World’s First 218 Scale Milling Job - Asia’s First Scale Milling on Wireline  

October 2008

A dual completion oil producing well offshore Malaysia had been having CaCO3 scale build up problems since the well was put on production in 2004. In the middle of 2008, the scale had accumulated severely in the short string of the well, almost choking oil production. It had reduced the 3-1/2” x 9.2 lbs/ft tubing internal diameter from 2.992” to approximately 1.50” all the way to the end of tubing. In total, the expected scale interval was 2,522 ft, which had reduced Q oil from the short string from an average of 2,000 bbl/day to almost 70 bbl/day.

 

Due to the limited deck space and crane capacity on the platform, mechanical milling of the scale by coiled tubing was not an option. The operator chose mechanical milling on electric wireline utilizing a combination of the Well Tractor and Well Miller with a customized 2.70” outer diameter scale milling bit. The decision to apply a 2.70” outer diameter scale milling bit was taken due to the permanent minimum restriction of 2.813” in the well.

 

During the scale milling operation, the well was being flowed with a choke size of 22/64 equivalent to a flow rate of approximately 120 bbl/day to flow the debris to surface as well as for heat dissipation purpose at the mill bit.

 

Achievements

•	An actual total of 1,631 ft CaCO3 scale interval was milled (vertical scrapping and rotational milling)
•	Total time from rig up-rig down: 14 hours
•	Time savings: Approximately 4 days (compared to CT)
•	A path with an internal diameter of 2.70” equivalent to the scale mill bit outer diameter was obtained
•	Increased oil production: 36 bbl/day (Immediate additional gain post scale milling)

The open path enabled further slickline operations such as tubing broaching to scrap remaining scale in the tubing or maintain the open path for the remaining production life of the well followed by changing out the gas lift valve. Q oil is expected to be further increased by another 300 bbl/day after changing out the gas lift valve.

Well Data

End of tubing of the short string:	8350 ft MD
Maximum deviation:	61.08° at 4,300 ft MD
Minimum restriction in the string:	2.813” at the X-Nipples
Bottom hole temperature:	241° F
Scale thickness:	Approximately 0.1” – 0.7”