Study group report 2007: slug flow (Norsk Hydro)
This is the final report on the problem of gas entrainment at a propagating slug front in oil pipelines, brought to ESGI59 by Norsk Hydro. Click on the link at the bottom to download the full report as a pdf document.

Report authors
Melvin Brown (Industrial Mathematics KTN)
Paul Dellar (Imperial College London)

Executive summary
The growing world deficit between oil discovery and consumption makes optimising production from existing fields increasingly important. When oil is produced, the reservoir pressure decreases and the oil flow rate decreases in proportion to the decreasing pressure difference between the reservoir and the processing facility. At low oil flow rates, a well becomes unstable and this leads to reduced production and processing problems. The formation of slug flow in pipelines, particularly at bends in the vertical plane is a manifestation of such instability.

Figure 1: Two-phase flow regimes.

The incidence of different flow patterns depends on the relative flow rates of each component. The characteristics of slug flow are intermittency and gas entrainment at the front of the propagating slug.

Figure 2: Two-phase flow map.

In this case multi-phase fluid slugs may travel at velocities of up to 2 - 3 ms^-1 along the inside of a pipe, each filling the full cross-section over a finite length and each bounded before and after by stratified flows. There is a strong dependence on initial conditions, and on pipe inclination, as shown in figure 3. In this context, the total flow comprises a mixture of oil, water, gas and solid particles; it is almost always fully turbulent with complex free boundaries and subject to multi-scale effects. The presence of slug flow at a given point along a pipeline may be manifested by apparently random variations, with respect to time, of pressure (188 - 196 bar), gas volume flow rate (1 - 20 Sm^3 hr^-1), and oil volume flow rate (0 - 80 Sm^3 hr^-1), where the exemplified numerical ranges are from figure 3. Large slugs are difficult to handle and potentially damaging for processing units.

Figure 3: Transients for a bend in the vertical plane.

The front of a slug may be regarded as a propagating, continuously breaking wave, which continuously entrains gas. The amount of gas entrainment has a large impact on flow pattern, pressure drop, and the slug length and propagation velocity, each of which affect decisions about how to optimise production. Hence the interest of Hydro in gaining better understanding of gas entrainment in the slug flow regime. In particular, the Study Group was asked to

• explore alternative or new ways to treat the entrainment problem;
• improve understanding of physical processess governing entrainment;
• suggest models for the various phenomena.

Click on the link below to view the full report.

Download 'NorskHydro-SlugFlow.pdf' (767 Kb).