Direct Hydrocarbon Detection of Produced Formation Water Discharge on the Northwest Shelf, Australia

Rapid and continuous direct hydrocarbon detection (DHD) of benzene, toluene and total C1-C6light hydrocarbons was used to map the dispersion of produced formation water (PFW) discharged from a petroleum production platform into near-surface waters of the Northwest Shelf of Australia. These hydrocarbon components are sensitive tracers of PFW discharge. Analyses at 2 minute intervals at a typical ship speed of 3 knots provided a C1-C6benzene and toluene sample every 180 m of ship travel. The PFW discharge was mapped in a NNW and SSE direction on both the ebb and flood tides to about 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 Is., rather than toward the east and the mainland. Measured dilution of hydrocarbons at distances less than 1 km from the platform, indicated that dispersion rates appear to be benzene> toluene>C1-C6, where typical dilution of total C1-C6 hydrocarbons was over 2500 times, and benzene and toleune, over 10 000 and 20 000 times respectively. Vertical profiles of PFW hydrocarbons showed that the PFW plume was primarily a surface to mid-depth feature with most hydrocarbons confined to the top 12 m. Beyond 1 km, all hydrocarbons were well mixed vertically at only slightly elevated levels with characteristic dispersion half-distances of about 3·5 km on a spring (3 m) tide.