Well interventions in depleted gas wells are often seen as not being economically viable. Since the production rates in these wells are generally low, the pay-back time on the cost of a coiled tubing (CT) intervention could be rather long, making cash flow and earnings not attractive for gas producing companies. Furthermore, due to the low reservoir pressure, introducing fluids in depleted gas wells may cause a water block, formation damage and liquid loading, which can kill the well. Small diameter Coiled Tubing, often called Mini Coil, can overcome these problems, making interventions in depleted gas wells both economical and feasible.

Mini Coil operations with ¾″ coiled tubing were re-introduced in Europe, after 25 years of absence, to offer cost-effective intervention methods for depleted gas wells. Typical Mini Coil operations include small acid treatments, salt plug washes, fluid lifting, inhibition of tubing, etc. For clean outs in wells with a coiled tubing velocity string completion or on offshore installations, where crane limitations do not allow for a standard coiled tubing reel to be used, Mini Coil can be an useful option1 .

In this paper the design criteria, applications and the limitations of Mini Coil operations are discussed. Equipment design features and principles for the choice of coiled tubing size and material are presented. Candidate well selection criteria and the benefits of these types of operations are illustrated by case histories in the Netherlands and Germany.

Depleted gas wells, which might not be considered for any further interventions, can now be treated economically again. Mini Coil allows for the use of a compact equipment spread, resulting in cost savings through faster rig-up and rig-down times, smaller footprints, fewer personnel required and fewer truck loads. The small internal volume of the Mini Coil increases the control over the amount of fluid injected into a well, thereby reducing the risk of killing the well during a treatment.

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