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.

Effects of non-aqueous fluids-associated drill cuttings discharge on shelf break macrobenthic communities in the Campos Basin, Brazil

“…The effects of drilling activities located around the drilling well, with drill cuttings influencing oxygen consumption and oxygen penetration depth in the sediment, presence of black sediments indicating anoxic conditions, with high number of capitellid polychaete, genus Capitella, the more specific fauna replaced by original fauna, and the establishment of a recovery process in a short time frame have also been found by several authors, such as Smith and May (1991), Candler et al (1995), Daan et al (1996), Terrens et al (1998), Fechhelm et al (1999, Dalmazone et al (2004) and Trannum et al (2010), corroborating the results obtained in the present study.…”

“…Various studies have been done to detect the effects of oil and gas drilling discharge on benthic communities, but most have been restricted to continental shelf, such as the North Sea (Dicks 1976;Hartley 1984;Addy et al 1984;Davies et al 1984;Kingston 1992;Daan et al 1996), southern Gulf of Mexico (Hérnandez Arana et al 2005), northern Gulf of Mexico (Middleditch 1981;Rabalais et al 1992; Montagna and Harper 1996), northwest shelf of Australia (Oliver and Fisher 1999), and southeast Australia (Terrens et al 1998). On the other hand, the potential impact of the drilling activities on the shelf break benthic ecosystems is scarce.…”

Impact of Drilling Activities in Warm Sea: Recolonization Capacities of Seabed

“…At many sites where SBFs have been used there is a previous history of WBF and LTMBF cuttings discharge; it is therefore often difficult to differentiate the respective impacts of each type of cuttings. Furthermore, few field studies have documented the impact on biological communities: Terrens et al (1998) (esters, Australia), Oliver and Fisher (1999) (linear paraffins, Australia), lower impacts than with OBF cuttings. -Areas that recover most rapidly are those with highenergy seabed conditions.…”