Transition Flow

Transition flow (also called churn flow) is a multiphase flow regime in which gas and liquid are mixed in a violently unstable, chaotic pattern with no clear continuous phase. It occurs at gas velocities higher than those that produce slug flow but lower than those that produce annular flow. In slug flow, alternating plugs of liquid and gas move in an organized way through the pipe. In annular flow, gas moves as a fast central core with liquid clinging to the pipe walls. Between these regimes, the liquid slugs begin to break up and the gas core has not yet stabilized, creating the turbulent, chaotic mixture that defines transition flow. The regime is a design problem in gas-lift systems, gas-condensate pipelines, and electrical submersible pump installations because the irregular pressure fluctuations in transition flow create hydraulic loading that is difficult to predict and can destabilize downhole equipment.

Key Takeaways

Multiphase flow regimes in a vertical tube are typically classified as bubble flow (low gas fraction, bubbles dispersed in liquid), slug flow (alternating liquid plugs and gas slugs), churn or transition flow (breakdown of slug structure, chaotic mixing), and annular flow (gas core with liquid film on walls). The transition between these regimes depends on gas velocity, liquid velocity, pipe diameter, and fluid properties.
The Orkiszewski, Hagedorn-Brown, and Beggs-Brill correlations are commonly used flow regime maps that predict whether a given combination of gas and liquid rates will produce slug, transition, or annular flow in a vertical pipe. Flow regime maps were originally developed empirically from laboratory measurements and have been adapted for downhole conditions.
Gas lift design specifically tries to avoid operating an installation in the transition or churn flow regime because the chaotic pressure oscillations can cause the gas lift valve to hunt (open and close irregularly) rather than operate stably.
ESPs operating in wells with high gas-oil ratios can encounter transition flow above the pump, causing intermittent gas slugging that loads and unloads the pump unpredictably. Gas separators installed above the pump intake help by separating free gas before it enters the pump stages.
In horizontal pipes, transition flow has a different character than in vertical pipes because gravity does not act perpendicular to flow. Stratified-wavy flow (where the liquid rides on the bottom with a wavy gas-liquid interface) and slug flow are the dominant horizontal regimes, with churn flow appearing only in near-vertical sections.

What Is Transition Flow?

Pour water into a clear plastic pipe and blow air through it. At low air flow, you get bubbles rising gently through the water. Increase the air flow and the bubbles organize into distinct alternating plugs of liquid and gas: slug flow. Keep increasing the air and the slugs begin to break up. The liquid becomes choppy, the gas core splutters and collapses and re-forms. This chaotic, violent state between organized slugs and a stable gas core is transition flow.

The engineering challenge with transition flow is unpredictability. Slug flow has a regular, if sometimes disruptive, pressure signature. Annular flow is stable and amenable to calculation. Transition flow produces pressure oscillations that are irregular in timing and amplitude, which stresses mechanical equipment and makes flow rate measurement difficult.

In production engineering, the goal is usually to design wells and surface lines so that the actual operating conditions fall clearly in one of the stable regimes (bubble, slug, or annular), and to understand where the boundaries are so that changes in operating conditions (gas rate changes, liquid rate declines, pressure changes) do not push the system into transition unexpectedly.

Fast Facts

The Taitel and Dukler model (1976) is the most widely used theoretical framework for predicting flow regime transitions in vertical and near-vertical multiphase flow. It identifies the physical mechanisms responsible for transitions (Kelvin-Helmholtz instability for slug-to-annular, turbulent disruption of slugs for slug-to-churn) and predicts transition boundaries from dimensionless fluid parameters. Despite being nearly 50 years old, the Taitel-Dukler model remains the reference framework in most multiphase flow simulation programs, including PIPESIM and OLGA, which are the industry-standard multiphase flow simulators used by operators worldwide.

Transition Flow in Producing Wells

In a producing oil well with a gas-oil ratio (GOR) that rises as reservoir pressure declines, the flow regime in the production tubing changes over time. Early in the life of a well with a low GOR, the tubing may be in bubble flow regime. As GOR rises, the well transitions through slug flow to increasingly disordered flow. Wells that were comfortable to produce under natural flow can develop liquid loading (inability to lift liquid to surface) when GOR drops back at later life, or can develop stability problems in the churn flow regime before annular mist flow stabilizes them at high gas rates.

