A new application of well tractor robotics in combination with well milling technology has been proven in the UKCS.
In June 2007 on a North Sea platform, well tractor robotics in conjunction with a well milling rotational unit, using an electric wire line cable, were used to remove a cast iron bridge plug at 16,000-ft.
A cast iron bridge plug run for a water shut off was set at an incorrect depth. Through evaluation, milling tests onshore confirmed that the concept of using a well miller with a well tractor was viable. The total operational time from rig up to rig down was 67 hours, with an actual milling time of 16 hours. During this unique operation all milled material was recovered and the challenges of controlling rate and milling penetration were achieved.
This new application for milling well completions hardware and other well bore obstructions offers a cost efficient alternative technology to existing methods. The proven technology, normally used for production logging in highly deviated wells, also offers HSE benefits and logistical advantages.
The article will describe the actual process from testing to completion of the offshore operation.
The East Brae is predominantly a gas condensate producing platform. E21 is a gas producer and a PLT run in March 2007 indicated water ingress from the lower perforations at 16,295 ft. From this data it was proposed to set a permanent bridge plug at 16,280 ft in the 5" liner. During April 2007, an attempt at setting a cast iron bridge plug was thwarted by a gear box failure on the wire line unit.
The toolstring fell over 50ft before being brought under control with the drum brake of the wire line unit. However the inertia associated with the freefall of the toolstring and the rapid deceleration associated with the braking, allowed the permanent bridge plug to preset at 16,200ft mdrkb. Several attempts were made to pull the tool string free but there was a concern that the wireline weak point would release leaving a toolstring in hole. As a result, a decision was made to set the bridge plug which would allow retrieval of the tool string and leave only the bridge plug down hole (Figure 1).