Statoil strives for improved yield through accurate wellbore placement

Baker Hughes Inteq's rotary closed loop system is one of several rotary steering drilling systems close to commercial application. A few years ago, the 6,863-meter Gullfaks West well 34/10-B29 just missed its final target. With today's technology, it would probably have hit it. Developing the ability to drill accurately in thin oil zones is one of Statoil's current priorities.

Developing the ability to drill accurately in thin oil zones is one of Statoil's current priorities.

The company operates a number of undeveloped fields containing thin oil zones, including Tyrihans North, Heidrun North and Snoehvit. Such zones are also to be found in fields under development, such as Smoerbukk and Midgard (both part of the Asgard project), and in the Sleipner West producing field.

Production of the oil zones should enhance field economics. As thin zones are mainly found in fields containing predominantly gas and condensate, they must ideally be produced early, before gas production lowers the reservoir pressure.

The ability to precisely place a wellbore is also applicable to the later years of a larger field's life, when many small pockets of reserves remain to be vacuumed up - if it can be done economically. Statoil's Gullfaks and Statfjord Fields fall into this category.

For new fields, particularly the smaller ones, issues of drilling and well completion are crucial to getting good economics, according to Helge Tjoetta of Statoil's drilling engineering sector.

One of the important factors in thin oil zones is to reduce the uncertainty in well positioning. There are many sources of uncertainty. For example, there are uncertainties associated with different ways of interpreting a seismic map. Again, when a wellhead is placed on the seabed, its exact position is not known to the nearest metre. Nor is the exact trajectory of a borehole known.

The magnitude of survey uncertainty can be very large. In extended horizontal drilling, the location of a horizontal section could have an uncertainty in lateral position of up to 100 metres, Tjoetta says. This was the case when Statoil drilled seven- and eight-km extended reach wells from Statfjord C.

It has now often become customary to drill a pilot hole to locate the top of the reservoir. The hole is then plugged back and deviated into the reservoir. To counter inaccuracies in the readings of downhole geomagnetic survey instruments used to track the course of the borehole, Statoil is pursuing a system that can be correlated against readings taken from a monitor placed near the wellhead.

But geo-magnetic readings are influenced by the varying magnetic field strength, and a magnetic storm can disrupt efforts to position a borehole in the desired location. Hence work is under way to develop gyroscopic MWD techniques, which will not be influenced by the magnetic field and has a higher accuracy.

New error models of the survey instruments have also been developed to achieve greater accuracy of measurement. The current model was developed several years ago for tracking simple well-paths and further work is needed to suit it to the more complex needs of current horizontal well technology.

Statoil is also actively involved in current industry efforts to refine horizontal drilling techniques. It has been working on the development of a seismic geo-steering tool for several years, first in-house and latterly with contractors. One outcome is the POSLOG geo-steering tool, which Baker Hughes Inteq is now developing into a commercial product. In June the prototype is due to undergo trials on a test field in Tulsa.

POSLOG represents a significant addition to the drilling armoury, giving feedback on the geological conditions around the drill-bit to a distance of 20 metres, compared with around four metres for currently available tools. It tells you where you are in relation to the formation, which is the important thing, Tjoetta says.

Statoil has also been closely involved in the Advanced Wells project which is part of the EU's Thermie program, and has played a leading role in the drilling and completion work, including the development of rotary steerable drilling techniques.

Six rotary steering drilling systems have already undergone full testing. Among those which have emerged with success, and are now close to commercial application, are Baker's Rotary Closed Loop System, the Cambridge Drilling Automation and Camco's D&S Steerable Rotary System.

The Baker system has been successfully tested on Agip's Cortemaggiore Field in Italy, where it was used to locate a horizontal bore within one metre of the target. Prototypes of both the Cambridge and Camco systems were expected to be ready for testing in March.

These technological advances give the oil companies much greater control over the optimal placement of their well-paths. They will be particularly important for the drilling of horizontal and multilateral wells.

Statoil has five multilateral wells planned for this year, and sees great potential in them for improving recoverability. In the future, when the necessary expertise is fully developed, a large number of production wells are likely to be multilaterals, Tjoetta says.

On the large producing fields, coiled tubing drilling will be the technique used to access small pockets of oil, enabling sidetracks to be drilled from existing wells without pulling the tubing. It will be used in conjunction with 4D seismic techniques, which will enable the exact location of remaining oil pockets to be identified

Statoil spudded its first CTD well on Statfjord in February.

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