Previous assessment of dispersion modelling, toxicity testing, and characterisation of produced formation water (PFW) discharges into Bass Strait indicated a very low environmental risk from PFW to the marine environment. Peak PFW concentrations can exceed the effect levels (EC50 or LC50) measured in 24-96 hr laboratory toxicity tests only within distances of tens of metres from the discharge point. In this assessment, the field monitoring of aromatic hydrocarbons in the water column (which are in low concentrations in PFW) was undertaken to directly assess dispersion and predicted fate mechanisms. Very low concentrations of both light and heavier aromatic hydrocarbons are likely in any PFW discharge. A high volume absorption sampler was deployed 20 m from the discharge point to continuously sample ocean concentrations of aromatic hydrocarbons for up to one week, providing large volume (1,000 L) water samples. Gas Chromatography/Mass Spectrometry (GCMS) was used to measure aromatic hydrocarbon concentrations. The concentrations measured with the ocean sampling device provide time integrated samples over approximately one week, and the results showed that the ratio of discharge concentration to ocean concentration was approximately 20,000:1. Compared to dispersion modelling predictions, the ocean sampler indicates lower environmental risk. This is because dispersion modelling predicts ocean concentrations within the plume whereas the sampler is measuring concentrations at a fixed point over the long term and is exposed to the plume only intermittently, similar to a sessile marine organism. Therefore the ocean concentrations provided by the large volume sampler are more representative of longer term ocean concentrations which can be experienced by marine organisms. Further assessment of prevailing operational and oceanographic conditions in Bass Strait suggests that there does not appear to be a water column accumulation of PFW aromatic hydrocarbons adjacent to the discharge. Introduction There are currently 12 offshore produced formation water (PFW) discharges from Esso/BHPP's oil and gas production facilities in Bass Strait, Australia (Figure 1). Total volumes of the discharges are approximately 90 ML.d-1. The volumes have increased significantly in the past five years from the minimal discharges in the early 1970s when oil production was brought on-line starting with Halibut in 1970. PFW discharge volumes are likely to increase further as fields mature. Environmental risk assessment of PFW discharged into Bass Strait suggests the discharges present a low potential for impact on marine organisms due to low acute toxicity and high dilution rates. Terrens and Tait reported discharge plume dispersion modelling predictions where the dilution of a typical PFW discharge in Bass Strait was 30:1 within 10 m of the discharge point, and that acceptable acute toxicity could be obtained with dilution of less than 4:1. The dispersing discharge plume is spatially limited as a narrow band, and mobile due to local tidal conditions. The length of any water column organism's exposure to concentrations in excess of acute effect levels measured in laboratory toxicity tests would be less than about 30 seconds for median current conditions, a period much less than that of the 24 hr or 96 hr acute toxicity tests. P. 739

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.

An environmental effects study was undertaken to assess the potential environmental effect of discharged produced formation water from offshore oil and gas platforms in Bass Strait. The focus was on potential effects on the fisheries of Bass Strait. Produced formation water discharged to the ocean from the Halibut platform, one of the largest Australian oil fields and now the most prolific produced formation water field, was modelled for dilution versus distance from the platform. Characterisation of produced formation water from 10 Bass Strait platforms for chemical constituents and toxicity to local marine organisms was performed. It was found that produced formation water discharged into Bass Strait presents a very low risk to marine organisms due to very low toxicity and very high dilution rates. For the Halibut platform, acceptable acute toxicity concentration can be obtained with dilution of less than 2:1. Actual dilution rates are approximately 30:1 within l0m of the discharge point, and concentrations reach an indicative "no chronic" toxicity effect within 25m of the discharge point. The plume is also spatially limited and mobile and any organism would be exposed to potentially lethal concentrations of produced formation water for less than about 30 seconds for median current condition. Introduction Esso Australia Ltd (Esso), the operator for the Gippsland Basin joint venture in Bass Strait with BHP Petroleum Pty Ltd (BHPP), is Australia's largest oil and gas production operator, responsible for the production of around half of Australia's crude oil supplies and most of Victoria's natural gas (Figure 1). Esso/BHPP have operated 19 oil and gas projects within the Gippsland Basin, comprised of manned platforms, unmanned platforms and monotowers, and subsea completions. The major continuous discharge from Esso/BHPP Bass Strait platforms is produced formation water. As the name suggests, this is water that has existed over geological time in the oil bearing formation and is produced from the oil reservoir with the oil. In Bass Strait, produced formation water provides the natural water drive for all production fields. In major offshore oil developments in Bass Strait produced formation water is separated from crude oil on the platforms using hydrocyclone and dissolved gas flotation technology to reduce discharged oil levels below 30 mg/l. The produced formation water is discharged from the platform into the ocean. Snapper platform is the closest produced formation water discharging platform to the coast, 35 km offshore. The offshore operations are spread from west to east by over 100 km. This study assesses the produced formation water dilution rates which occur in Bass Strait. It then discusses data on the chemical characterisation and ecotoxicology of produced formation water discharges and the environmental risk to Bass Strait. Produced Formation Water Volumes Discharged Produced formation water is separated from crude oil on Bass Strait platforms and is discharged to the ocean. During the production life of a Bass Strait oil field produced formation water volumes peak in the mature period as the reservoirs are depleted.

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 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.

Previous assessment of dispersion modelling, toxicity testing, and characterisation of produced formation water (PFW) discharges into Bass Strait indicated a very low environmental risk from PFW to the marine environment. Peak PFW concentrations can exceed the effect levels (ECs0 or LC50) measured in 24-96 hr laboratory toxicity tests only within distances of tens of metres from the discharge point. In this assessment, the field monitoring of aromatic hydrocarbons in the water column (which are in low concentrations in PFW) was undertaken to directly assess dispersion and predicted fate mechanisms.Very low concentrations of both light and heavier aromatic hydrocarbons are likely in any PFW discharge. A high volume absorption sampler was deployed 20 m from the discharge point to continuously sample ocean concentrations of aromatic hydrocarbons for up to one week, providing large volume (1,000 L) water samples. Gas Chromatography/Mass Spectrometry (GCMS) was used to measure aromatic hydrocarbon concentrations.The concentrations measured with the ocean sampling device provide time integrated samples over approximately one week, and the results showed that the ratio of discharge concentration to ocean concentration was approximately 20,000:1.Compared to dispersion modelling predictions, the ocean sampler indicates lower environmental risk. This is because dispersion modelling predicts ocean concentrations within the plume whereas the sampler is measuring concentrations at a fixed point over the long term and is exposed to the plume only intermittently, similar to a sessile marine organism. Therefore the ocean concentrations provided by the large volume sampler are more representative of longer term ocean concentrations which can be experienced by marine organisms.Further assessment of prevailing operational and oceanographic conditions in Bass Strait suggests that there does not appear to be a water column accumulation of PFW aromatic hydrocarbons adjacent to the discharge.