Resaturation of previously drained compartments form attractive late-life targetsNorth Cormorant Platform. Photo: TAQA

Resaturation of previously drained compartments form attractive late-life targets

North Cormorant field in UK Northern North Sea proves concept of vertical oil migration from poor quality unswept reservoirs to overlying high quality previously drained intervals.

With over 130 drilled so far, the North Cormorant field in the UK Northern North Sea has been well developed. Discovered in 1972 and currently operated by TAQA, this Middle Jurassic Brent field is expected to produce approximately 470 MMbls at a Recovery Factor of 47% by the time the field is expected to cease production in 2023.

In contrast to expectations, a North Cormorant infill well drilled near a producer that was shut in 20 years ago found the Etive Formation close to initial saturation. Here is an account.

Infil target

In 2019, the PN01 structure was evaluated as an infill target in the North Cormorant field. The opportunity was considered to be a redrill of a historical producer, targeting the lower quality Brent intervals of the Lower Ness, Rannoch and Broom Formations which were believed to have been poorly swept during the historical production in the area.

The strategy of targeting bypassed oil in lower permeability Brent units has been adopted in several of the TAQA operated Brent assets as part of late life development towards achieving Maximum Economic Recovery (MER).

A previous producer, CN39, was online from 1986 to 1999 and produced nearly 17MMbbls oil which was believed to be predominantly from the Etive unit (based on a 1986 PLT). The CN39 well was at 93% water cut when it was shut in, which was interpreted to indicate the well-swept nature of the Etive Formation.

Location CN39 and PN01 target in North Cormorant field. Map from TAQA.

Perforating lower quality formations

Some Etive resaturation was predicted, due to gravity segregation but with recovery factors for the Etive unit having been calculated at 60-70%, this was expected to be minimal.  Perforation strategy for the PN01 well was planned for the better saturated, lower quality units with any Etive resaturation to be produced with a final intervention on the well closer to Cessation of Production (CoP).

When the PN01 target was drilled in late 2019, there had not been any offtake in the area of the field for 20 years.  The well targeted a crestal location near to the CN39 historical producer (see map below). The results of the well unexpectedly showed a fully saturated Etive interval, with oil saturation very close to initial saturations while the underlying Rannoch was more water bearing than expected.

Well results. The newly drilled CN39S5 shows similar neutron-density cross-over as CN39. From: TAQA.

Upward oil migration

The explanation of this was that due to the stasis seen during the long period of shut in since 1999, that the Rannoch and Etive had been in vertical communication – something which had not been seen before in the TAQA operated assets. The Rannoch unit as a middle to lower shoreface unit, is highly layered, and vertical permeability is generally poor.

As a result, the Rannoch Formation has been developed separately in North Cormorant, with dedicated Rannoch producer-injector pairs to maximise the recovery of this unit. However, at PN01, time and gravity appear to have worked together to have allowed some of the Rannoch oil to move into the Etive, which is much more attractive in terms of production efficiency and ultimate recovery.

The PN01 well started production in May 2020 and initial rates of over 13,000 bopd were seen, outstripping most of the late-life infill wells on the field. 430,000 bbls were produced between May and August this year.

Learnings

The above case history clearly proves the value of re-entering compartments that may have experienced re-saturation over a period of down time. Even intervals that do not sweep vertically during production may see vertical connectivity post-production. Identification of these targets need to be completed several years before COP, enabling drilling and hooking up to production facilities.

CARYS THOMAS (TAQA), HENK KOMBRINK

HK thanks Carys Thomas for presenting this case study at Devex 2020, which formed the basis for this article.

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