Skipper oil – heavier and more viscous than expected

Independent Oil & Gas(IOG) drilled its Skipper appraisal well in August this year, but despite initial indications that the heavy oil was moveable in the reservoir, the company has now announced that sample analysis points to a significantly higher viscosity than expected, forcing a rethink on potential development options.

Skipper sits in a heavy oil prone region to the south of heavy oil fields Bressay, Mariner, Kraken and Bentley and is estimated to contain 2C resources of 26.2 mmstb.

The key uncertainty prior to drilling the appraisal well was the viscosity of the oil, so the primary objective was to recover good quality reservoir samples from the well. Viscosity is seen to be a more useful indicator of flow properties for heavy oil fields, and a higher viscosity than expected would require different processing considerations and a higher well density. The discovery well, 9/21-2, was drilled in 1990 but failed to flow on test. Samples recovered from the well indicated the oil was 14 – 16 deg API, but uncertainty remained regarding the viscosity of the oil, since the samples were dead oil and there was no representative pressurised sample from the well. These samples indicated a dead oil viscosity of 750cP, which was considered to be pessimistic and taken as the low case for the well. In the 2013 CPR the oil viscosity at saturation pressure at reservoir conditions was then calculated, using a range of correlations, to be 160cP in the mid case.

A viscosity of 160cP would have put Skipper alongside Captain, the only heavy oil field producing in the North Sea to date and Kraken, due onstream in H1 2017. First sample results indicate that the oil is heavier than expected at 11 deg API and that the viscosity is significantly higher, though no actual figure is given. Although a higher viscosity would make a development more challenging, such oil can still be developed, with Statoil going ahead with the 508cP Heimdal reservoir in Mariner and Xcite demonstrating that commercial rates could be achieved in the 1500cP Bentley when it produced from the 9/03b-7 well at up to 3,500 bopd through an ESP. Both Bentley and Mariner are however significantly larger than the 26mmbbl Skipper.