube test, Oil & Gas Glossary

What Is a Slim Tube Test?

A slim tube test is a laboratory procedure used to determine the minimum miscibility pressure (MMP) of a gas injection enhanced oil recovery scheme by displacing reservoir crude oil through a small-diameter coiled tube packed with sand or glass beads at reservoir temperature while progressively increasing injection pressure, with the MMP defined as the pressure at which the displacement efficiency reaches approximately 94% of oil in place, above which the injected gas and reservoir oil achieve first-contact or multiple-contact miscibility.

Key Takeaways

Minimum miscibility pressure (MMP) is the critical design parameter for CO2, hydrocarbon gas, and nitrogen EOR flood design.
The slim tube simulates 1D miscible displacement; recovery above 94% OOIP at or above MMP confirms miscibility.
Slim tubes are typically 12-18 metres of 6-7 mm ID coiled stainless steel tubing packed with 100-200 mesh sand.
Multiple slim tube tests at different pressures generate a recovery-pressure curve; MMP is the knee of this curve.
MMP depends on reservoir temperature, crude oil composition, and injected gas composition; each combination requires its own test.

How the Slim Tube Test Measures Minimum Miscibility Pressure

The slim tube apparatus consists of 12-18 metres of small-diameter (typically 6.35 mm or 1/4-inch OD) coiled stainless steel tubing packed with clean fine sand or glass beads. The tube is coiled to fit inside a thermostatted bath or oven that maintains reservoir temperature throughout the test. The tube is initially saturated with dead crude oil (reservoir oil without dissolved gas) or recombined live crude under reservoir pressure. Injection gas (CO2, enriched hydrocarbon gas, nitrogen, or natural gas) is then injected at a controlled pressure from one end of the tube, and produced fluid is collected and metered at the other end. The displacement efficiency — the fraction of oil displaced from the tube at 1.2 pore volumes of gas injection — is measured as the primary test result.

The test is repeated at multiple injection pressures, typically five to seven pressures bracketing the expected MMP range. At pressures below the MMP, the displacement efficiency is limited (typically 50-80%) because the injected gas and oil remain as two immiscible phases, and significant oil is bypassed or trapped behind the gas front. As injection pressure approaches the MMP, the displacement efficiency increases sharply as the gas-oil interface effectively disappears and the gas sweeps progressively more efficiently through the tube. At and above the MMP, displacement efficiency plateaus at 94-99% of oil in place. The inflection point of the recovery-versus-pressure curve is defined as the MMP. This pressure is then compared to the reservoir pressure and the expected injection pressure range to determine whether miscible injection is achievable in the field.

Slim Tube Test Applications Across International Jurisdictions

In Canada, slim tube MMP determination is required for any CO2 EOR or hydrocarbon gas injection miscible flood scheme proposed to the AER under an enhanced recovery scheme application per Directive 065. The laboratory MMP from slim tube testing is compared to reservoir pressure to confirm that miscible injection is achievable at the proposed injection wellhead pressure and reservoir depth. Pembina Cardium CO2 EOR pilots in Alberta used slim tube tests to confirm MMP for the Cardium light oil at approximately 10-14 MPa, achievable at the shallow Cardium depths of 1,500-1,800 metres with surface injection pressures of 12-16 MPa. Weyburn Unit CO2 flood in Saskatchewan — one of the largest CO2 EOR projects in North America — used slim tube MMP determination as part of its design basis.

In the United States, the DOE's National Energy Technology Laboratory (NETL) has supported slim tube MMP research for CO2 EOR in the Permian Basin, where Wolfcamp and Delaware Basin light oils have MMPs of 14-25 MPa for CO2 injection, achievable in Permian reservoirs at 1,500-2,500 metres depth. BSEE does not specifically require slim tube testing for OCS enhanced recovery schemes, but operators document the technical basis for their EOR design in field development plans. In Norway, Equinor has conducted slim tube MMP studies for potential CO2 EOR applications in Ekofisk and Valhall chalk reservoirs, where the chalky crude's composition and temperature determine whether CO2 injection at attainable pressures can achieve miscibility. In the Middle East, Saudi Aramco's EXPEC research programme has used slim tube testing to evaluate hydrocarbon gas cycling and CO2 injection schemes for Arab Formation carbonate reservoirs at SHAYBAH and other locations.

Fast Facts

CO2 typically achieves first-contact miscibility with light crude oil (API gravity above 35°) at pressures of 10-20 MPa (1,450-2,900 psi) at reservoir temperatures of 50-80°C. Heavier crude oils (API 25-35°) require higher MMP values (18-30 MPa) for CO2, and very heavy oils may not achieve miscibility with CO2 at any practical reservoir pressure. The MMP decreases as reservoir temperature decreases and as the CO2 purity increases (contaminants like N2 and CH4 raise the MMP). For a given crude oil, the MMP with pure CO2 is typically 30-50% lower than with lean natural gas injection, which explains why CO2 EOR achieves better incremental recovery than natural gas injection in most light oil applications.

