Over the years, drilling technology in the Dutch sector of the North Sea has evolved in an attempt to overcome many drilling challenges. Gaz de France's 12–1/4" hole section, in the K12 field, has been a very active arena for technological advancement and new product development. Many new drilling systems and new drill bit designs have been tested and developed through this complex drilling interval. However, even with all of the advanced technology utilized in drilling this section, many operational difficulties remain, due to the variety of formations, the directional requirements, and the length of this interval. Until recently, this section was drilled using a variety of different products, including roller cone and PDC drill bits, on both downhole motors and rotary drive systems.
In an effort to improve the performance through this interval, another new technology was introduced to this drilling program, the geared Turbodrill. This new downhole drive system is capable of producing very high mechanical power at the bit, both high torque and relatively high RPM. The high power of this new tool, in conjunction with aggressive PDC drill bits, has proven to provide excellent performance in this challenging environment. This paper will detail the development and characteristics of this new geared Turbodrill, and also analyze recent North Sea case histories that included the 12–1/4" hole section through the upper Cretaceous chalk down to the upper Permian Zechstein formation.
The development of many new technologies, in combination with the proliferation of more powerful rig pumps, has allowed dramatic drilling performance improvements to be realized through the use of Turbodrills. Although Turbodrills have existed in the industry for over a century, their use has, historically, been somewhat limited. The main reasons that Turbodrills were not widely used in the past included problems with thrust bearings, incompatibility with existing drill bit technology, difficulties in monitoring the torque and RPM output while drilling, and a general lack of sufficient hydraulic power to properly drive the tools in many applications. Recently, new technologies have been developed to overcome all of these historical Turbodrill issues. Further, as drilling programs have become more and more complex, and the measured and total vertical depths (MD, TVD) of wells have increased, rig pumps have generally become more powerful. In the past, it was fairly common for land rigs to have limited available standpipe pressure, but in today's drilling environment, many rigs have plenty of hydraulic power to spare. This paper will discuss these new Turbodrill technologies, and detail the significance of more powerful rig hydraulics.1
Gear reduction is one of the technologies that has greatly expanded the applicability of turbodrilling to many applications. This new development takes advantage of the high power output of Turbodrills by converting high RPM into high torque at the bit. The Dutch sector of the North Sea is one area that has become an excellent example of the performance improvement made possible with the modern Turbodrill, in the form of the new geared Turbodrill, NGT. In the K12 field, the 12–1/4" section is typically from 6,000 to 7,000 feet long, and often includes a significant amount of directional work. It also includes a wide variety of formations, such as chalk, chert, dolomite, sandstone, marl, claystones, siltstones, shales and anhydrites, and has historically been drilled with PDC drill bits on positive displacement motors (PDMs) and on rotary.