Steerable drilling liner technique speeds progress through unstable formations
Nick Terdre
Contributing Editor
Statoil has successfully piloted steerable drilling liner (SDL), a new method to drill unstable formations which opens the way to significant time and cost savings. The method has been trialed in recent months on both the Brage and Statfjord fields in the Norwegian North Sea.
The technique, developed by Statoil with equipment supplier Baker Hughes, involves drilling and setting liner in the same run, with the liner shoe attached to the drilling assembly behind the reamer bit. Conventionally, the two operations are separate – first a hole section is drilled, then the drillstring is pulled and the liner is run. Shallow well tripping may only take a couple of hours, but in a deep well it may be two to three days’ work. And the trend these days is towards deeper wells.
The other major benefit comes when drilling in unstable formations with a danger that the well wall may collapse before the liner is set. SDL minimizes the time the well wall is at risk.
“Setting liner while drilling means we do not have to leave the borehole open and minimizes the risk of collapse,” says Geir Slora, head of drilling and well, Statoil.
This is important to Statoil because reservoir formations tend to become unstable as the reservoir is depleted, and many of the fields operated by the company in the Norwegian sector are at an advanced stage of depletion. In the case of the two fields on which the pilots were run, Statfjord has been in production since 1979 and Brage since 1993.
Drilling new wells is critical to prolonging the productive life of these fields, and SDL could boost returns from the company’s enhanced oil recovery efforts.
HP/HT depletion issue
Formation instability can also arise relatively early in a field’s producing life. The problem can affect high-pressure/high-temperature fields, some of which have undergone rapid depletion accompanied by a swift decrease in pressure in the early production phase.
One case in point was Statoil’s HP/HT Kristin field in the Norwegian Sea, which prompted the search for a new solution, says Arne Torsvoll, project manager for well construction and intervention, Statoil. Kristin has only been onstream since 2005.
A drill-in liner system already existed, but as it could only be applied to drilling very short intervals and lacks steering and logging capabilities, it was too limited for Statoil’s purposes. The SDL system consists of standard drill pipe as the inner string and a conventional rotary steerable system bottomhole assembly with full steering and logging capabilities, making its suitable for drilling long, complex 3D well trajectories with the same directional and logging capabilities as conventional drilling.
The liner is run into the hole as an outer string. The outer and inner strings are connected via a running tool located at the top of the liner. The running tool transmits torque required to rotate the liner and the axial force to run the liner into the hole and pull it out. It can be released by a hydraulic mechanism.
A unique feature of the system is that the BHA can be changed while the liner is in the hole. The liner running tool can be released by deactivating the reamer drive sub, after which the inner string can be pulled.
The first downhole tests were carried out at Baker Hughes’ test rig in Tulsa, Oklahoma. The objective was to ensure that the system specifications and functions were fully qualified ahead of the offshore pilots.
The Brage pilot was conducted in well 31/4-A-13A in April 2009, when a steerable drilling liner was used to drill and set liner in a 1,300-m (4,265-ft) section of 9 5/8-in. hole. The Statfjord test, which was last December, involved drilling and setting liner in a 280-m (918-ft) long section of 7-in. (17.8-cm) hole. The technology is therefore considered qualified for these hole sizes.
On Brage the test took place in three stages. First the section was drilled to total depth, then the inner string was pulled and the backflow valve run to the bottom of the liner, and cement pumped. Finally the drillstring was pulled and the liner pack-off set.
For the Statfjord pilot, this procedure was reduced to two steps, with the setting of the liner pack-off implemented in the same run as the cementing of the liner. The final aim of the technology development is to perform all the operations in a single run.
The rate of penetration was the same as in conventional drilling, and steerability was good, Torsvoll says. There were concerns beforehand over potential difficulties with the liner connections, but operations went smoothly with conventional liner.
During the Brage pilot, the running tool accidentally parted, leaving the inner string and the liner downhole. Two fishing trips were needed before the inner string and the BHA could be re-connected to the liner. Release of the running tool also proved problematic. The running tool has since been redesigned to avoid the recurrence of these issues in future applications.
Statoil now plans to pursue further opportunities to apply the new method, Torsvoll says. These will include trialing it in larger-diameter sections and also deployment from a floating rig, as several of the fields to which it is relevant, including Kristin, have been developed with subsea wells.