Milling Through an Inoperable Ball Valve at Record Depth Provides for Successful Completion of an Ultradeep Well in the Gulf of Mexico

Abstract

Abstract The Tahiti Field is located in the Green Canyon area of the Gulf of Mexico in water depths in excess of 4000ft. The discovery well was drilled in 2002 in Block 640 and, following appraisal drilling, was subsequently re-entered and successfully tested in 2004. Development of the field followed in 2006 with subsea completions from a drilling vessel but the combination of the water depth, the ultra deep producing zone and the high reservoir pressure presented some major hurdles to the field's exploitation. Because of the weight of the completion fluid being used (14.8 ppg Zinc Bromide), fluid loss to the formation during completion was a major consideration. To prevent this, a ball-type fluid loss control valve was routinely set in the top frac packer assembly in the production liner, above the producing zone, prior to running the tubing. In January 2007 a well in Green Canyon block 596 was drilled and prepared for completion when the ball valve was damaged which prevented its mechanical actuation process from functioning and put the successful and economic completion of the well in jeopardy. After a review of the alternatives it was decided to perform a milling operation to mill as large a hole as possible through the ball valve and thereby provide access for production. A bottomhole assembly had been designed as a contingency, based on the use of suitable milling technology and a positive displacement motor (a first for this proposed operation), and it was mobilized to the rig, on an urgent basis, along with the required drill pipe work string. The milling job was successfully performed, to drill a 3.800in ID hole through the ball, in a time frame of 2 hours from reaching bottom with the mill, at a considerable savings when compared to other alternatives considered. In this paper the authors will describe both the completion techniques used and the decision process that resulted in undertaking the milling job. They will go on to describe the procedures and equipment used to achieve its successful completion. Though such a milling job had been performed before, it had never been attempted at anywhere near the record depth of approximately 26,000' and bottomhole pressure in the order of 20,000psi at which the valve was set.