Pluto 4D Monitor Two – a second outstanding success

Abstract

A second 4D seismic monitor survey acquired over the Pluto gas field has been an outstanding success despite three tropical cyclones, the start of a global pandemic, and technical subsurface risks. At the time of the Pluto 4D Monitor Two (M2) survey (December 2019 till February 2020), the Pluto field had been in production for 7 years and 9 months and had produced approximately 2 Tcf of gas. The M2 4D survey was acquired between December 2019 and February 2020 over a 20 × 30 km area, on a 12.5 m square grid. Data was processed by CGG to pre-stack depth migration, took 6 months to deliver, and required considerable collaboration with Woodside geoscientists. Use of modern processing technologies, including least squares migration, full waveform inversion and wave equation based amplitude versus offset inversion facilitated a high quality interpretation. On the 4D difference volumes, hardening responses were interpreted as water ingress. Hardening responses were seen in multiple Triassic reservoirs. A pronounced response was seen in a large Triassic valley within a predominantly mudstone background unit, which showed continued water ingress into the valley, upwards from the gas water contact towards the producing well. This confined hardening response contrasted with unconfined, water flood front responses seen in other reservoirs. Both responses enabled detailed interpretation of geological features and provided useful control points for history matching. Interpreted water ingress also gave additional depth control.