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.


In 2002, across all of BP, well intervention added nearly as much oil as new wells and has become an important aspect of the business. However, problems and challenges still remain. In the UKCS and Norway alone, there were four cases of stuck pipe involving coiled tubing and hydraulic workover. Three, and possibly all four, of these were as a result of poor hole cleaning. Only one string was recovered without loss of bottomhole assembly (BHA), and the total cost of these events was several million dollars.

Numerous worldwide well intervention operations require viscous sweeps for scale and cement milling, sand cleanouts, and gel for excess cement slurry contamination during squeeze operations. A particular challenge for 2003 was a barite mud solids cleanout from the horizontal section of Well 108 in the 148°C gas condensate Marnock field in the UKCS.

Hole cleaning is known to be particularly troublesome with coil tubing due to the lack of pipe rotation and the difficulty in getting sufficient flow underneath an eccentric pipe.1 Solids suspension and cuttings transport can be correlated to a Low-Shear-Rate Viscosity (LSRV),2 generally measured at 0.06 s−1.

Case History I

BP Ula A-05 has a tapered 7-in./5-in./41/2-in. completion reaching 69° deviation, in a 150°C reservoir. Twenty-three barrels of cement were to be milled following a cement squeeze using an aggressive bit on coiled tubing. Most of the cement was in the 41/2-in. liner, although some extended into the 5-in. tubing. A 33/4-in. bit sized for the 41/2-in. liner was run with 2 lb/bbl welan gel used for viscous sweeps.

During the initial milling, circulating pressure indicated that the motor was clearly being worked with the motor stalling three times. A 10-bbl welan sweep was circulated after 26 m were milled. After 30 m milled, a drilling break was encountered. Lower circulating pressure led the crew to believe that no cement was being milled, and the rate of penetration (ROP) was increased to approximately 32 m/hour for the next nine metres at the end of which the motor suddenly stalled. The string was stuck and no circulation was possible. The BHA could not be pulled free and fishing attempts were unsuccessful.

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