By Joanne Liou, associate editor
Utilizing the forces of nature, an electrically powered mid-level riser pumping technology can give existing sixth-generation rigs the capability to drill difficult wells in the next 20 to 30 years, PETRONAS’ Robert Ziegler stated. Retrofitting rigs with the pumped riser provides a dual-gradient solution to drill in challenging deepwater environments. “With hundreds of sixth-generation newbuilds out there that will have to drill all these difficult wells, we need to find a system that is easily retrofittable,” Mr Ziegler stated. The pumped riser technology extends the benefits of riserless drilling by taking away mud overbalance at the mudline in deepwater drilling operations, he explained at OTC in Houston on 5 May.
The pumped riser system re-establishes riser margin, which Mr Ziegler called the panacea of deepwater drilling. “We gave it up because we just couldn’t maintain it from shallow-water floater drilling to deepwater floater drilling. Now, we have a system again where we can disconnect from the well, and we’re not relying on multiple functions working flawlessly in order to shut in this underbalanced well when we disconnect.”
Saipem’s Scarabeo 9 semisubmersible was retrofitted with a pumped riser and was the first successful commercial application of the pumped riser in the ultra-deepwater Gulf of Mexico (GOM) in 2012. The industry had tried to drill wells in about 7,500 ft water depth in similar GOM formations and had never able to reach TD; they were not able to keep the riser full because the formation was very weak. “Oil-based mud was not an option because of logistics,” Mr Ziegler said, and “with water-based mud, you couldn’t get the drilling fluid light enough to maintain circulation.”
With the pumped riser system, the mud gradient can be maintained between the formation fracture gradient and the pore pressure gradient, mimicking the pressure regime on land operations, he explained. “If we have strong enough pumps, we can reduce the mud level enough in the riser to create full dual gradient – same pressure inside and outside the riser and the mudline.”
The retrofitted system on the Scarabeo 9 did not create a full dual-gradient system; however, the system was able to drop the fluid level by about 1,000 ft in the riser. “When we go to pumped riser drilling, we actually use heavier mud than normal,” Mr Ziegler said. “When you design this system, you can design a mud system that covers both the high-pressure scenario and low-pressure scenario in an exploration well. Therefore, you most likely will not have to roll over the mud during drilling of this well, which translates into significant time savings.” In case of pressure influxes, “kill mud” is already in the well so that the fluid in the riser can be raised for overbalance.
The ability to also drop the fluid level according to the pump capacity allows for downward and upward adjustments within 10 to 15 minutes. “We can adjust mud weight by several points instead of having to roll it over, which takes a day or even longer to get the new pump in place on a typical rig with several thousand barrels in the riser.”
For more information, please see SPE 25267, “Dual Gradient Drilling Is Ready for Primetime: The Benefits of a Retrofit System For Better Well Control, Enhanced Water Depth Capability and Flat Time Reduction.”