Drilling costs account for roughly half of a field’s total development costs. Bringing that number down has been the target of a lot of research and development in recent years.
While the oil price crash has slashed day-rates for even the most sophisticated drilling rigs on the market, most experts agree that technological advances will do more to rein in drilling costs over the long term than fluctuations in the drilling services market.
That accounts in part for renewed interest in slender wells. Advocates say a substantially smaller well diameter could provide significant savings.
The technology is one of the target research areas at DrillWell, the Norwegian research partnership that includes the Research Council of Norway, International Research Institute of Stavanger (IRIS), the University of Stavanger, SINTEF and the Norwegian University of Science & Technology (NTNU) in Trondheim.
Some of DrillWell’s early research supports the slender well concept as a way to reduce the amount of casing, drilling fluids and cement required during drilling and well construction. The technique could also greatly reduce the amount of cuttings that have to be recovered at the surface, reduce carbon dioxide emissions, make use of less bulky blowout preventers (BOPs) and marine risers, and be carried out by lower-specification, less expensive drilling units.
A typical exploration well begins with a 30-inch diameter conductor. A slender well may start with the same conductor size but reduces some of the intermediate casing stages or casing diameters required to reach target depth.
“A slender well is simply about reducing the clearance between each casing string,” says NTNU professor Sigbjorn Sangesland, who heads up DrillWell’s slender well project. In Norway, seven-inch liner with seven-inch completion string has been more or less a standard, he says.
“We may end up with the same seven-inch liner, and start with the same 30-inch conductor, but then run a smaller casing — not necessarily the standard 20-inch, but go directly to 14, then 11¾ inch, then 9 5/8 to seven-inch.”
In many field developments, slimmer completion string of five-inch diameter or less will be sufficient, he says. Reducing the casing diameters allows for increased pressure rating of the tubular. This allows for increased use of liners without tie-back string to the seabed wellhead, which reduces operational time and steel consumption.
Advances in top-hole drilling using return mud recovery, managed pressure drilling (MPD) and seismic-while-drilling have made it more feasible to safely increase the length of open- hole section and reduce well diameter, Sangesland says.
“The operational time is the most important factor for well cost reduction. Savings in consumables like fuel, steel, mud and cement is another issue contributing to significant cost savings.”
But that has not eliminated industry resistance, which stems in part from the fact that the vast majority of rigs are based on the 18¾-inch BOP and 21-inch riser standards. Smaller BOPs with a 13 5/8 and 11-in diameter exist, but are relatively rare.
Drilling cost savings of 30% or more could eventually turn the tide, Sangeslan says. “For a slender well, you can slim down the BOP to 13 5/8-inch. Then the weight is significantly reduced, you can reduce the size of the riser accordingly, and the rig can be smaller. You may use a lower specification rig to drill a deepwater well.”
Another issue is the risk that, if a problem in the well arises, the smaller diameter casing allows fewer options to intervene.
“The pressure, geology and hole condition dictate how many casing and points you need to reach the target. And if you run into problems you may not have enough contingency casing, or end up with something smaller than initially planned at the target.”
But that would be an uncommon occurrence, Sangesland says. As the cost savings in a multi-well drilling campaign add up, so what if one well out of many fails to reach its target? The risk could be reduced with downhole diagnostic tools compatible with smaller diameter wells, an area ripe for further research and development.
R&D needs
The opening of previously unexplored Barents Sea licences in Norway’s 23rd licensing round has sparked interest in a number of enabling technologies, including slender wells.
Last year, DNV-GL prepared a report for the Norwegian industry group OG21 that laid out the technology challenges to year-round oil and gas production in the northernmost Barents Sea blocks on offer (the licences are expected to be awarded in the first half of 2016). Among the report’s recommendations is the development of slender well technology.
While the OG21 report is specific to Norway, many of its conclusions apply to northern oil-producing offshore areas in the US and Canada, says Per Olav Moslet, senior principal engineer at DNV-GL and the report’s lead author.
