BHPB enters the fourth dimension

IN January this year, PGS Asia Pacific acquired Australia’s first-ever customised baseline 4D seismic survey.
BHPB enters the fourth dimension BHPB enters the fourth dimension BHPB enters the fourth dimension BHPB enters the fourth dimension BHPB enters the fourth dimension

Undertaken for BHP Billiton Petroleum, the survey used high density 3D seismic acquisition technology in a time-lapse format to produce a baseline for future generations of 4D monitor surveys, allowing the understanding of small changes in reservoir state during the production of the Pyrenees oil and gas fields.

Located in the North West Shelf’s Exmouth Sub-basin, the “Pyrenees Development Area” includes several finds in two adjacent permits with estimated oil reserves totalling more than 120 million barrels.

These discoveries are in WA-12-R, which BHPB operates with 71.43% interest with Apache Energy (28.57%), and in WA-155-P(1) where BHPB operates with 39.99% interest in partnership with Apache (31.51%) and Japanese major Inpex (28.5%). The fields include the Pyrenees discovery itself, Ravensworth, Crosby, Stickle, Harrison and Moondyne.

Ravensworth, Crosby and Stickle are the initial focus, with the draft environmental impact study suggesting these fields could form a potential Phase 1 of the development program.

The plan is to produce from subsea wellheads on each of these three fields and connect them with flow lines to a floating processing storage and offtake vessel. Water depths are around 220m.

Other discoveries in the area may then be connected to the system in a later Phase 2.

With oil being the primary goal, associated gas is likely to be re-injected rather than flared.

BHPB and its partners undertook the new seismic to clarify the structural make-up of the area and to look for a suitable location for a gas disposal well. They also wanted to lay the ground for 4D reservoir monitoring over the predicted 20-year life of the field.

“The three fields that make up the Pyrenees development have excellent direct hydrocarbon indicator [DHI] support, including seismic flat-spots coincident with fluid contacts and elevated seismic amplitudes associated with hydrocarbons,” BHP Billiton geologists Jack Woodward, Mark Stanley and Guy Duncan wrote in the abstract of a presentation given at the 2006 American Association of Petroleum Geologists International Conference and Exhibition in Perth early this month.

“Based on this strong DHI support it was recognised that these fields may be candidates for 4D reservoir monitoring. Rock physics modelling using oil saturations predicted from reservoir simulation showed that after five years of production you would expect to detect changes in seismic properties caused by oil and gas production.”

Before this latest survey, the Pyrenees fields were covered by three different seismic surveys. These surveys were reprocessed in 2005 to produce a single seismic volume that gave improved data quality and a relatively seamless merge between the surveys. But differences related to the original acquisition parameters remained and portions of the seismic were deemed sub-optimal for the development of these fields.

In the Pyrenees survey the Ramform Victory used 12 streamers – the vessel has actually towed up to 16 streamers in other operations, according to PGS.

The 3600m streamers were only 50m apart. BHPB said it found this approach relatively cost effective as the streamer redundancy is partially balanced by reducing the requirement for infill seismic.

PGS and BHPB said the HD3D-4D acquisition strategy delivered both high repeatability and high resolution.

In laying a strong base for further seismic, it would allow the recovery of hydrocarbons from the reservoir to be maximised over the production lifetime.