Stag Development - Challenges and Successes in the First Two Years

Stag Development - Challenges and Successes in the First Two Years J. Goodacre; J. Goodacre Apache Energy Limited Search for other works by this author on: This Site Google Scholar A.M. Ion; A.M. Ion Apache Energy Limited Search for other works by this author on: This Site Google Scholar I.R. Alexander; I.R. Alexander Apache Energy Limited Search for other works by this author on: This Site Google Scholar K.J. Aitken K.J. Aitken Apache Energy Limited Search for other works by this author on: This Site Google Scholar Paper presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Australia, October 2000. Paper Number: SPE-64439-MS https://doi.org/10.2118/64439-MS Published: October 16 2000 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Goodacre, J., Ion, A.M., Alexander, I.R., and K.J. Aitken. "Stag Development - Challenges and Successes in the First Two Years." Paper presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Australia, October 2000. doi: https://doi.org/10.2118/64439-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE Asia Pacific Oil and Gas Conference and Exhibition Search Advanced Search AbstractStag oilfield commenced production in May of 1998 and reached peak rates in excess of 30,000 barrels of oil per day in July 2000.Stag development is an $A200 million oil development on the Northwest Shelf of Western Australia (Fig 1) designed to produce in excess of 50 million barrels of oil over the life of the field. The field was discovered in mid-1993, delineated in 1994 to 1996, and tested with a horizontal well production test in 1996. The development consists of a Central Production Facility containing wells and processing equipment, a 2km subsea line to a CALM buoy and FSO storage and export tanker. Designed to have five producers and three water injectors producing around 30,000bopd oil, the field did not meet initial expectations. Gas-oil ratios were higher than forecast, peaking in excess of 1000scf/bbl and causing significant operating problems for the process plant and electrical submersible pumps. Oil production was lower in the east of the field than anticipated, due to the thinning or absence of high productivity sand units, reservoir pressure support was poorer than modelled leading to more rapid pressure decline than anticipated, and one well produced significant quantities of sand and clayey solids. These challenges led to a significant re-work of the development. subsea water injectors were added, wells were recompleted in better quality reservoir, pump designs were significantly changed, the reservoir model was rebuilt and more producers were added (Fig 2).At year-end 1999, Stag came of age - production reached design basis levels, reliability approached 100% and overall reservoir performance met expectations.The paper reviews three key elements of the development -Reservoir Description and Performance, Completion Design and, Facility Design and Performance - and presents the lessons learned.Reservoir Description and PerformanceExploration & Appraisal HistoryThe Stag-1 discovery well was drilled in June 1993 and intersected a 16-metre column of biodegraded 19° API oil within the Lower Cretaceous M. australis Sandstone at a subsea depth of 680 metres. Geochemical tests indicated that the oil was similar to oil from the nearby Wandoo Field. Approximately 5,700km of 3D seismic data was acquired over the Field in November 1993 and subsequent mapping indicated sufficient reserves to justify further appraisal.A further eight appraisal wells were drilled over the next two and a half years to delineate the structure, establish fluid contact depths and evaluate well productivity. Two wells encountered free gas with different gas-oil contact depths, suggesting the presence of localised, perched gas caps. PVT tests confirmed a saturated crude with low GOR (120 scf/stb) and relatively high viscosity (9 cP at reservoir conditions).The Stag-6H appraisal well was drilled in May 1995 to test the deliverability of a horizontal well in a thin, saturated, heavy-oil system. The well flowed 2,800 bopd under natural lift, in excess of 7,000 bopd using an electric submersible pump (ESP) and confirmed the commerciality of field development. Stag-6H was then suspended as a future production well.Geological BackgroundThe Stag Field is a combination structural/stratigraphic trap with the M. australis Sandstone bounded above by the Upper Muderong Shale, and below by a base seal comprising argillaceous, bioturbated, glauconitic sandstone/siltstone. The M. australis Sandstone was deposited as a shelfal sheet sand which was subsequently modified by erosion. The sand pinches out along the south and eastern edge of the Field and is believed to extend downdip into an aquifer to the north and west. The structure has minor four-way dip closure; the maximum oil column thickness is interpreted to be 16 metres and the areal extent of the initial oil-water contact approximately 22 km2 (5,500 acres).Exploration & Appraisal HistoryThe Stag-1 discovery well was drilled in June 1993 and intersected a 16-metre column of biodegraded 19° API oil within the Lower Cretaceous M. australis Sandstone at a subsea depth of 680 metres. Geochemical tests indicated that the oil was similar to oil from the nearby Wandoo Field. Approximately 5,700km of 3D seismic data was acquired over the Field in November 1993 and subsequent mapping indicated sufficient reserves to justify further appraisal.A further eight appraisal wells were drilled over the next two and a half years to delineate the structure, establish fluid contact depths and evaluate well productivity. Two wells encountered free gas with different gas-oil contact depths, suggesting the presence of localised, perched gas caps. PVT tests confirmed a saturated crude with low GOR (120 scf/stb) and relatively high viscosity (9 cP at reservoir conditions).The Stag-6H appraisal well was drilled in May 1995 to test the deliverability of a horizontal well in a thin, saturated, heavy-oil system. The well flowed 2,800 bopd under natural lift, in excess of 7,000 bopd using an electric submersible pump (ESP) and confirmed the commerciality of field development. Stag-6H was then suspended as a future production well.Geological BackgroundThe Stag Field is a combination structural/stratigraphic trap with the M. australis Sandstone bounded above by the Upper Muderong Shale, and below by a base seal comprising argillaceous, bioturbated, glauconitic sandstone/siltstone. The M. australis Sandstone was deposited as a shelfal sheet sand which was subsequently modified by erosion. The sand pinches out along the south and eastern edge of the Field and is believed to extend downdip into an aquifer to the north and west. The structure has minor four-way dip closure; the maximum oil column thickness is interpreted to be 16 metres and the areal extent of the initial oil-water contact approximately 22 km2 (5,500 acres). Keywords: upstream oil & gas, drillstem testing, injection, completion, water injection, reservoir simulation, injection rate, goodacre, valve, recovery Subjects: Formation Evaluation & Management, Drillstem/well testing This content is only available via PDF. 2000. Society of Petroleum Engineers You can access this article if you purchase or spend a download.

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