The paper describes Statoils experiences in drilling extended reach (ERD) wells, to primarily serve remote parts of widespread reservoirs and experiences in drilling horizontal and complicated wells, specially designed for optimal drainage of complex reservoirs. Several world records in extended reach drilling, conducted from the Statfjord "C" platform are summarized. The latest ERD well drilled has a horizontal reach of 7290 m. Included are also the latest plans of implementing these techniques in developing new fields. A brief summary is given on the achievements within the industry regarding other special well types, such as dual lateral, snaky, multiple arm, etc. Furthermore the paper describes technical challenges of drilling complex wells and how planning is done to cope with these challenges. Some limitations to the drilling are highlighted and solutions to drilling problems related to torque, drag, buckling, hole cleaning and surveying are given. Cost-Benefit considerations regarding ERD, horizontal and complex wells are discussed. Reducing number of platforms and/or leave out subsea templates due to ERD wells are considerable cost reducing factors. Reducing the total number of wells from a platform due to horizontal and/or complex well profiles is another major cost reducing factor. The oilfields developed in the 1970-80s had relatively significant limitations to the drainage area from each platform. The Statfjord and Gullfaks fields were initially planned to be developed with maximum 60 degree inclination wells, resulting in a maximum horizontal reach of 3000 m at the Statfjord field and 2100 m at the Gullfaks field reservoir depths. To drain the fields in an optimum way based on the technology available, called for three GBS platforms on each field. Developments and improvements in drilling technology, engineering research and field experiences have, since then, pushed the limits for available well profiles and horizontal reach distances dramatically. The purpose of drilling extended reach, horizontal and complex design wells is to drain the field in the most cost effective way. The use of these new techniques can make old nonprofitable fields profitable, prolong an existing field's economic life and make new questionable field discoveries worth developing. Today's techniques enable the reservoir, dip angles, faults and structural geology to be design factors for optimal placement of the wellpath. P. 505^
Summary This paper addresses the various aspects of torque and drag problems encountered in drilling extended-reach wells. It discusses how to use torque and drag calculations and measurements to plan long-reach well profiles, to execute drilling operations that minimize torque and drag effects, to monitor hole cleaning, and to plan jarring operations. Introduction In extended-reach drilling, a limitation on the horizontal displacement occurs because of frictional forces between the drillstring and the borehole wall. Drag is measured as the difference between the static weight of the drillstring and the tripping weight. Similarly, a difference between the torque applied at the rig floor and the torque available at the bit occurs owing to friction. Torque and drag problems are often associated with each other and maybe profound in extended-reach and horizontal wells. As Sheppard et al. stated, a variety of sources of drag and torque loss exist: differential sticking, key seating, hole instabilities, poor hole cleaning, and the general frictional interaction associated with side forces along the drillstring. Therefore, drag and torque measurements may be used to monitor operations to optimize performance. In extended-reach drilling at Statoil, torque and drag problems have initiated use of more sophisticated well profile and use of torque as an indicator of hole-cleaning problems. Understanding of torque and drag problems has been applied to the well planning process. As a result, problems are often not found in wells with horizontal displacements up to 5000 m. Another interesting implementation of drag knowledge in operational procedures is described in a paper on the influence of drag on hydraulic jar efficiency. In this paper, we discuss torque and drag problems in extended-reach wells, how knowledge of torque and drag is used in operational procedures, and to what extent the planning phase can help avoid operational problems. Although always referring to extended reach, the same principles are valid for horizontal,'S'-shaped, and designer wells. Well Profiles Optimizing well profiles to minimize torque and drag problems has been discussed in many publications (e.g., Refs. 1, 4, and 8 through 10). Sheppard et al. thoroughly discussed the catenary curve principle for well drilling. Alfsen et al. discussed a modified catenary principle; Banks et al. included the concept of tortuousity and reached the important conclusion that making a smooth well path is key for successfully drilling extremely long-reach wells. To reduce friction in any well, a good mud program design is important. Friction factors down to 0.16 simulations have proved to give a best fit with measurements. The torque and drag program used in the work described here has been used extensively at Statoil together with measurements of actual data. Confidence in the calculations has been achieved, and they have been used to monitor and improve operational practice. Minimizing dogleg severity and even making changes in dogleg severity have been implemented in our procedures. Several papers have been published on long-reach well drilling from the Statfjord C platform. After a 6000-m horizontal displacement was reached in Well 33/09-C03, it was recognized that the well profile would need to be optimized to reach the planned depth for Well C02-7200-m horizontal displacement. The catenary curve, proposed as a possible solution to the torque and drag problems, is the solution to the following problem. A cable with weight per length, W, has a horizontal force at left Point A, FH, and a tangential force at right Point P(x, y), FT. The horizontal component of the force at Point P is in the opposite direction of the force at Point A. The solution to the above problem is given in the x-y plane as where An interesting feature of the catenary curve is the zero contact force between the drillstring and the borehole wall. Consequently, the catenary curve could theoretically give zero friction between the borehole wall and the drillstring. Several difficulties exist in using this approach for drilling a well. First, the effective force at the bottom of the well results in drillstring compression as opposed to the tension given