An innovative device adopted by Statoil has improved the job of drilling a well which winds through the bedrock like the sinuous curves of a snake.
This rotating AutoTrak drilling machine is designed for use with demanding and twisting production wells that can optimise reservoir drainage.
"We've taken a lead within the industry in adopting new technology to boost drilling efficiency, and were the first to use AutoTrak off Norway," says directional drilling adviser Morten Hyvik in Statoil.
The group found in the mid-1990s that it needed better downhole drilling machines which allowed the angle and direction of a well to be altered more effectively.
So Statoil began to work with suppliers of such technology, and this cooperation has helped to improve drilling processes and enhance their efficiency.
AutoTrak is marketed by the Baker Hughes Inteq oil services specialist, and was originally developed in partnership with Italian oil company Agip.
Statoil has subsequently been involved in enhancing the drilling machine, reports Zvonimir Djerfi.
He is a specialist in such systems for Baker Hughes Inteq. Other companies have subsequently followed Statoil's lead in adopting AutoTrak off Norway, where the device has found its widest application.
Measuring about 10 metres long, the machine is attached to the end of the drill string just behind the bit. An engineer on the surface controls its direction with the aid of a computer. Three steering ribs are mounted on a non-rotating stabiliser at AutoTrak's front end.
Applying pressure to these vanes creates friction with the sides of the borehole, and putting unequal weight on them causes the machine to turn in the desired direction. The computer on the rig communicates with the "brain" of AutoTrak, a minicomputer located behind the steering ribs.
Before the machine changes direction, this brain reports back that the command has been received. By comparison, a conventional drilling machine is driven forward with the aid of a downhole "mud motor", which gets powered by the mud circulated through the well.
Its direction can only be changed by stopping the rotation of the drill string, with just the bit turning. Drilling takes longer and steering the well accurately towards its sub-surface target is often difficult.
By allowing the drill string to continue rotating, AutoTrak has eliminated the steering problems experienced by operators with conventional equipment. The drill bit may have to be directed at a small area measuring just 50 by 50 metres and located no less than 6,000 metres beneath the sea, for instance.
Over such long distances, AutoTrak can control the vertical deviation of a well to an accuracy of half a metre. "That makes this machine an excellent tool for placing the well with great accuracy when the reservoir targets are small," observes Mr Hyvik.
Like a worm eating its way through an apple, the device winds towards its target in response to the commands of the guiding engineer. AutoTrak advances faster than a conventional machine because the drill string continues to turn during steering operations, and time saved is money earned.
But Mr Hyvik notes that a rotating drilling machine is significantly more expensive to operate than a conventional unit. So both operator and contractor must carefully assess the economics of each well.
So savings in time and reduced rig hire must be set against greater hire charges for the equipment. The equation naturally looks most favourable when rig rates are high and accurate well placement is critical.
"We're making profitable use of AutoTrak on such projects as the Gullfaks satellites in the North Sea," says senior drilling engineer Ina Garvik in Statoil.
The Gullfaks area features hard rocks which lie deep and are difficult to penetrate with conventional drilling gear. A rotating drilling machine makes it easier to guide wells to the optimal point in the oil-bearing strata.
Because the machine can be steered continually while it advances, the borehole is effectively cleaned and ends up nice and smooth. Roughly 30 wells are due to be drilled on the three Gullfaks satellites.
With rig hire costing roughly NOK 1.5 million per day, Ms Garvik expects to save up to NOK 276 million by using AutoTrak. She emphasises that this saving shows how good the device is already. Eventually, the number of breakdowns should be reduced - giving a further gain in speed with consequent saving in costs. Tech in Depth.
© 2000 Mena Report (www.menareport.com)