Appraisal And Development-Sanction Challenges For A Small, Stranded, Offshore Heavy-Oil Project In The North Sea
- Gerard Philip Coghlan (Venture Production PLC)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference, 3-6 May, Houston, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2010. Offshore Technology Conference
- 2.3.3 Flow Control Equipment, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 5.1.1 Exploration, Development, Structural Geology, 3.2.5 Produced Sand / Solids Management and Control, 4.1.5 Processing Equipment, 2.2.2 Perforating, 4.3 Flow Assurance, 2 Well Completion, 4.2 Pipelines, Flowlines and Risers, 4.5 Offshore Facilities and Subsea Systems, 1.6 Drilling Operations, 2.4.3 Sand/Solids Control, 5.4.1 Waterflooding, 5.4.2 Gas Injection Methods
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The Central North Sea Pilot field lies in UK license block 21/27. In 1989 a discovery of heavy oil - by North Sea standards - was made in Eocene sands at a depth of 2700ft TVDSS. Three vertical appraisal wells determined relatively extensive, thin package of turbidite sands (ca.100ft) of limited relief (ca.150 ft). Early flow-testing was constrained by difficulties in sustaining flow because of separation and flaring problems due to low rates, water coning, also sand production. Sampling was problematic with oil properties ranging from 17-22 degAPI and 90-160 cP.
In 1998, a 'prover', horizontal well was completed with an ESP with the intent to prove up sustainable, commercial rates, however this was not achieved, because the oil samples pointed to an oil of much lower quality than expected, 13 degAPI and 1900 cP. Venture Production acquired the field in 2001 as part of a package of acquisitions.
Subsequently all historic data were reviewed and a major effort to move field development forward resulted in the identification of several innovative approaches, centred on a cylindrical hull FPSO to commercialise this geographically stranded (nearest infrastructure 25miles), technically and commercially challenging opportunity. A postulated, developable OIIP of 110 MMstb with a viscosity of ca.100 cP or less was to be targeted by horizontal, hydraulically pumped wells and a waterflood.
Key uncertainties were a) the oil quality distribution and b) the need to recover samples of sufficient volume to allow flow assurance issues (such as calcium napthenate and sodium soap) to be assessed. A further appraisal well was proposed to acquire the necessary, large-volume, representative samples on wireline. An innovative appraisal strategy utilising a geotechnical drillship with much-reduced drillex was planned but the program could not clear all necessary HSE hurdles - resulting in a delay of 1 year and a conventional drilling program (and costs). The appraisal well was drilled in 2007 and determined that oil quality was variable and unpredictable and the necessary minimum OIIP/ reserves for project sanction could not be assured. Follow-up studies resulted in a decision to relinquish the acreage after 20 years of work.
The Pilot oilfield is located on the UKCS in the Central North Sea in license block 21/27, ca.100 miles east of Aberdeen in a water depth of ca.265 ft. The reservoir is located in Middle Eocene age, deep water sands which were deposited as turbidite flows from a shelf immediately to the west of the accumulation. The reservoir interval was deposited by turbidity currents that ranged from high density to dilute low density.
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