Westwood Insight: TTH, an important step but is it a game changer?
Through-tubing rotary drilling — known in the industry by the abbreviation TTH — has been generating steady interest as a potential route to cheaper intervention and sidetrack operations on mature fields. The technology allows operators to drill through existing completions without pulling the production tubing, saving rig time and avoiding the cost and risk of a full workover.
On paper, the economics are attractive. A TTH sidetrack can be executed in roughly 60% of the time required for a conventional operation, with cost savings of 30-40% on a like-for-like basis. For mature North Sea fields running marginal economics at current oil prices, that kind of saving could be the difference between drilling and not drilling.
Where it works
TTH has been deployed successfully in the Middle East and onshore US, where wellbore geometries tend to be more forgiving and the operating environment less demanding. Saudi Aramco has drilled several hundred TTH sidetracks over the past decade, primarily to access bypassed pay in mature carbonate reservoirs.
The North Sea is a different proposition. Deeper water, higher pressures, more complex wellbore architectures, and tighter regulatory requirements all raise the bar. The first North Sea TTH operations, conducted on the Norwegian Continental Shelf in 2016-2017, showed promising results on relatively straightforward wells but encountered complications on more challenging candidates.
The limitations
TTH works best in specific circumstances: vertical or low-deviation wells, reservoir targets within a few hundred metres of the existing wellbore, and formations that don't require large-bore completion hardware. For the growing proportion of North Sea wells that are high-angle or horizontal, with complex multi-zone completions, TTH remains a poor fit.
There is also a workforce challenge. TTH requires a different skill set from conventional drilling. The number of crews with genuine TTH experience is small, and training takes time. Until the supply of competent operators catches up with demand, deployment will be constrained.
Important, not transformative
DW's view is that TTH will become a useful addition to the mature-field toolkit but is unlikely to be the step-change that some advocates suggest. We estimate the addressable market at roughly 80-120 candidate wells per year across the North Sea, generating incremental expenditure of $300-500 million annually by 2020.
Useful? Yes. A game changer? Probably not. The real gains in mature-field economics will come from broader improvements in well construction efficiency, digital surveillance, and production optimisation — unglamorous work, but work that applies across the full well population rather than a narrow subset of candidates.
Steve Robertson, Drilling & Production Analyst, Douglas-Westwood