Oil based muds formulated from diesel oil have proven to be quite expensive and environmentally unfriendly. This current study focused on investigating the suitability of castor oil in formulating oil based muds. The experiment in this work was conducted at different temperature conditions ranging from 40 to 80°C. The density and specific gravity of castor oil based mud were found experimentally to be 0.3 ppg and 0.4 higher than the density and specific gravity of diesel oil based mud, respectively. Plastic viscosity and gel strength of castor oil based mud were found to be 2 cp and 2 lb/100 , respectively found to be less than those obtained for diesel oil based mud (15 cp and 4 lb/100 , respectively) indicating the ability of castor oil in enhancing rate of penetration and efficient in hole cleaning. Results also show that a thinner filter cake was obtained for castor oil based mud in comparison to diesel muds. This shows that with castor oil based mud, undesirable effects such as differential sticking, loss circulation and poor primary cement jobs are minimized. The result from toxicity test also implied that the drilling mud formulated with castor oil can be easily disposed with less post drilling treatment. In conclusion, oil based mud formulated with castor oil can be used as an alternative to diesel oil based mud for drilling operations because its properties show that it can effectively perform its functions as a drilling fluid. It is also environmentally friendly. Key words: Oil based mud, castor oil, diesel oil, rheological properties, drill cuttings.
