Impacts of Gas Injection, Reservoir Pressure Recharge and Maintenance in the Usano Field
When Oil Search assumed operatorship of all of Papua New Guinea’s producing oil fields in 2003, the Usano field was producing at 850bopd, and 6.5 MMscf/d. The field was on decline, having produced 8 MMstb and the outlook was less than 0.5MMstb remaining reserves. At this rate, abandonment was imminent. By late 2007, Oil Search had kicked off a new drilling campaign in Usano and by the end of the year the oil production rate had more than doubled, to 2500 bbl/d. By late 2009, the Usano field reached a peak oil rate of 9000 bbl/d, brought about by the drilling of seven new production wells and one new injector. With increased production offtake from the field, reservoir pressure declined rapidly, and high oil rates could not be sustained. Reservoir Engineering studies indicated that higher EUR’s could be achieved through maintenance of reservoir pressure and effective voidage replacement. The theory is that stable gravity drainage, supported by constant gas injection, promotes piston like displacement along the highly permeable beds. Other PNG oil fields have larger gas caps which assist in maintaining reservoir pressure whereas Usano Main and Usano East have relatively small gas caps. As such, gas reinjection and ongoing pressure maintenance is key to maximising EUR through this recovery method. With the drilling of UDT 11 in 2009 and the resulting increased oil rates seen in Usano East, it justified the need for an injector to arrest the reservoir pressure decline. Two years later UDT 13 was online and injecting at ~20-30 MMscf/d. This resulted in pressure recharge and a period of stable production. Meanwhile, Usano Main pressures and oil rates were on decline. A decision was taken to shut in the field to focus on reservoir pressure recharge. When the field was brought back on line in late 2014, a year later, the reservoir pressure had climbed more than 500psi, and the initial online rate exceeded pre-shut in rates by more than 100%, and ultimately resulted in an extended period of higher sustained production between through to 2017. Gas reinjection has been an effective tool in maximising production from the Usano field. It is widely available given current gas export requirements, is of sufficient specification and is also an effective means of unloading wells and producing high water cut wells under gas lift. There is no doubt the significant impact that gas injection, reservoir pressure recharge and maintenance has had on arresting the decline to oil rate in the Usano field, as well as many other aging oil fields in PNG. Work is underway to further optimise this strategy and identify sign posts to determine effectiveness in ongoing attempts to maximise EUR.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020