in the theoretical curve. Furthermore, the catenary curve will lead to a much longer well path than more traditional well profiles. Thus, a slight modification of the catenary curve must be made. An important feature of the catenary curve was kept in the well plans for Wells 33/09-C24 and 33/09-C02 in the Statfjord field: the very slow build rate in the shallow part of the well with a slowly increasing build rate as well depth increases. The sailing angle of 80 to 84 is therefore much higher than the traditional 60 . Figs. 1 and 2 describe the well-path planning process with the resulting torque calculations. The catenary curve is compared with traditional constant-build curves with 1.5 /30- and 2.5 /30-m build rates. A much lower sailing angle is achieved with the traditional curve design. As a result, as Fig. 2 shows, the measured depth (MD) of the actual well path is longer than with traditional shapes. The friction along the drillstring is lower, however, and a higher torque at the bit is a welcome result. The success of reducing wall contact and thereby the total friction was reported in Ref. 4 and is shown in the simulations of comparison of wall contact force in Fig. 3. Well 33/09-C03 has a standard profile; Well 33/09-C02has a modified catenary profile. Note the difference in scale in the two parts of Fig. 3. The very high normal force in Well 33/09-C03 compared with the33/09-C02 profile will give similar marked higher friction and thus higher torque loss. The well profile used in Statfjord Wells C24 and C2 may lead to enhanced problems with formation stability and differential sticking owing to the high sailing angle. However, wherever these problems can be handled, the modified catenary curve will give a lower friction than traditional well profiles. Monitoring Hole Cleaning The confidence in torque and drag simulation programs may give unexpected benefits. When long-reach wells are drilled, the torque and drag simulation curves may be used to monitor hole cleaning. Deviations from properly modeled torque and drag simulations may indicate hole-cleaning problems. Fig. 4 shows torque simulations in Well 33/09-C02 and actual measured torque in the 12 1/4-in. section. The three smooth curves are the acceptable, planned, and actual torque simulations, respectively. The marked change in simulation curves at about 2600 m was caused by a bit change. An aggressive bit must be simulated with a higher torque on bit than a less aggressive bit. P. 800^
Summary Well 33/9-C2 with its horizontal reach of 7290 m is a new world record in Extended Reach Drilling. The well was drilled from the Statfjord C platform in the North Sea, Norwegian sector, and has a total length of 8761 mMD. This paper will describe the planning techniques used, with focus on coping with limitations in equipment and formation. It will also present a case history of the drilling operations including operational experience gained and comparisons between simulated drilling parameters and field data. Introduction The Statfjord field was discovered in 1973. Based on the knowledge and experience at that time, it was expected that it would be possible to drill wells with a sail angle of up to 60 degrees in this area. This resulted in the establishment of three platforms with approximately 5000 m separation. These 60 degree boundaries around each platform resulted in a horizontal reach of approximately 3000 meters at reservoir depth. The northern part of the Statfjord-field, known as North Statfjord, is separated from the main field by a large fault (fig.1). The horizontal distance from the Statfjord C platform to North Statfjord is approximately 5000 meters. Accordingly it was planned to develop this part of the field utilising sub-sea technology. Such a development would cost about three times as much as an Extended Reach solution. More than 100 wells have been drilled from the three Statfjord platforms. The platforms are producing from two reservoirs; the Brent Group and Statfjord Formation of Jurassic age. The first extended reach well with a horizontal reach of more than 5000 meters was drilled from Statfjord C in 1989, well C10[1]. The first well to exceed 6000 m was completed in 1991, well C3[2]. Since drilling began in November 1978 techniques have been developed to push the limit beyond maximum 60 degree inclined wells (fig. 2). The evolutionary process has been based upon a combination of engineering research, application of new technology and field experience. FORMATION RELATED DRILLING PROBLEMS. Because of the many wells drilled in the Statfjord field the possible problem areas are known and attention can be focused on how to overcome these problems, and on which alternative operations can be used to avoid them. Highly Reactive Clays and Shales. Statfjord field stratigraphy shows a high shale and clay content with large proportions of Montmorillonite and Illite which have a large surface area. These characteristics indicate a high potential for the shale to hydrate and swell.
Drilling in the Statfjord Field has become increasingly challenging as remaining oil pockets require designer wells, horizontal wells, or Extended Reach Drilling (ERD) wells with great demand for reservoir navigation in three dimensions. Until recently this has been performed with steerable motor assemblies, with frequent steering difficulties in the reservoir sands. Low rate of penetration (ROP) while sliding has given an overall low drilling efficiency, and in some cases inadequate steering ability has resulted in a less than optimum wellbore placement. Long sliding intervals have also resulted in poor hole cleaning, which has contributed to stuck drill string and lost circulation problems. This paper presents the introduction of a rotary steerable system in the multilateral horizontal well C-23 in the Statfjord Field, and how this improved drilling performance over comparable wells in the reservoir. The following topics are discussed in the paper:–Field description and brief overview of directional drilling challenges in the Statfjord Field.–Statoil's active role in the development of rotary steerable systems.–Functional description of the rotary steerable system, highlighting the technology's benefits compared to traditional directional drilling methods.–Why well C-23 was chosen as a suitable candidate for therotary steerable system, becoming the first successful utilization of this promising technology in the Norwegian Sector.–Case history, emphasizing directional control, drilling efficiency improvement, and other valuable experiences from utilizing the rotary steerable system. P. 313
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