Extended oil recovery from Gullfaks

Statoil’s latest well on the Gullfaks field provides access to reserves that cannot be reached with standard drilling techniques. Four SINTEF scientists helped to open up this new section of Norway’s money machine.

By Svein Tønseth
Photo: Øyvind Hagen, Statoil

Statoil was responsible for an unusual drilling operation that was carried out from the Gullfaks C platform this summer – the first of its type on the Norwegian continental shelf.

With the help of a technique known as underbalanced drilling, the drilling team had no problems in penetrating a geological stratum that cannot be drilled using normal drilling technology.

Increased oil recovery
Statoil will probably drill somewhere between five and ten more wells on the mature Gullfaks field with the aid of the new method.

“On Gullfaks alone, the new drilling technology will give us oil worth NOK 1.6 billion that could not have been produced using conventional drilling “ explains Statoil‘s project manager Johan Eck-Olsen.

Fragile rock
During normal drilling, the well is filled up with heavy drilling fluid or “mud”. The idea is to maintain sufficient pressure in the well to prevent oil, gas and water from flowing into it during drilling operations.

In some places on the continental shelf, however, the formation is so fragile that it will fracture if heavy mud is used. Fracturing can lead to a loss of drilling fluid, which makes continued drilling more difficult.

Weak ceiling rock
There is a danger of fracturing of this sort in one area of the Gullfaks field, because a ceiling formation that was destroyed by the extensive use of water injection lies over parts of the remaining petroleum reserves.

However, this does not mean that the oil will have to be left in the formation. Underbalanced drilling has enabled Statoil to send a production pipeline down to it without destroying the weak ceiling rock.

“Light” drilling
A lighter type of drilling fluid is employed in underbalanced drilling. The return flow of mud is also led through an adjustable valve, which allows the drilling crew to modify the well pressure quickly during drilling.

This enables the well pressure to be kept high enough to prevent the well from collapsing, but not so high that it will fragment damaged formation strata. Although oil and/or gas can now enter the well during drilling, this happens under controlled conditions.

Complete control
During drilling operations of this sort, the crew needs to have complete control of the pressure produced by the drilling fluid. The contribution of the SINTEF scientists to the drilling operation was a set of numerical simulations that could predict the pressure within a well while it was actually being drilled.

Statoil’s Johan Eck-Olsen explains that this type of computer model improves the likelihood of success on the first attempt. “This is no small gain, considering that a day of drilling costs two million kroner”, as he points out.

Successful premiere
When underbalanced drilling had its Norwegian premiere on Gullfaks, no extra trials were necessary. Halliburton, a company with long international experience of underbalanced drilling, assisted Statoil. The underbalanced operation was carried out in collaboration with Statoil’s drilling contractor Prosafe.

Eck-Olsen says that underbalanced drilling will be an important technique for Statoil, both in other Norwegian fields and abroad.

Contact:  Rolv Rommetveit, SINTEF Petroleum Research