This paper presents the development and field trial of a novel biodegradable invert emulsion drilling fluid.

Responding to environmental concerns and legislation restricting the discharge of mineral oil base drilling fluids, research into biodegradable alternatives commenced in the mid 1980's. Laboratory testing found that some complex esters form stable invert emulsion fluids and are readily biodegradable in both aerobic and anaerobic environments.

After reviewing the laboratory and environmental data, the Norwegian Government agencies (SFT and NPD) granted their approval for a limited field trial of the new ester mud system. The Ester mud system was successfully used in an Ula Field appraisal well. Hole conditions and drilling rates were comparable to nearby wells drilled with conventional Oil Based Mud. Compared to an offset well drilled with a water base mud, normalised drilling time savings of 13.2 days were realised over two hole sections.

Toxicity levels on new and used mud samples have been well within Norwegian government statutorylimits for drilling muds. An independent laboratory biodegradation test on used field mud found it81% degraded after 28 days, allowing the ester mud to be classed as "readily biodegradable" by European Community (EC) protocol.

To assess the environmental impact, a long-term seabed survey programme at the wellsite is ongoing.

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