Slim Tube Test Limitations and Alternatives

The slim tube test has well-known limitations as a predictor of field-scale miscible flood performance. The test simulates 1D piston-like displacement at laboratory scale, without the gravity, viscous fingering, reservoir heterogeneity, or capillary effects that control actual field-scale sweep efficiency. Field recovery factors in miscible floods rarely achieve the 94%+ efficiencies measured in slim tube tests because gravity override (gas rising to the top of the reservoir), viscous instability (gas channelling through high-permeability streaks), and reservoir heterogeneity all reduce the volumetric sweep. The slim tube MMP is nevertheless used as the design target for injection pressure because achieving miscibility at the contact scale is a necessary (though not sufficient) condition for good miscible flood recovery. Alternative MMP determination methods include the rising bubble apparatus (RBA), which tests phase behaviour visually by observing CO2 bubbles rising through crude oil at different pressures, and equation-of-state compositional simulation, which predicts MMP from fluid phase behaviour calculations without laboratory measurements.

Tip: When using slim tube test results to set the injection pressure target for a field-scale miscible flood, add a safety margin of 1-2 MPa above the slim tube MMP to account for uncertainties in crude oil composition variation across the field, impurities in the injection gas (methane or nitrogen reduce CO2 purity and raise effective MMP), and measurement uncertainty in the slim tube procedure itself. Running slim tube tests with both the target reservoir crude oil and with cruder oil samples from adjacent areas of the field helps identify whether MMP varies significantly across the flood target and whether a single injection pressure will achieve miscibility throughout the pattern.

Slim tube test is also referenced as:

Slim tube displacement test — the fully descriptive name used in formal EOR technical reports and regulatory submissions; used when the test method needs to be distinguished from other MMP determination methods
MMP test — shorthand used in EOR project documentation and reservoir engineering discussions, focusing on the output (MMP) rather than the apparatus; "the MMP from slim tube testing" is a standard phrasing
Miscibility test — the broadest category encompassing slim tube, rising bubble apparatus, and EOS calculations; used when the specific method has not been specified or when results from multiple methods are being compared

Frequently Asked Questions

What does it mean for injected gas to be "miscible" with reservoir oil?

Two fluids are miscible if they mix in all proportions without forming two distinct phases. When injected CO2 or enriched hydrocarbon gas is miscible with reservoir oil, the interfacial tension between the gas and oil drops to zero — there is effectively only one fluid phase rather than a separate gas and oil phase. With zero interfacial tension, there is no capillary trapping of oil by the invading gas; the gas sweeps all of the oil it contacts without leaving residual oil behind. This is why miscible displacement achieves near-100% recovery in the slim tube (where the geometry ensures good contact) and significantly higher recoveries than immiscible displacement in field applications. First-contact miscibility occurs when the gas and oil are immediately miscible at the injection point; multiple-contact miscibility develops over several contact and equilibration steps as components transfer between the gas and oil phases progressively enriching the gas until it becomes miscible.

Can the slim tube MMP be replaced by equation-of-state calculations?

Equation-of-state (EOS) compositional simulation can predict MMP from fluid composition data and thermodynamic correlations without conducting a physical slim tube test, and the two methods agree within 5-15% in most cases when the EOS is properly tuned to PVT data from the specific reservoir fluid. EOS MMP predictions are faster and less expensive than slim tube tests and are routinely used for screening multiple gas injection candidates, sensitivity analyses, and scoping studies. However, slim tube tests remain the standard for final design confirmation because EOS predictions are only as good as the PVT data and tuning quality used to generate them, and slim tube tests capture phase behaviour nuances (near-critical phenomena, asphaltene precipitation) that EOS models may not accurately represent. For major capital investment decisions like full-field miscible flood development, both EOS predictions and slim tube confirmation tests are typically performed.

Why Slim Tube Tests Matter in Oil and Gas

CO2 enhanced oil recovery is projected to play a major role in recovering the approximately 60-70% of oil remaining in conventional reservoirs after primary and waterflood production. The economic viability of CO2 EOR depends entirely on whether the injection pressure achievable at a given reservoir depth can exceed the MMP for the specific crude oil-CO2 system. A slim tube test that confirms MMP below reservoir pressure validates the technical feasibility of the miscible flood design; a test that shows MMP above achievable injection pressure rules out miscible CO2 flooding for that reservoir. This laboratory measurement — conducted on a coiled tube barely thicker than a finger and costing USD 20,000-50,000 — determines whether a USD 100 million or more field development investment in CO2 supply infrastructure, injection wells, and surface facilities is technically justified. Few laboratory measurements in oil and gas engineering have a higher return on investment than the slim tube MMP test that correctly scopes a miscible flood project.

Slim Tube Test: Definition, Minimum Miscibility Pressure, and EOR Design