A method of acidizing a subterranean geological formation by injecting an emulsified acid into a wellbore, wherein the emulsified acid includes at least an aqueous phase comprising a hydrochloric acid solution, an oil phase comprising waste oil, and an emulsifier, and wherein the waste oil contains 45-75% by weight of aromatic compounds and preferably contains less than 0.5% by weight of non-hydrocarbon compounds. Various embodiments, and combinations of embodiments, of the emulsified acid and the method of acidizing are provided.