Benthic Impacts Resulting from the Discharge of Drill Cuttings and Adhering Synthetic Based Drilling Fluid in Deepwater

Abstract

To address the potential for modification of discharge regulations concerning residual oil on cuttings concentrations (OOCC), a scientific study was conducted in 2011 to evaluate the fate and effects of drill cuttings resulting from completion of a development well using a non-aqueous drilling fluid (NADF). The design of the study conducted in deepwater offshore Ghana was statistically defensible and OSPAR-compliant, utilizing a stratified random sampling design. The study integrated various sampling techniques and matrices to provide a comprehensive assessment of the impacts of cuttings discharges on the marine environment and to verify the results of dispersion modeling. Survey results were used to develop recommendations for regionally applicable OOCC discharge limitations. The physico-chemical and macroinfaunal sampling of sediments at varying distances from the wellsite offered insight into the benthic fate and effects of NADF-based cuttings discharges. Statistical analyses of the data, including correlation, analysis of variance, Tukey's analysis, and pattern analysis, revealed the following results. Benthic impacts were limited to within several hundred meters of the wellsite and included reduced numbers of species, reduced species diversity and evenness, increases in hydrocarbon levels (including NADF tracers), and elevated levels of drilling-related metals. Macroinfaunal and physico-chemical metrics return to ambient levels within approximately 500 m of the wellsite. On a worldwide basis, most regulations pertinent to residual OOCC have limits ranging from 1 to 9.4%. An OOCC level of approximately 5% was achieved for the NADF-based cuttings discharges during this development drilling program. Survey results indicate minimal and localized environmental impacts to the sediment chemistry and macroinfaunal community structure from NADF-based cuttings discharges, consistent with findings from other studies. Based on this survey, it would be reasonable to apply the results of this study to establishing appropriate drill cuttings discharge guidelines for deepwater operations that utilize NADFs.