ENVIRONMENTAL ASSESSMENT OF SYNTHETIC-BASED DRILLING MUD DISCHARGES TO BASS STRAIT, AUSTRALIA

Abstract

Esso Australia Ltd. (Esso) on behalf of the Esso/BHP joint venture undertakes exploration and development drilling activities to produce oil and gas from the Gippsland Basin in eastern Bass Strait. As part of the environmental management of the operation, a field monitoring program was undertaken to verify the predicted limited effect on the seabed of the discharge of residual synthetic based mud (SBM) adhered to drill cuttings.

Reservoir studies of the mature Fortescue oil field identified scope to more efficiently deplete existing reserves as well as develop undrained oil pools within the known reservoir system. As such, 18 additional wells were drilled from the Fortescue platform in 1994-1996 following the original development drilling program when 21 wells were drilled in 1983-1985.

Esso's standard drilling fluid in Bass Strait is an environmentally acceptable water based mud (WBM). However, due to the high frictional forces involved the greater lubricating properties of a non-water based drilling mud were required to drill the long reach, high angle sections of seven of the additional wells. The ester SBM was chosen on the basis of its lubricity and biodegradability, to reduce any potential environmental effects from the persistence of non-water based mud in seabed sediments.

A seabed monitoring program was undertaken around the Fortescue platform by taking seabed samples from August 1995 to August 1997 at sites along a transect following the predominant ocean current and at control sites, before, during and after the period in which SBM cuttings were discharged. The seabed sediments were measured for esters, barium, biological changes and grain size. Results show clearly the increase in esters concentration in sediments during drilling, then the rapid decrease after completion of cuttings discharge. After a recovery period of 11 months from completion of SBM drilling, SBM esters were not detected in sediments. Sediment barium concentration was also found to be elevated, but only after SBM had been used, indicating greater dispersion of WBM than the more cohesive SBM. The barium concentrations found generally do not have measurable effects on biota. The impact on the sediment biology was found to be limited to the sampling site 100 m from the platform discharge point with recovery evident within four months of completion of drilling.

Mechanisms for recovery of the minimal zone of effect are thought to be a combination of the biodegradation of the ester SBM used and the physical seabed dispersion process evident in eastern Bass Strait generated by the continuous series of storms which pass through especially in winter.