In gas-lift wells on the Norwegian Continental Shelf (Equinor, Aker BP, Vår Energi) and in the Gulf of Mexico (Shell, Chevron, bp), engineers use nodal analysis combined with multiphase flow correlations to predict the operating flow regime and design gas injection rates that keep the well in stable slug or annular flow. Operating in churn flow reduces lift efficiency and can cause valve instability.

In coalbed methane (CBM) wells in Alberta (Horseshoe Canyon, Mannville formations), the produced gas-water mixture in the annulus and tubing passes through different flow regimes as gas production increases and water rate declines over the life of the well. The transition from liquid-dominated to gas-dominated flow involves a period of churn flow that can create unsteady wellhead pressures and production rate swings.

Transition Flow in Gas-Condensate Pipelines

In a subsea or surface pipeline carrying gas with condensed liquids, the flow regime depends on the gas velocity, condensate rate, pipe diameter, and terrain profile. In a long, relatively level pipeline, stratified flow (liquid on the bottom, gas on top) transitions to slug flow at lower gas velocities and to annular mist at high gas velocities. Terrain-induced slugging occurs where the pipeline dips downward and then rises again: liquid accumulates in the low point, builds into a slug, and surges to the slug catcher at the receiving facility.

Designing a slug catcher to handle the volume of a single terrain-induced slug, which can be several hundred cubic metres in large-diameter lines, requires accurate flow regime prediction. Undersizing the slug catcher leads to liquid carryover into the gas processing facility, damaging compressors and separation equipment. Transition flow instabilities make slug size prediction uncertain, which is why designers apply conservative margins.

Transition flow is also called churn flow or churn-turbulent flow. Related terms include slug flow (a multiphase flow regime in which alternating elongated plugs of liquid and gas move through the pipe; more organized than transition flow but responsible for significant pressure surges and liquid carryover at receiving facilities), annular flow (a multiphase flow regime in which gas travels as a fast central core and liquid moves as a film on the pipe walls; the highest-gas-velocity stable regime; typical of high-GOR wells in late life), gas-liquid ratio (GLR, the ratio of gas volume to liquid volume in a multiphase stream; the GLR is the primary factor determining which flow regime will occur at a given set of flow conditions), nodal analysis (a reservoir and wellbore engineering method that balances inflow and outflow performance to determine the natural flow rate of a well; predicts which tubing flow regime applies at the expected operating point), and liquid loading (a condition in a gas well where gas velocity has fallen below the minimum needed to continuously carry liquid droplets to surface, causing liquid to accumulate in the wellbore and further reduce production; associated with the transition from annular to slug or churn flow).

How Transition Flow Caused Unstable Production From a North Sea Gas-Condensate Well

An operator on the Norwegian Continental Shelf had a producing gas-condensate well in the Åsgard field in the Norwegian Sea. The well produced through 4.5-inch tubing at 8 million standard cubic metres per day (MMscm/d) with a condensate rate of 450 cubic metres per day. A nodal analysis at initial conditions placed the tubing flow regime firmly in annular mist flow, with stable production and predictable wellhead pressure.

As reservoir pressure declined over three years, the total production rate dropped to 5.5 MMscm/d gas and 320 m³/d condensate. A repeat nodal analysis showed the tubing was now operating near the transition between slug and churn flow at current conditions. Wellhead pressure began showing oscillations of 0.8 to 1.2 MPa over 20 to 40-minute cycles, consistent with chaotic slug behavior. The compressor at the host platform was designed for a steady inlet pressure within a 0.3 MPa band; the 1 MPa oscillations caused repeated compressor surge trips.

The operator installed a gas lift system at 2,200 metres depth, injecting 0.4 MMscm/d of lift gas to increase tubing velocity and push the flow regime back into stable annular flow. The oscillations stopped and compressor trips fell to zero. Compressor surge trips had been costing approximately NOK 180,000 per event in downtime and maintenance. With four to six surge trips per month, eliminating them recovered the cost of the gas lift installation in under eight months.