Conventional drilling fluids such as diesel and mineral oil have posed some environmental and health challenges in their drilling applications but the introduction of synthetic-base fluids over the past two decades has considerably reduced such challenges. In some cases, a bottom hole temperature above 300°F (150°C) can cause significant instability in the rheological properties of these drilling fluids. Vegetable oils or pseudo oils are known to be environmentally friendly drilling fluids, but have not received much attention because of their instability in High Pressure Temperature High (HPHT) environments. The antioxidant potentials of Citric Acid (CA), Red Onion Skin Extract (ROSE) and Propyl Gallate (PG) on the oxidative stability of seven vegetable oils were examined. The additives (antioxidants) were able to protect the stability of the oils up to 250°C which is beyond the range for the 1st tier of HPHT environment (150 - 205°C). Though it was also observed that the peroxide values (PVs) of the oils as temperature increase also increase, it did not follow and defined pattern (no pattern was established). The applications of combined antioxidants improved the stability of the oil samples when compared with using individual antioxidants. The applications of appropriate antioxidants in local vegetables oils (esters) have revealed the potentials geothermal stability of the local esters to with stand the 1st tier of HPHT environments.
Key words: Drilling, high temperature, stability, antioxidant, vegetable oils (Esters), oil base muds.