When scale problems occur into the tubings, without hardly affecting the well productivity, it can be interesting to find new downhole injection means, representing an alternative to classical squeezes.

On some wells of block 3 in Angola, the squeezes turned out difficult because of the reservoir heterogeneity and temperature (164°C). The reservoir being dolomitic most of the inhibitors show a strong incompatibility and precipitate when injected. The operator of these fields preferred to turn to an inhibitor injection via the gas lift system.

This injection is made possible, because scales, made by calcium carbonate and barium sulphate, do not affect the well bottom but only the tubing. The gas lift valves are located under the area where scale begins to form and the inhibitor injection through the gas lift system permits the prevention of scale precipitation into the wells without having to perform squeeze treatments.

Injection via the gas lift began with standard inhibitors dissolved in water to which VPD (vapor pressure depressants) were added.

In spite of the presence of VPD, water evaporation caused the formation of gums and a rapid clogging of the gas-lift valves. Therefore a solid scale inhibitor was used and diluted in solvents with a very high boiling point, so that it would not evaporate at well temperature, i.e. 164 °C.

The case history we are presenting, describes the design of the scale inhibitor, the injection and the follow-up which have been carried out in Angola on 11 wells in Block 3 fields.

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