Closed-bottle biodegradation test for synthetic-based drilling fluids under Australian conditions

Abstract

Biodegradation rate, along with acute and chronic toxicity, is an important factor in determining the overall environmental performance of drilling fluids (SBFs) used in synthetic-based mud (SBM) formulations. The closed-bottle anaerobic biodegradation test (CBT) is one of the three criteria set by United States (US) Environmental Protection Agency that drilling fluids have to meet in order to be permitted for use in offshore exploration. This project, based on US methodologies, was aimed at optimising the CBT to Australian conditions. Phase one (CBT1) included a comparison study for the relative biodegradability of selected SBFs including esters, internal olefins (IO), linear alpha olefins (LAO) and paraffin in Australian and US sediments. Apart from dissimilar characteristics, Australian sediments demonstrated very high metabolic activity relative to US sediments, most likely due to the presence of higher levels of sulfate reducing bacteria (SRB) and organic content (% volatile solids, VS). The results suggest the relative ranking from fastest to slowest anaerobic biodegradation rates of SBFs as esters > LAO > IO > paraffin. CBT2, the second phase, focused on further refinement in the selection of marine sediment along with comprehensive characterization of five selected sediments and resulted in successful completion of CBT in two sediments among five inoculums. Results suggest that increasing number of microbes including SRB and VS correlate well with biodegradation rates.This study has shown that CBT has the capacity to provide information on the relative biodegradability of SBFs and sediment recovery period under marine conditions. CBT data may assist in improvements to SBM technology including the selection of SBFs, quality control of base fluid batches, and subsequent minimization of environmental impacts associated with SBM discharge at sea under Australian conditions.