Environmental impacts related to drilling fluid waste and treatment methods: A critical review

The oil Exploration & Production (E&P) industry provides an important energy source for the world. However, there is a worldwide concern about the environmental impacts of E&P activities. Spent drilling fluid is one of the drilling wastes generated by the oil and gas industry's activities. Drilling fluid and drill cuttings together form the second-largest volume of residues generated by the E&P industry. Drilling fluids are responsible for many important functions in the well drilling process. They are recirculated between the well and the platform several times during the drilling of an oil well. When the drilling reaches the reservoir phase, spent drilling fluid return to the surface contaminated with oil. The resulting residue has several compounds containing potential pollutants, which if incorrectly disposed of can pose several risks to terrestrial, aquatic, and aerial environments, including reducing soil fertility, affecting negatively the flora and fauna and causing health problems due to the volatilization of hazardous oil components such as benzene, toluene, ethylbenzene and xylene into the atmosphere. In this sense, regulators have established that the disposal of non-water based and water-based drilling fluid containing free oil is not allowed above 1% in volume. Therefore, the treatment of oily residues generated in E&P activities is an essential task. Studies around the world presented a variety of physical, chemical, and biological methods to treat drilling waste: bioremediation, thermal, physicochemical, supercritical fluid, electrokinetic and stabilization/solidification treatment. All these methods showed promising results while presenting a series of advantages and limitations. It is also important to note that most treatment methods can recover and/or recycle oil, the main contaminant of the wastes generated. In choosing the best method, some relevant factors need to be considered, such as the drilling waste's characteristics and the costs to implement the process on a large scale. It was observed that only bioremediation and stabilization/solidification proved to be easy to apply on a large scale. However, the bioremediation treatment process is of long duration, and high biodegradation efficiency is difficult to be achieved in uncontrolled environments. In the stabilization/solidification treatment, in turn, encapsulating all contaminants from the waste is difficult. Therefore, the development of new physicochemical methods for the large-scale treatment of drilling waste have been proposed, as the physicochemical treatment presented the highest treatment efficiency when compared with other methods, while also enabling the recovery and reuse of all separated phases.