Turbodrills have been used in the oil and gas industry since the 1920s. The general characteristics of a turbodrill are higher power, higher rotary speed (revolutions per minute expressed in RPM) and relatively low torque, due to the high RPM. For applications in which diamond impregnated drill bits are used, hard and abrasive formations for instance, high RPM is desirable. When drilling medium-soft to medium-hard formations, however, using higher torques with lower RPMs can be a more efficient way to drill. Many of the latter applications call for PDC or roller cone drill bits that often produce a higher rate of penetration (ROP) through application of higher weight on bit (WOB). When lower speeds and higher torque are required, turbodrill speeds must be reduced. This is accomplished through use of a gearbox.
For decades, attempts have been made to develop gearboxes that run in conjunction with the turbodrill. These developments have had limited success for two primary reasons: failures resulting from axial thrust loading on the gears, and longevity failures resulting from inadequate sealing for lubrication in the gear train. This paper provides insight into current technical developments addressing these two issues. Further discussion defines the applications in which geared turbodrilling will benefit drilling processes.
The concept of using gearing in conjunction with turbodrills is not new. The inherent design of a turbodrill lends itself to high power, high speed and a corresponding output torque. Turbodrills derive their power from the fluid flow through the power section (also referred to as a turbine section). Flow rates in drilling are, in large part, dictated by the need to overcome frictional losses in the drill string and annulus, while balancing fluid velocities for sufficient evacuation of cutting from the wellbore. Turbodrill needs cannot, in most cases, dictate system flow parameters, so tools are designed to be as efficient as possible over a range of hydraulic parameters. Optimizing turbodrill configurations for an application is accomplished by adding or removing stages to vary power output, and by utilizing turbine blade arrangements that produce desirable characteristics in anticipated hydraulic conditions. Output RPM and torque are inversely proportional in downhole hydraulic motors. Although turbodrills are very powerful tools, the power is primarily used in the form of high RPM rather than high drilling torque.1 Displayed in the following equation, mechanical power (shaft power) produced by a turbodrill can be represented as a function of torque and RPM: Mechanical horsepower = (Torque × RPM) / 5252
The mechanical power produced by any turbodrill (or positive displacement motor, PDM) results from the conversion of hydraulic energy in the drilling fluid into mechanical energy in the drive shaft. This conversion carries with it a corresponding efficiency, so mechanical power can also be represented by: Mechanical horsepower = hydraulic horsepower × efficiency
Turbodrills are unusually powerful tools. This is particularly true relative to PDMs. The primary reason turbodrills are so powerful is that they are capable of operating reliably with a large pressure drop across the power section.