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 140C. 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.
Oil-based muds are among the best performant and cost effective fluids in hostile conditions. They are used in particular when drilling water-sensitive shale, or when are encountered high temperatures, risk of important differential pressure sticking, exposure to acid gas, or long directionnal intervals requiring minimum torque and drag on the drill string. Even though they may be two or three times more expensive than water based muds, their use is justified by better performances and savings on mud maintenance.
However their intensive use make mineral oil based muds an important source of pollution. During the last decade, environmental regulations have severely restricted the use of mineral oil based muds, mainly because of their impact in marine environment during offshore operations (1-6).
All these general environment concerns have led to an extensive industrial research aiming at designing non toxic substitution fluids that could replace mineral oil based muds but have the same performances in a wide range of drilling conditions.
Low toxicity mineral oils containing substantially lower concentrations of aromatic or naphtenics were used to replace diesel as the base fluid in these muds. However, the legislation has become more and more restrictive and even these classical low toxic fluids like kerosene have been contested and will progressively leave the market under the pressure of environmental requirements.