Riserless subsea scale milling in three hours

CLE.SUB.NO.10.06.V1 Riserless subsea scale milling - Well Intervention - Welltec
  • Well type Subsea
  • Water depth 950 ft
  • Max. temperature 307.5°F
  • Well head pressure 2,204 psi
  • Max. deviation 33°


Welltec® has recently completed a successful calcium carbonate scale milling operation in the North Sea at a record setting time. The job was executed on a subsea well from an intervention vessel as a riserless eell intervention.

The operator was experiencing reduced production and was not able to run a production log to total depth to locate the problem. Previously, they had attempted to pass 17,034 ft MD with a PLT but were hindered due to a restriction. According to the operator, the restriction was rather large as they could not pass it with a 2.3” toolstring on previous attempts.


Welltec ran into the well with the Well Tractor® and Well Miller® configured with a novel 5.7” convex bit and tagged the scale at 17,034 ft MD. The toolstring was pulled up 33 ft where the Well Miller was started and then descended gradually until the scale was tagged. For every 1.5 ft of scale milled, the tools were pulled back up 35 ft and flushed with 30 liters/minute with injection fluid from the surface. Milling continued for 18 more feet down to 17,052 ft until there was no indication at surface of milling in scale.

The Well Tractor and Well Miller were stopped and the Bottom Hole Assembly (BHA) ran to total depth (17,142 ft MD) with the Welltec CCL activated. Due to some increased weight passing the scale area on the way out of hole, a decision was made to perform two additional passes with the Well Tractor and Well Miller. This resulted in no increased weight pulling out of hole, confirming the tubing was scale free.


Welltec performed the entire operation on e-line as a RLWI quickly and without incidents.

In only 3 hours of downhole operations, our tools successfully removed 18 ft of calcium carbonate scale. Led to a significant increase in oil production.