In late 1996, BP Norge UA deepened two wells on the Ula platform with a 33/8-in. slim hole turbodrill on coiled tubing. Neyfor-Weir, a subsidiary of the Weir Group PLC, Glasgow, U.K., manufactured the turbodrill.
The Ula field, discovered in 1976, is in the southern part of Norway's North Sea sector and has three conventional steel platforms (processing, drilling, and accommodations) in 30 m of water. Ula, during 1996, produced about 47,000 bo/d and 58 MMcfd.
In the first well, the 10-m long turbodrill powered the bit through about 158-m (518 ft) of new formation below the well's original 5-in. completion string.
According to Neyfor-Weir, after drilling the cement shoe and rathole, a single slim hole turbodrill successfully drilled the section to the logging point without any operational difficulties.
Previous coiled tubing drilling attempts in the North Sea, according to Neyfor-Weir, had to be abandoned after repeated motor failures, attributed to high bottom hole temperatures, in excess of 150° C. (302° F.).
A second well on Ula has also been deepened with the turbodrill.
Drilling procedure
In Well AO7A, the 5-in. casing shoe had been set at 3,893 m (12,773 ft) in the Jurassic Ula sandstone. Well deviation was 18°.
The coiled tubing unit had 5,095 m (16,716 ft) of 2-in. tapered tubing. The drilling fluid was treated seawater, pumped at about 2 bbl/min.
Neyfor-Weir describes the bottom hole assembly as consisting of a surface-diamond bit, turbodrill, measurements-while-drilling (MWD) tools, jar, circulating valve along with the required coiled tubing disconnect, valves, joints, etc.
After the cement shoe, rathole, and 1 m of new formation were drilled, the turbodrill continued drilling for another 158 m through the Ula, Bryne, and Triassic formations that were mostly interbedded mudstones and sandstones, according to Neyfor-Wier.
Neyfor-Wier says the average penetration rate was 7.5 m/hr (24.5 ft/hr) and was significantly higher than the penetration rate with positive displacement motors (PDM) over the same formations.
Turbodrill
Neyfor-Weir describes the turbodrill as consisting of an all-metallic construction capable of drilling in temperatures of over 200° C. (392° F.). It says unlike PDM and vane motors, the turbodrill does not require sealing between the rotor and stator drive elements.
The 10-m turbodrill has 150 drive stages that run concentrically. Neyfor-Weir says that this allows for smooth vibration-free operations, ensuring a clear signal from the MWD. It also describes the drive stages as having a varying pressure characteristic (pressure profile) so that the onset of tool stalling can be easily detected at surface; although, unlike other drilling motors, stalling does no harm to the turbodrill.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.
