For many years, oil based muds have shown proof of efficiency towards hostile drilling cases especially water – sensitive shales or high temperature wells. However, their use is subjected to more and more constraints due to the fast evolution of environmental legislations. One of the possibilities to avoid this problem while keeping the advantages of oil muds consists in substituting mineral oils by biodegradable vegetable oils such as fatty acid esters.

The esters used in these new oil-based muds have an aerobic biodegradability of more than 85 % after 4 weeks and an anaerobic biodegradability of more than 60 % after 2 months.

An original formulation has been developed, presenting very good properties up to 140°C. Its performances are based on a properly choice of ester quality, emulsifying system and additives types, and on a severe control of lime concentration to avoid any risks of ester hydrolysis in conjunction to pH and temperature.

Its behavior has been studied under bottomhole conditions, using an home made HT/HP flow loop, during aging. Contamination tests demonstrated the ability of ester-based muds to resist to possible contaminations i.e., sea water, drilled solids, cements during the drilling process.

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