Extended reach wells are becoming increasingly more common. These wells have long sections of high inclinations when the target reservoir is at shallow true vertical depth (TVD). Justification for this type of well includes increased production efficiency due to more contact with the reservoir and/or reduced capital cost by eliminating the need for additional platforms. These high angle, extended reach wells present difficult design and operational requirements for drill pipe.

Union Oil Company of California (Unocal) is continuing to drill extremely high departure wells (over 10,000 feet), including those from Platform Irene. Problems with drill pipe in earlier wells, primarily due to fatigue effects, served as the impetus for a thorough review of the string design.

This paper describes an integrated approach to extending the useful mechanical life of a drill string in high angle, extended reach wells. This approach draws on engineering analysis, material specifications, operating guidelines and inspection schedules. Illustration of this approach is made through a discussion of requirements for a new drill string to be used in drilling operations on Platform Irene.


To achieve maximum mechanical benefit from a drill string used in high angle, extended reach wells, the pipe must be designed, fabricated, operated and inspected in one interactive program. Engineering analysis provides insight into anticipated stress levels and their locations under a variety of operating conditions. These insights can then be used to control material specifications and operating procedures so that they mitigate or minimize stress-induced mechanical damage. These engineering insights can also be used to establish inspection and maintenance procedures to help prevent costly drill string failures. Data collected during the drilling operations can be used to calibrate the engineering predictions and to guide revised inspection requirements, if necessary. Inspection data can also be used to further validate some aspects of the analytical model. An iterative process develops between engineering prediction and field results leading to better control and enhanced mechanical life.

Since 1986, Unocal has been developing the Pt. Pedernales field off of Platform Irene. Some of these wells have set records for horizontal reach for drilling off the California coast. Mueller1 provided a thorough review of the technical issues encountered and how they were successfully addressed. Partially as a result of the successful completion of these very long reach wells, Unocal planned more wells with similar horizontal departures.

A large portion of these wells are drilled at high angles due to the high departures and shallow vertical depth of the reservoir. Consequently, large sections of drill pipe are run in compression to provide sufficient weight to the bit. In previous wells it was necessary to remove joints of drill pipe from the string due to fatigue cracks. In one case the string twisted off and resulted in a costly fishing job.

A study was undertaken to recommend changes in string design, material specifications, operating procedures and inspection requirements which would reduce the likelihood of fatigue damage and prevent costly pipe failures.

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