STATOIL CLAIMS WORLD RECORD IN EXTENDED REACH

Innovative downhole engineering lies behind Den norske stats oljeselskap AS's claim to a world record for extended reach drilling.

A combination of techniques enabled Norway's national oil company to control downhole drag and torque so successfully that a new mark of 23,917 ft horizontal reach could be set Jan. 26 with its 33/9-C2 well (OGJ, Feb. 1, p. 24).

Statoil drilled the water injection well from the Statfjord platform in the Norwegian North Sea to boost production from the northern portion of the reservoir.

The company spudded 33/9-C2 last Oct. 28, achieving the record extended reach depth almost 13 weeks later. Statoil's drilling engineering manager, Oystein Haaland, said key elements in the success were wellbore geometry, friction control, hole cleaning, drillstring design, and field geology.

GEOMETRY

The well's planned profile and 83 sail angle were modeled with computers. The ideal was a catenary profile, the curve achieved if a semisubmersible rig towed a 29,000 ft anchor chain that was fixed at one end.

"The catenary curve gives the minimum side forces for a high deviation well," Haaland said. "We drilled ahead mainly on the basis of measurements of drag and torque taken from surface equipment. It was a matter of matching readings with theoretical curves and making adjustments. Curvature of the well bore was measured downhole."

Reducing friction through choice of drilling fluid also was important. An esther based fluid was chosen, which had a friction factor of about 0.17 compared with about 0.18 for an oil based drilling mud.

"Such a difference may seem small," Haaland said, "but in long holes the friction factor is a very important part of the equation."

Cuttings from esther based muds can be dumped after passing through shakers under Norwegian pollution regulations. Compared with oil based fluids, esther muds are biodegradable and of low toxicity.

HOLE CLEANING

Statoil conducted several experiments to improve hole cleaning on the record well. Rotational speed of the drillstring was the most critical factor. Haaland said the best cleaning was achieved at about 180 rpm.

Because of the linear relationship between torque and the volume of cuttings in the hole, measured torque proved a good indicator of cleanliness of the hole.

"Drillstring design also was crucial," Haaland said. "We had to have reliable equipment for measurement while drilling because shutdowns would be costly with such a long drillstring."

Choice of mud motors was critical for reliability and for helping maintain mud circulation in a 24,000 ft hole. Also, Statoil used relatively few drill collars, having chosen to drill with the drillstring in compression.

THEORY AND PRACTICE

Long experience in drilling Statfjord wells enabled Statoil to build up reliable hole stability curves from a wide knowledge of geology, pore pressures, and formation stabilities.

"This enabled us to balance theory with practice, particularly when choosing mud weights," said Haaland. "You need reliable information on a project like this. We could not have done so well in a new field unless it was due to luck."

The rig was a two-gear TDS 4 Varco topdrive built in the mid-1980s. It was modified to handle 6 5/8 in. drill pipe but otherwise little changed.

Statoil ran 7 in. liner Feb. 8 in readiness for cementing next day. The intention is to complete the well in the next few weeks.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.