An invert emulsion drilling fluids having a combination of fatty acids derived from waste vegetable oil (WVO) and a fatty alcohol as a rheology modifier. An invert emulsion drilling fluid may include a water in oil emulsion, an invert emulsifier, a fatty alcohol having six to thirty carbon atoms, and a fatty acid having six to eighteen carbon atoms. The fatty acid is provided by esterifying a waste vegetable oil to produce a methyl ester of the waste vegetable oil and cleaving an ester group from the methyl ester of the waste vegetable oil. The invert emulsion drilling fluid may be formulated free of organoclay. Methods of drilling a wellbore using an invert emulsion drilling fluid are also provided.