TRACKING PRODUCED FORMATION WATER DISCHARGE FROM A PETROLEUM PRODUCTION PLATFORM TO THE NORTH WEST SHELF

Abstract

During September 1995, the Australian Geological Survey Organisation and the Australian Institute of Marine Science took part in a joint survey to monitor the dispersion, fates and effects of Produced Formation Water (PFW) discharged from the Harriet A production platform. Continuous Geochemical Tracer(CGT) analysis of benzene, toluene and C1−C6 hydrocarbons was used to map the spatial distribution of the PFW discharge in near surface waters. Hydrographic parameters were also continuously measured and four hydrocarbon vertical profiles were completed at various distances from the platform.

PFW collected at the Harriet A production platform contained 3.75 mg/L of C1−C6 hydrocarbons; 1.59 mg/L of benzene and 1.72 mg/L of toluene. Benzene, toluene and total C1−C6 hydrocarbons are sensitive tracers of PFW, with concentrations of benzene and toluene in the PFW plume about 100 fold higher than typical 'detectable background' levels. Maximum concentrations measured within the PFW discharge plume were 140 ng/L for benzene, 250 ng/L for toluene and 1500 ng/L C1−C6 hydrocarbons. Minimum dilutions of about 2,500 for C1−C6 hydrocarbons and less than 10,000+ for benzene and toluene were measured at distances 1 km from the platform. Thus, close to the platform, benzene and toluene are apparently more 'reactive' than C1−C6 hydrocarbons and disperse more rapidly.

The PFW discharge was mapped in a NNW and SSE direction on both the ebb and flood tides to a maximum distance of 10 km from the production platform. A net westerly drift of the plume was evident on both the ebb and flood tides toward the Montebello Islands and Varanus Island, rather than toward the east and the mainland. A one dimensional dispersion model found hydrocarbons are dispersed in seawater, with characteristic half-dispersion distances of about 3.5 km on a spring tide. Vertical profiles of PFW hydrocarbons suggest that within 1 km of the platform the PFW plume is primarily a surface feature but beyond 1 km all hydrocarbons are well mixed.