Assessment of the Fate and Ecological Risk of Synthetic-Paraffin-Based Drilling-Mud Discharges Offshore Sarawak and Sabah (Malaysia)

Abstract

Abstract The environmental fate and effects of four paraffin and one olefin synthetic based drilling mud (SBM) cuttings discharge offshore Sarawak/Sabah Malaysia were compared approximately 3 and 15 months post discharge. Shallow and deepwater sites were assessed for physical, chemical and biological properties to determine whether paraffin based drilling mud cuttings discharge effects were comparable to olefin based cuttings discharge, and would potentially be acceptable for the discharge of Gas-to-Liquid (GTL) paraffin based cuttings. Prospective ecological risk assessment at each site was used to predict projected depositional area concentrations and toxicity of discharges. Those data were used to select field collection sites. Average retention on cuttings (ROC) for all five sites was 5.3 % (4.2-8.3%). The SBM, measured as Total Extractable Hydrocarbon (TEH) concentrations, were correlated to distance from discharge and were typically near analytical detection beyond near field stations (60 to 400 m) from discharge. Detectable TEH concentrations were predominantly found in 0-4 cm depth of the sediment samples collected from all sites. Both paraffin and olefin cuttings were degraded three months after discharge and continued to show increased degradation 15 months post discharge. Composition of the paraffin SBM changed over time with a loss of n-alkanes and some isoparaffins indicating that paraffin degradation was occurring in the field. Sediment toxicity conducted on several samples from two sites indicated toxicity at sites with high TEH. Pisagan samples containing n-alkanes showed toxicity whereas samples that showed no toxicity did not have n-alkanes that have largely degraded. A modeling study determined that with existing bottom oxygen concentration ~2 mg/L, estimated currents at 5-10 cm/s, and aerobic sediments having positive (+) oxidizing reduction potential within 2 cm of surface, aerobic degradation was possible for paraffin (and olefins) and could explain degradation of the paraffin. Both modeling and actual field sampling verified a thin cuttings deposition layer that allowed oxic degradation to occur. Redox correlated with TEH and redox was significantly affected by distance at some sites. Benthic macroinvertebrate surveys limited to two deepwater sites indicated no differences between reference sites having below detection TEH and those sites within the zone of cuttings discharge, although differences were observed between these two sites. Benthic community parameters do not appear to be related to discharged SBM cuttings as measured by TEH concentrations. Degradation of paraffin over time and decreasing concentrations, aerobic surface sediment conditions, and no apparent differences in benthic communities confirm that paraffin SBM cuttings are having minimal environmental impact as presently used in offshore Malaysian coastal environments.