In the late 1800s, the drilling industry recognized the inherent benefits promised by downhole drive systems. Turbodrills, and later, positive displacement motors (PDMs), were introduced to capitalize on these benefits. Both Turbodrills and PDMs convert hydraulic energy in the drilling fluid into mechanical power at the bit. With a Turbodrill, this power conversion is accomplished through the use of turbine blades, and with a PDM, it is accomplished through a seal between an eccentrically rotating steel rotor and an elastomer stator. While there are many benefits to the use of turbine blades to create mechanical power from hydraulic power, their superior efficiency at relatively high output speeds stands out. The rotational speed generated at the bit is not ideal for certain applications, however. The industry recognized this situation in the 1920s, and the development of the geared Turbodrill was initiated. By including reduction gearing in a Turbodrill, high rotating speeds generated by the turbine can be converted into lower RPM and higher torque at the bit.

For decades, there have been attempts to develop a successful geared Turbodrill. Until recently, these efforts have realized little success. Design work performed with the latest software technology and modern materials have now resulted in a very successful, high power, high torque geared Turbodrill. With the new possibility of decreasing output RPM, and increasing the available torque, the range of applications in which Turbodrills perform favorably is significantly expanded. The focus of this paper will be to identify applications in which the deployment of a gear reduced Turbodrill will be advantageous.

This paper will also analyze three recent North Sea case histories, in which geared Turbodrills, together with PDC bits, were run successfully to drill a 12-1/4" hole section through the upper Cretaceous chalk down to the upper Permian Zechstein formation. One of these cases involved running the new geared Turbodrill (NGT) with a rotary steerable system. Analysis of these runs will detail proposed versus actual drilling curves including an in-depth discussion on all parameters encountered during drilling.

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