Turbodrilling in the Hot-Hole Environment
Authors: Herbert Pat 
Published: December 2007

Summary
Historically, geothermal and other types of hot-hole drilling have presented what seemed to be insurmountable barriers to efficient and extended use of downhole drilling motors, particularly those containing elastomeric bearing or motor components. Typical temperatures of 350 to 700 deg. F (177 to 371 deg. C) damage the elastomers and create other operating problems, reducing the life of the motors and their ability to drill efficiently. Recent innovations in turbodrill design have opened heretofore unrealized potentials and have allowed, for the first time, extended downhole drilling time in hot-hole conditions.
The unique feature of this turbodrill is the lack of any elastomers or other temperature-sensitive materials. Its capabilities are matched closely to the requirements of drilling in elevated-temperature environments. The bearing assembly can withstand conditions encountered in typical geothermal formations and provides the performance necessary to stay in the hole. The result is increased rate of penetration (ROP) and more economical drilling.

Introduction
Typical hot wells drilled in the U.S. present formidable technical difficulties in the effective use of such standard tools as downhole motors, bits, and surveying instruments that are used every day in the petroleum drilling industry. The foremost obstacle to the downhole life of these tools is elevated-temperature environments. All positive-displacement motors and turbodrills currently used contain elastomeric components that cannot survive in the temperature ranges of 350 to 700 deg. F (177 to 371 deg. C). This disadvantage provided the impetus for development of a new generation of turbodrills capable of performing under these conditions. Turbodrills have been developed that can withstand high operating temperatures while providing output power needed to drill the most commonly encountered formations (e.g., graywackes, granite, siltstone, and claystone), which, by their lithology, present difficult drilling conditions.
These conditions also play havoc on traditional forms of drilling equipment, adding importance to the development of downhole motors. Standard rotary assemblies used for drilling geothermal and hot petroleum wells do not realize the same life as their counterparts in most other normal drilling operations. This is true for two major reasons:doglegs or sharp bends in the hole accelerate wear on the rotating assembly because of wall friction with these hard formations, andthe higher stresses, both bending and thermal, reduce the fatigue life of the material.
Therefore, all drilling must be done by placing all available rotational power at the bit with downhole motors for economy. Another important factor to consider is escalating costs associated with drilling, making the potential savings available with downhole motors a major factor in the increased use of these tools.

Design Features
A turbo drill consists of a multistage motor, each stage comprising a rotor and stator. The stator, the stationary part of the motor, is attached rigidly to the housing. The rotor is attached rigidly to the main shaft and makes up a rotating assembly (Fig. 1). The complete motor assembly is a multitude of stages stacked one upon the other in sufficient number (usually more than 100) to develop the power dictated by the blade profile design. The turbodrill develops this power by directing the hydraulic flow of drilling fluid passing through the stator to the rotor blades, causing rotation. Fig. 2 shows a typical blade configuration of 1 1/2 stages and the flow of drilling fluid through the motor. Visualizing the impact of the fluid on the individual blade segments reveals how the flow is deflected.
JPT
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