Optimization of Bit Performance for Qatar's Offshore North Field

Abstract

This paper describes the methodology and the results of a successful partnership between operator and service companies that improved the drilling performance and economics for the development of the North Field in Qatar. Fifteen high rate gas wells were delivered within designated directional targets, ahead of schedule and under budget. The systematic use of a new methodology for bit selection, linked with an excellent level of communication and confidence between different parties involved in the drilling operations, was a major reason for the drilling success. The different sections drilled were mainly made up of limestone, marl, and shale, but also anhydrite in the top portions of the well. In the lower sections, hard and soft limestone alternates with highly compacted shales and dolomites. Penetration rates were greatly enhanced and bit performance improved by more than 100% in some instances. The optimization of the performance took into consideration three aspects:A very tight schedule, where the optimization phase had to be performed within a limited time frame: Four wells were planned to be drilled back to back with two jack up rigs from two production platforms. After this short phase, batch drilling was expected to be performed for more than 20 deviated wells.The formations to be drilled presented a high complexity and heterogeneity where hard stringers alternate with a soft or a very compacted rock, and where in the lower section of the hole, a typical pseudo plastic behavior of the rock has to be anticipated.The well designs, casing depths, and bit types had to be in harmony with the drilling environment, the formation, and provide effective performance. New bit designs, more adapted to the technology available had to be developed or identified. The use of roller cone bits and motors was planned and used in the top hole sections. The use of PDC bits associated with extended steerable motors and/or turbines was considered as potentially providing the best performance in the intermediate and bottom sections. The early bit designs were modified based on wear patterns of dull bits. These enhancements and modifications were one of the major reasons in improved drilling performance and helped reduce the total drilling time from 75 days to 56 days per well. The challenges were met successfully, the most economical options selected and carried out ahead of time and under budget. Introduction The North Field Development - Phase 2, North Field Bravo (NFB), is the second development of the giant North Field, offshore Qatar (Fig. 1). Qatar Liquefied Gas Company (Qatargas) is responsible for this development, and to date, 15 deviated production wells have been drilled from two centrally located wellhead platforms. Following the field discovery in 1971, 18 vertical exploration/appraisal wells were drilled across the field to provide information on structure/reservoir development. A further 16 deviated production wells were drilled and completed for the first phase of gas production from the North Field Alpha (NFA) Complex. Records from these latter wells were used to benchmark the drilling performance in the NFB series of wells. The NFB wells are high departure directional wells with an average inclination of 55 degrees. The well bores can be split into two main sections, the upper section consisting of 24" and 16" phases and the lower section covering the 12 1/4" and 8 1/2" hole sizes. The use of tricone bits on steerable motors was planned for the upper section, and PDC bits on turbines or extended motors were considered to provide the best performance potential in the 12 1/4" and 8 1/2 phases. P. 91^