Impact of Drilling Activities: Seabed Recolonization. A French experiment.

Abstract

Due to large constraints on environmental regulations, discharge of drilled cuttings at sea has to be controlled. It is thus necessary to have basic information to contribute to the selection of drilling fluids respectful of the environment. The present study was intended to appreciate under real conditions the impact of cuttings, the capacity of regeneration of the ecosystem and the processes of recolonization. In a first phase, standardized (OSPAR) and experimental toxicity tests, aerobic and anaerobic degradation measurements and lixiviation tests were performed on various types of muds and cuttings (containing diesel oil, two different paraffinic cuts, and glycol). The results of the study demonstrated that:OSPAR tests show a low toxicity of cuttings. Those containing diesel oil are more toxicThe experimental toxicity test using mollusc's larvae is very sensitiveThe presence of a toxic fraction other than the hydrocarbons in muds has been demonstrated, in particular in muds containing diesel oilThe decomposition of hydrocarbons in aerobic conditions is fast (40 days approximately) and slower on the cuttings because of accessibility to hydrocarbonsThere is no anaerobic degradation after two months on basic oils (diesel oil and paraffinic base oil). In a second phase, the most suitable measurements were taken within the framework of an in situ experiment at a discharge site in Congo (Africa). Together with this experiment, a complete analysis of benthos was performed after 8 years of deposits. The preliminary results demonstrated:The zone of extension of the toxicity and the modifications of the benthos is less than a few hundred meters around the discharge siteOriginal fauna is replaced by a more specific fauna In the more general context of the environmental constraints in the oil industry, our work demonstrated the containment of the impacted zones, the absence of significant risks of modification of the adjacent ecosystems and the possible application to other discharge sites. Introduction Since the onset of undersea drilling, the oil industry has been concerned by the impact of drilling cuttings and drilling fluids on the sea bottom. Numerous studies carried out in the laboratory on their foreseeable fate have led to the search for new formulations with the aim of achieving better intrinsic biodegradability. This research is based on the results of various tests of biodegradability and tests of toxicity that have been undertaken in the laboratory by teams of specialists. About 2 Millions tons of cuttings are generated each year by the offshore industry. Although the study of the impact associated with the discharge of drilling cuttings impregnated with fluid has already given rise to numerous works (1), it is clear that further information is required on the monitoring of cuttings discharged today and on the fate of old cuttings. We therefore propose to apply the principle of the measurement of chemical, biological and toxicological parameters for the monitoring and recolonization of the contaminated sediments.