Through tubing rotary drilling (TTRD) should help Statoil lift recovery from its many subsea fields. The company has developed the technology with FMC Kongsberg Subsea (FKS), and expects its first application offshore in 2006.
Statoil operates around 300 subsea wells, which account for more than half its overall production on the Norwegian shelf. But its subsea recovery rates vary widely, from a mere 20% in some wells on Gullfaks to over 55% on Norne. The average across all the wells is 45%, however the target is to raise that figure to 55% by 2008, equivalent to extra production of 1.4 Bbbl. To this end, Statoil is working on a raft of technologies in addition to TTRD under its Subsea Improved Oil Recovery (SIOR) program, including subsea processing and light well intervention.
TTRD makes it possible to enter a well and drill a sidetrack without having to pull the tubing, saving considerable time compared with sidetracking by conventional means. In March, Statoil signed a contract under which FKS will provide TTRD equipment and services over a two-year period, via its affiliate FMC Production Services.
TTRD should help reduce the cost of constructing new drainage points in existing subsea wells, according to Steinar Strøm, SIOR project manager. The eventual target is to drill a sidetrack for NKr60 million. Using conventional methods, the cost can exceed NKr100 million, but even that is substantially less than the NKr250-300 million set aside typically to drill a new well.
Rig hire accounts for the bulk of the drilling cost, especially with day-rates being exacerbated by high utilization rates. Reducing drilling and completion times is therefore a priority. Statoil plans to use the semisubmersibleStena Don, which it has taken on a long-term contract, with dedicated offshore and onshore teams. As they gather experience, the teams should improve their performance until they can deliver an optimized TTRD operation.
Stena Don will employ its dynamic positioning system for station-keeping, saving around three days taken up otherwise with anchor-handling operations. However, TTRD technology may also be applied from anchored mobile drilling units, Strøm says.
Statoil's concept for through-tubing rotary drilling, for which it plans to use semisub Stena Don.
The TTRD equipment spread should also help speed up operations. For example, the time spent preparing for drilling once the rig is on location can probably be halved, equivalent to a saving of three or four days. A major factor will be a new design of blow-out preventer (BOP).
BOP handling is time consuming, involving installation of a conventional drilling BOP and marine riser from the rig’s moonpool to the wellhead, followed by assembly of the inner string and subsequent lowering through the riser/BOP to facilitate plug retrieval and well-killing operations.
The BOP selected for the TTRD program is based on a design developed by Aberdeen-based Enovate. It weighs only 60-70 tons, compared with around 200 tons for a conventional drilling BOP. And unlike the rams on conventional BOP actuators, which are mounted horizontally, the Enovate BOP uses circular vertically actuated rams, which in turn give it a circular shape. This allows running of the BOP through the rotary table on the drill-floor, avoiding weather-sensitive moonpool operations. The net effect is a two-day saving on BOP running/retrieval.
The new BOP assembly has a split design - the section containing the safety devices and cutting functions is on the seabed, while the work rams used in normal operations are in or below the splash zone. A high-pressure riser fitted with a telescopic slip joint connected the two parts to absorb rig motions. Use of a high-pressure riser is a new concept for BOPs, Strøm says. Coincidentally, Statoil already had a suitable riser in stock.
The downhole drilling operations will be conducted as normal, but with smaller diameter pipe. In the case of wells with horizontal trees, 3 1/2-in. drill-pipe will be run down to the point where the sidetrack is to be drilled, and the new borehole will be 5 3/4-in. For wells with vertical trees, the drill-pipe will be 2 7/8-in. and the borehole 4 3/4-in.
The TTRD equipment package should be delivered by next March, with TTRD operations under way by summer 2006. Three wells in the Halten/Nordland area of the Norwegian Sea are possible candidates, but a final decision will be made nearer the time, Strøm says.
Statoil classified TTRD as a medium intervention operation. Some of the equipment, such as the lower riser package, may also be used for other well intervention operations. FKS has previously supplied light well intervention equipment and services for Statoil, using Prosafe’sRegaliaplatform. Earlier in the project, the partners looked at extending this equipment to the TTRD package, but eventually opted instead for a newbuild spread.•