“In parts of the Barents Sea where we operate now there is so little sea ice that it is possible to do both exploration and production well drilling in what we could call the open-water season, when there is no ice on the water,” Moslet says.
“That makes things easier in respect to choice of drilling units, because there is a much bigger market of units that do not have the capability to operate in ice conditions. As far as we’ve seen with this analysis, it is possible to use generic units like semisubmersibles that are already on the market.”
However, costs tend to escalate in remote areas. “I think a challenge from a cost side is the mobilisation time required in some of these Arctic areas. You need to bring the unit up to the location. But that is easier in the Barents Sea than some places in the Chukchi Sea, which is really remote from port facilities and infrastructure. The Barents Sea is not so remote.”
Per Jahre-Nilsen, a drilling expert at DNV-GL, says cost is the primary driver behind the renewed interest in slender wells.
“It’s definitely a hot topic for the Barents Sea, and there are a number of reasons for that. One of them has to do with cost,” he says. “There is tremendously less volume that you need to drill out while using a smaller diameter bore when you drill.
“That will result in less cuttings coming out of the bore that you need to handle. You use less drilling fluid when you drill. These are all aspects that will affect load and weight on the mobile unit that you use.”
Slender wells could improve logistics as well. The need for less casing and fluids means a reduction in the number of supply vessels needed to support a drilling campaign.
“I have seen studies that claim slender wells can reduce costs by as much as 50%,” Jahre-Nilsen says. “I think this is something that can be attributed to the volume of steel casings and liners needed as well. This is quite a lot.”
Among the challenges, he says, is “the number of actors in the field delivering these services. That is currently a bit limited.” Another is the requirement to be prepared to drill a relief well if it is necessary to kill a well.
With the shallow reservoirs common in the Barents — 200 to 300 metres below the mud line in water depths of 200 to 300 metres — it could be difficult to pump enough kill fluid through a slimmed-down well fast enough. A new proven technique should be on the industry’s agenda, he says.
Business case
There are other technical hurdles to overcome, including the challenge of maintaining the proper torque while drilling with thinner drill string. A typical slender well might be drilled with a six-inch bit and a drill string with a diameter of four inches or less, Sangesland says.
“You can imagine, with these monster rigs and top drives, it is easy to twist off this inner string,” he says.
“I think it’s a question of better control through instrumentation and drilling automation. Well bore hydraulics may also cause limitations regarding hole cleaning in small diameter bore holes. However, proper selection of casing programme, drilling fluids or using MPD can overcome this challenge.”
At the moment, he adds, the logging tools that can be deployed in a slim bore hole may not be as advanced as those made for standard 8½-inch holes. But developing them is simply a matter of industry will.
“There are no real technology gaps that prevent us from using this technique,” he says.
Slender wells may prove especially suited to the Barents Sea’s shallow reservoirs. “Since the distance is so short, you will need fewer casing points to reach the target,” Sangesland says.
“In this case a six-inch hole to the reservoir with a five-inch liner would most likely be sufficient. And since the distance is so short, well, you start with a traditional 30-inch conductor and you may be able to drill to top of reservoir in one go.
“Let’s say you start with a 30-inch conductor, drill an 8½-inch hole, set a seven-inch casing to the top of reservoir, then drill reservoir with a six-inch hole. Then you’re talking two casing strings after the conductor casing is set.” In this well plan, a much smaller BOP such as an 11-inch standard BOP could be put to use, as well as a slimmed-down drilling package and lighter rig.
Slender wells could also enable extended reach and horizontal drilling where shallow reservoirs make it difficult to achieve sufficient low radius of curvature.
The same applies for relief well drilling, Sangesland says. “A small diameter, low radius of curvature borehole may be the only viable option to intersect a blowing well in these areas.”
Even a field developed with vertical wells could benefit, and for relief well drilling, smaller diameter borehole may be the only viable option Sangesland says.
“There are a number of reservoirs located a few hundred metres below the seabed. It has been suggested that it would take many wells to drain these reservoirs. This could be a business case for drilling this type of well.”
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