Tubing String Design for a 35,000-ft Extended-Reach Well

ABSTRACT Completions of high angle' extended reach drilling (ERD) wells present unique problems for the production tubing string design. Loading conditions resulting from the trajectory of these wells should be considered along with the traditional design load conditions. This paper discusses the procedures used by Conoco Inc. to develop a tubing string design for an ERD research well having an approximate measured depth of 35,000', a true vertical depth of 12,000', and an average inclination of 74 degrees. The design was based on the general conditions and operating practices typically encountered on the Murchison and Hutton platforms, located in the U.K. sector .of the North Sea. The tubing size was selected on the basis of well flow analysis calculations and the planned ERD casing program. Described are the methods for analyzing the behavior of the tubing string, calculating the running and pulling loads, using the Von Mises triaxial stress design technique, and predicting the onset of buckling. Effects of doglegs, instantaneous changes of direction, buoyancy, and wellbore friction were also considered. Not surprisingly, the controlling design condition was the tensile load encountered while pulling the tubing off the bottom of the well. The design approach used in this paper shows that API grade tubulars will satisfy the expected load conditions. INTRODUCTION For Conoco and many other operators the greatest near term application for extended reach drilling (ERD) wells is in the North Sea. In this area, offshore platform costs are high, reservoirs are very large in area, and faulting often complicates the exploitation plans. Sometimes during development drilling, it is discovered that the reservoir is different than, originally modelled, and that the platform is not optimally located to drain the reserves using conventional directional drilling techniques. For these conditions, ERD wells offer viable alternatives to multiple platform installations and subsea completions. Unfortunately, the practical limits of ERD technology are largely unknown. Presently, management can only ponder the limits of Elm when planning these types of development projects. In 1988, Conoco proposed a joint industry project to define ERD limits. A research well was to be drilled from a jack up rig in Block 29/2a of the U.K. North Sea. This well was planned with a six mile departure - almost double the previous drilling records. Figure 1 shows the planned' casing program for this well. A companion paper explains the goals and objectives of this project1. The completion was planned to provide the same productivity and serviceability as conventional platform oil production wells. Obviously the nearly horizontal inclination creates some unique problems for installing, operating, and retrieving completion equipment. One of the primary considerations for the completion was the tubing string design. Of particular concern was the unknown behavior of the tubing in the high angle section (ramp) of the well and its effect on the design. Basic questions had to be addressed to identify any unusual axial load conditions not encountered in more conventional directional wells.