A downhole electro-mechanical power-unit tool was used to provide anchored power to pull crown plugs in a single run and in reduced time. This setup has the capability to pull subsea crown plugs that often require greater force than is possible with traditional conveyances without requiring time and resources to reduce hydrostatic pressure. In deepwater environments, reducing time can significantly impact costs because of the extra equipment and logistics necessary to properly operate in those conditions. The downhole electrical power generator tool proved to be a viable and cost-effective option.
A Gulf of Mexico job for Statoil required the removal of the upper and lower crown plugs just below 8,000 ft with 10.6 lbm/gal fluid in the riser. While on location, the crew ran a conventional pulling tool to attempt pulling the upper crown plug. While latched into the plug, an integrated workover control system (IWOCS) pumped fluid in between the two plugs to decrease the differential across the plug to assist in pulling. The crew was unable pull the plug on two separate attempts. A 3.59-in. extended-stroke downhole electro-mechanical power-unit tool with a crown plug latch tool capable of an effective stroke of 36 in. and a linear pulling force of 60,000 lb was the program's contingency. It was deployed and pulled the crown plug.
There were indications that the IWOCS system had failed, and the downhole electro-mechanical power-unit tool pulled the upper crown plug with the full differential of the hydrostatic head. A conventional pulling tool was used to attempt to pull the lower crown plug, again without success. The downhole power tool was then used to retrieve the lower crown plug successfully.
In a subsea tree, crown plugs are used to isolate the wellbore from the environment. Often, one of the first required tasks of a subsea well intervention is to pull the subsea crown plugs from the wellhead to gain access to the wellbore. Hydrostatic pressure associated with fluid in the riser creates a large pressure differential across these wellhead plugs that seal the cross-sectional area of the tree. If the plugs cannot be removed from the profile conventionally, the fluid is displaced to lighten the hydrostatic head before the plug can be pulled. This operation requires a minimum of 24 hours of rig time.
Slickline and coiled tubing have limited constant pulling force because of the finite-strength limits of the conveyance. Deep water and debris often compound the required pulling force. These forces are well above the tensile-strength limit of slickline wire and even the pulling strength of coiled tubing. A subsea wellhead plug requires a steady pull along the entire length of the sealbore.
Conventional slickline methods are limited to creating extremely high, but short-duration, impact loads; however, brief impact loads are not suitable because the seals tend to reseat after each impact and are forced back on seat by hydrostatic pressure from above. Therefore, using mechanical or hydraulic jars to simplify the delivered force does not effectively retrieve the plug from the wellhead.
