Through-tubing drilling and completion is a cost-effective technology for increasing tail production, and shows promise for improving oil recovery from subsea fields.
This through tubing drilling and completion (TTD&C) technology permits offshoot wells (sidetracks) to be drilled sideways from a parent well by cutting through the production liner and sometimes through production casing.
Since sidetracks are drilled below the production packer, with the drill pipe conducted through the tubing, neither tubing nor Christmas tree need to be removed.
Such wells are particularly useful for accessing pockets of isolated oil and gas in mature fields approaching tail production.
It is also possible to minimise borehole lengths and avoid drilling problems in overlying formations by sidetracking in reservoir intervals.
Statoil currently uses small-diameter but conventional drill pipe for this purpose, which reduces the cost per operation to about USD 4 million, including completion and perforation.
This represents a cost benefit of about USD 1.5-3 million compared with conventional sidetracking.
The exact value depends on the platform in question, the distance or well path taken to the target, and the completion solution.
Having completed 12 TTD&C operations – seven of them in 2003 – Statoil is the leading player with this solution in the North Sea.
Recent technical advances also show that open (uncased) holes might be extended to 2,000 metres using present methods, and that multilaterals can be economically installed. This permits simultaneous production from a parent well and its sidetrack.
The next challenge is to extend the technology to improve oil recovery from Statoil’s subsea developments.
With an estimated saving per operation of around USD 6-10 million, the benefit of TTD&C for subsea wells promises to be even greater than for platform-based drilling.