The deepest well in the UKCS – a Rotliegend target at 9 km?

The deepest well in the UKCS – a Rotliegend target at 9 km?

A search for the deepest well in UK waters exposes the dangers of using data without QC.

It is a mistake every geologist probably makes multiple times in her/his career: writing down a depth in metres instead of feet. This must have gone wrong when populating the table of UKCS wells that is available through the OGA website.

When sorting the data on TD depth, the deepest well that comes up is 49/20a-7, with a depth of 9,153 m below sea level. Situated in the Rotliegend play of the Southern North Sea, it would be quite surprising to see a Leman sandstone at this depth. In most cases the Rotliegend is found at depths of around 3 km. A look at the composite log indeed reveals that meters should be feet, with a TDSS at 2712 m.

This is the third article in a series dedicated to the deepest wells drilled in the countries around the North Sea. Last week we reported on the well Tjuchem-02 in the Netherlands and the week before Belgium’s deepest borehole was discussed.

The same problem occurs with the second deepest well in line, 30/29a-3 drilled by AGIP in 1996. TD depth should be 2657 m instead of 8720 m.

The third well in line is well 112/19-1, drilled by Total just northeast of the Isle of Man in the Irish Sea in 1997. Again, the true vertical depth should be 1983 below sea level.

These mistakes are understandable and easy to fix, but it does nicely show the need to perform a basic quality control on any subsurface dataset.

Cross-section through the Franklin field. Please note that 29/05b-H2Z was drilled into the Franklin West compartment that is even slightly deeper buried than the neighbouring Franklin accumulation itself (see map above). Redrawn after Irving et al. (2020).

NCS Exploration – Recent Advances in Exploration Technology, May 19-20

Fourth time lucky

Will we be lucky the fourth time? Yes! It is well 29/05b-H2Z on the West Franklin field in the Central North Sea, drilled by Total in 2015. It was completed at an impressive depth of 6368 m TVDSS, with a TD in Upper Permian Zechstein salts.

Franklin is one of Total UK’s HPHT fields in the Central North Sea. It is a true North Sea work horse, with 466 MMboe produced so far and production to continue for decades to come. At the same time, it is also a challenging field that experienced a complete shutdown in April 2012 due to gas escaping to surface at the nearby Elgin field with which Franklin shares many similarities. Ultimately, the problem was caused by stress changes in the overburden leading to casing damage and gas ingress in the annulus.

The Franklin and West Franklin fields primarily produce from a thick Upper Jurassic Fulmar succession, but the underlying Pentland Formation and Triassic sandstones also contribute (see cross-section above).

United Downs

The deepest onshore well in the UK is the United Downs geothermal well recently drilled in Cornwall, SW England. It reached a depth of 5,057 m in 2019 after penetrating a fault zone in igneous basement. Water will be produced from the fault zone to produce electricity and subsequently re-injected in the same fault zone at a shallower level.

HENK KOMBRINK

Kristina Helland-Hansen (VP EXP NUKE Assets North Sea at Equinor) who will give a keynote at the forthcoming conference NCS Exploration – Recent Advances in Exploration Technology, May 19-20, entitled “Greater Fram – The Ship That Never Sinks!”

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