|THE STELLA FIELD - A RISING STAR|
|Field Developments and Case Studies|
1Ithaca Energy Plc
The Stella Field – A Rising Star
John Horsburgh – Ithaca Energy (UK) Ltd, 7 Rubislaw Terrace, Aberdeen, UK.
The Stella Field lies in the Central North Sea and was discovered by Shell/Esso in 1972 in Block 30/6 under a first round licence. The discovery has lain dormant until Ithaca Energy recently gained operatorship of the licence and fully appraised the structure. Field development planning of Stella and her satellites is now well and truly underway.
In 1972, gas condensate was encountered in relatively thin, distal turbidite sand of the Palaeocene Lista formation by Shell/Esso with well 30/6-2, drilled within a four way dip closed anticline. The primary objective of the well was to intersect a dipping structure in the Jurassic but the well was prematurely abandoned due to the high overpressures encountered. The 20ft of pay logged in the Palaeocene Lista interval was largely dismissed. A second attempt to drill the HPHT play in the Jurassic and Triassic reservoirs of the area was attempted by Shell/Esso in 1984 with well 30/6-3. While the results in the deeper interval were inconclusive, this appraisal campaign confirmed that the Lista reservoir was laterally extensive and discovered an additional oil reservoir in the Ekofisk chalk interval below. The Lista “Andrew Sand” was cored and drill stem tested, flowing at over 20MMscf/d and confirming the presence of rich gas condensate in the reservoir. The Ekofisk chalk interval was also tested and flowed light oil at respectable rates considering the low permeability. Over 700ft of untested closure remained below the existing penetrations and the estimates of in-place volumes were insufficient to rank the discovery highly in Shell/Esso’s portfolio. Consequently the field returned to dormancy. As part of a strategic move into the UK sector in 2004, Danish player Maersk farmed in to the Stella block as operator and discovered the Harrier field 10km south of Stella. Harrier was found to contain gas condensate in the Ekofisk and Tor chalk reservoirs, trapped in a low relief salt induced structure. Maersk then moved to Stella and appraised the Stella Ekofisk reservoir with three further penetrations. The Andrew sand was repeatedly found to be present and hydrocarbon bearing in all three additional Stella penetrations. However, given Maersk’s focus on the chalk section, over 500ft of untested Andrew closure within the Stella structure remained at the end of their campaign. The uncertainty in recoverable volumes remained too great for the incumbent Joint Venture to progress further at that time.
The latent potential of Stella was recognised by Ithaca Energy in 2007. Ithaca initially purchased Shell/Esso’s majority non-operated interest and Maersk and Ithaca collectively progressed the planning of a final appraisal well targeting the Andrew reservoir. However, following the economic turbulence of 2008, Maersk decided to farm out of 30/6a. Ithaca then stepped in and purchased Maersk’s interest and operatorship in October 2009 and quickly secured a farm-in deal with Challenger Minerals, a subsidiary of the Transocean drilling company, to execute the appraisal well. Appraisal drilling commenced in March 2010 and confirmed the structure to be full to spill and the continuity of the reservoir sand, proving volumes at the upper end of expectations. A planned sidetrack further reduced the uncertainty in the fluid contacts. The appraisal well has secured an extension to the production licence under the new licence extension process for first round licences.
Ithaca Energy is a Canadian oil and gas exploration and development company with offices in Calgary, London and Aberdeen, which is focused on the exploration and development of oil and gas reserves in the North Sea. Our objectives are to develop existing discoveries, explore for new discoveries and acquire assets by purchase, farm-in or licence aquisition. Ithaca Energy is dual listed on the London AIM and Toronto TSX-Venture stock exchanges. Ithaca, on behalf of our JV partners, is now rising to the challenge of developing Stella and her satellite fields. Since appraisal drilling ceased in May 2010, Ithaca has kicked off numerous subsurface and engineering studies to define the range of recoverable volumes in Stella and satellite discoveries and to cost and design their development. Tight integration of regional geological modelling, seismic reprocessing and inversion, reservoir simulation modelling, wellpath optimisation, surface facilities design and export host negotiations are now required to bring the Stella area on stream within the next three years.
Significant subsurface challenges include seismic imaging beneath a gas chimney, reservoir characterisation of thin distal turbidites and tight chalk, compartmentalisation risk management and the characterisation and prediction of retrograde gas condensate production behaviour.
It is envisaged that Stella will be developed as a subsea tieback to a third party host. Access to third party infrastructure is a challenge for the project and numerous offtake options are currently being pursued, including standalone concepts. Development plans are continually being tested economically against evolving infrastructure and host constraints as the commercial process matures.
The development concept for the Andrew reservoir calls for horizontal wells. These are required to be landed in a 20ft thick reservoir interval at a pre-defined standoff from the fluid contacts. Advanced geo and bio-steering techniques will be employed to maximise the reservoir contact and minimise unnecessary footage and associated costs. Similarly, the chalk reservoirs will require long horizontal wells with fracture stimulation to enhance productivity and will require careful placement and design.
The Stella area project is a key component in the growth aspirations of Ithaca Energy. We relish the challenges associated with developing previously undervalued assets and over the next few years, aim to turn an overlooked asset into a true rising star.
I would like to thank the Stella JV partners, Dyas UK and Challenger Minerals North Sea, for allowing me to present this material. I would also like to thank my colleagues at Ithaca Energy who contributed to the presentation.