Pluto 4D—Australia's first 4D over a gas field is an outstanding success

Abstract

The Pluto 4D seismic survey is Australia’s first 4D survey acquired over a gas field and has been an outstanding success despite the prior high technical risk of not being able to detect 4D differences above the background noise. At the time of the Pluto 4D monitor survey, the Pluto field had been on production for three years and nine months and produced approximately 1 Tcf of gas. The first monitor survey was acquired by PGS between November 2015 and February 2016 using dual sources and 12 streamers of 7050 m (geostreamer configuration). Also of note was the use of steerable sources to assist with repeatability, and towing the streamers at 20 m to minimise noise. Data was processed by CGG to pre-stack depth migration (PreSDM), took 12 months to deliver and required significant interaction between Woodside geoscientists and CGG to ensure an excellent 4D product. 4D feasibility studies carried out before acquisition showed that saturation changes (related to water ingress) were expected to show strong 4D responses near the gas water contact (GWC) and would be interpretable if the monitor survey was run after approximately three to four years of production. If the monitor survey was delayed, it showed that the pressure response became too large, swamped the 4D differences and made interpretation difficult or impossible. Strong ‘hardening’ responses on the 4D difference volumes are interpreted as water ingress into the field. Hardening responses are seen in all reservoir sequences in the Triassic from the TR27 to the TR32 units and range from strong (obvious) to weak (possible). Several examples are shown – the strongest response is seen in the large Triassic (TR27.3) valley within an essentially shale prone unit, which shows water ingress into the valley and moving upwards from the GWC towards the producing well. The results/insights from the 4D data have provided excellent control points for the history matching of the Pluto Field.