Bottomhole Assembly Design and Response on Statfjord CO2 Extended Reach Well

Abstract

Well 33/9-CO2 with its horizontal reach of 7290m is a new world record in extended reach drilling. The well was drilled from the Statfjord C platform in the Norwegian sector of the North Sea and has a total length of 8761m. The partnership established between the operator and the service company was key to the development of a successful directional drilling strategy. This paper examines bottomhole assembly design and performance in the key sections of this technically challenging well. INTRODUCTION Following the discovery of the Statfjord field in 1973, the operator elected to develop it using three concrete gravity base platforms. At that time it was believed that the maximum reach for platform wells would be around 3000 meters. This equated to a maximum drilled inclination of 60 degrees. The development scenario for Statfjord has previously been described in a number of papers1,2. The northern part of the Statfjord field, known as North Statfjord, is separated from the main field by a large fault (fig. 1). In planning the development of this part of the field, the horizontal distance from the Statfjord C platform had to be taken into account. Given that this departure is over 5000 meters it was originally decided to utilise subsea completions to drain North Statfjord. The rapid advance in extended reach drilling provides a more cost effective way to pursue such a development. Utilising a subsea system would cost on the order of two to three times as much as an extended reach solution. Pushing the limit beyond 60 degrees has been an evolutionary process based on experience, technological advances and research. Equally important has been the development of a forum at which all parties can contribute to both the planning and execution processes. Through this the operator has been able to call on the global knowledge of all the service companies involved. Four extended reach wells have now been drilled from Statfjord C, C10 which was the first to exceed 5000 meters, C24 which was also over 5000 meters, C03 which was the first to exceed 6000 meters and now C02, the first to exceed 7000 meters. Three of these profiles are illustrated in Figure 2. The limit is still to be reached and will probably be governed by the capacity of the existing platform-based equipment. The drilling problems asociated with the Statfjord field are generally well known and can in most cases be mitigated by quantitative well planning and the application of experience. These problems are described in a previous paper on the C02 well3. One of the key factors to be addressed when plnning extended reach wells is hole cleaning, particularly when considering the larger diameter hole sizes. On Statfjord C the rig pump capacity limits flow rate in the 17 ½" hole resulting in hole cleaning problems for long, highly deviated sections. In the smaller hole sizes this has not normally been a difficulty, provided that the drillpipe is rotated at high speed in order to reduce the effects of the lowside cuttings bed. High speed is defined here as 15–180 rpm. Such rotation in high angle holes with steerable assemblies creates unique difficulties for the directional drilling company, whose tools would normally be rotated at much lower speeds. The design criteria pplied to meet these requirements will be discussed in a later section of this paper. The field experience gained on Statfjord which calls for such rotational speeds is supported through the results of laboratory testing,4 although the results are not always as clear as the field observations.