Effects on the Marine Environment of Produced Formation Water Discharges from Offshore Development in Bass Strait, Australia

Abstract

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.

Produced Water in the North Sea: a Threat for the Environment or a Threat for the Industry?

“…These include the Gulf of Mexico, the Bass Strait between the Australian continent and Tasmania and the west Java Sea 16,17,34,35,42,48 . These data provide a general overview of fate and effects of production waters.…”

“…A number of 3-dimensional models has been developed 5,16,22,29,31,34,35,36,39,37,65 . Some were initially built for other oceanographic areas (the OOC model was used primarily in the Gulf of Mexico), but have been adapted to the North Sea and other areas 42 . Others are more specific, such as the IKU/SINMOD-3D model used for the Haltenbank 31 .…”