Milling of Scale to Retrieve Safety Valve  

November 2007

The buildup of scale deposits is an operational challenge to maximizing the production whether they are located in the reservoir, in the perforated interval, in the production tubing, including the SSV (Subsurface Safety Valve), or in the surface process system. 

 

In the North Sea, scales consist mainly of either sulfates, carbonates or a mixture of these two. The precipitation of scale deposits is difficult to predict and many parameters must be taken into consideration. In normal depleted reservoirs, the scale forming at SSV depth usually consists of more carbonate scale than sulfates. This problem with scale buildup is in fact a costly and energy-consuming process to remove.

 

At the Norwegian field called Gullfaks, one well had scale form on an insert SSV at 1,715 ft. The operator, StatoilHydro, performed several traditional slickline and pump down attempts without being able to pull the insert valve or make the valve stay open during production. The contingency method would have been to utilize coiled tubing, but in this case the mobilizing costs were excessive.

 

Instead, StatoilHydro chose the Well Tractor and the Well Miller to remove the scale deposits. Using a newly designed scale milling bit, the Well Miller milled from 910 ft to 1,715 ft in one day.

 

Achievements

• Reduced costs due to light intervention method
• Job completed 1 day ahead of scheduled time
• Reduced required personnel
• Total operation cost reduced by 20 Mill NOK (≈ $ 3 mill) compared to coiled tubing
• Eliminating heavy lifts
• Reduced operational hazards
• Short rig-up time
• Well quickly back on production