This paper describes an investigation into the characteristics of various biopolymer and mixed metal oxide fluids for potential application in well intervention operations. Poor hole cleaning is experienced on many well intervention jobs with coiled tubing and even with hydraulic workover units because of lower circulation rates compared with conventional drilling operations. This can lead to stuck pipe (illustrated by two case histories) and is an expensive problem worldwide. Currently used products are poorly characterised in terms of their viscosity at elevated temperature and at the low shear rates encountered under the pipe in deviated, eccentric annuli.

The investigation involved rheological measurements on xanthan, welan, diutan and scleroglucan polymers on a Bohlin rheometer at shear rates of 0.001–1000 s-1 and temperatures 50-175°C. Two mixed metal oxide fluids were also tested as a comparison. The fluids were tested on a Chan rheometer (with identical geometry to the common Fann 35) to investigate correlations.

These experiments were complimented by hydraulics analysis and by tests to determine the resistance of the various fluids to contamination by cement. A simple suspension test was also performed using material simulating common cuttings produced in well intervention operations.

We also describe the results of treatments designed to break the viscosity of the various fluids prior to flow through the production separators. Finally, a case history is presented of a successful well intervention operation on a high-temperature well using viscous mixed metal oxide pills.

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