Embodiments of the invention provide alkyl esters useful as base fluids in oil-based mud compositions, and processes of preparing such alkyl esters. The alkyl esters are produced from raw material waste oil that include vegetable oil. The raw material waste oil can be obtained from the food industry, such as from food chains. The raw material waste oil is purified by removing impurities from it. The raw material waste oil is then esterified with an alcohol in the presence of a catalyst. The resulting alkyl ester products are then separated from triglycerides. The alkyl ester products are then washed and dried.