Gas lift wells in the Clyde field have a history of problems, described in this paper.

Due to high temperatures (300 degrees F), the use of corrosion inhibitors and asphaltene dissolvers, it was necessary to use gas lift valves containing high temperature elastomers. These were not always effective at maintaining the nitrogen charge at the correct pressure which resulted in unloading valves opening intermittently causing unstable flow.

Valve failures were not the only for reason unstable flow, other causes included:

  • the use of an orifice valve with a port size which was too large;

  • opening of unloading valves because tubing pressures were higher than expected.

The latter effect occurred because the multiphase pressure drop correlation, while reliably predicting bottom hole pressure, was less accurate in predicting pressures at the depth of the unloading valve. When the tubing pressure profiles were measured with gauges a further problem became apparent. The pressure at a given depth did not remain constant but oscillated about a mean due to normal slug flow. If the opening pressure of an unloading valve is set too close to the tubing pressure, these oscillations can cause unloading valves to open unexpectedly.

These and other problems have been corrected. Firstly, spring loaded valves with factory sealed bellows have been installed. The elastomers’ integrity in these valves are less critical for reliable valve operation. In addition the lack of temperature sensitivity of their opening and closing mechanism makes unloading more reliable. Secondly, a more conservative gas lift design policy is now used, so that despite fluctuating or uncertain tubing pressures, or slight changes in the valve set pressures, unloading valves do not open.

Finally, to ensure that planned gas lift designs are stable, they were also checked using a transient multiphase flow simulator specially constructed to model gas lift wells. The simulator appears effective at predicting when stability problems can be expected.

The paper also comments on corrosion in the gas lift wells, a problem that was anticipated. Calliper surveys and other data suggest that it is unlikely to result in workovers in the near future.

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