New Developments in Down-Hole Motors for Improved Drilling Performance
Authors: Tschirky J.E. 
Published: December 2007

This paper discusses improvements in design, materials, and manufacturing of components for down-hole, positive-displacement drilling motors. Significant increases in tool life expectancy result in Moineau-type, down-hole motors that perform much longer under optimum drilling conditions. Research continues to improve the performance of down-hole motor elements.

Introduction

In its simplest form, a rig-floor rotary table turns a long string of drillpipe and drill collars to transmit rotation and thrust to the drilling bit. The bit usually is powered this way regardless of depth, hole size, or adverse hole conditions.

Putting the rotary power at the bit, on the other hand, eliminates the need for fuming the drillpipe and, thus, helps to reduce surface power, and minimizes wear, torsional stress to the drillpipe, and damage to the hole or its protective casing. Using a down-hole motor to power a drilling bit is not new. Indeed, the idea was considered during the days of cable tools, when operators were looking for more efficient drilling methods. This approach to mechanical rock destruction uses forces normal to the rock, with emphasis on bit rotation. The trend is toward more normal bit force and rotation speed, while increasing equipment life.

Development of the positive-displacement, down-hole motor using the Moineau principle began in 1956 in the U.S. (Fig. 1). This motor supplies bottom-hole rotational power at greater speeds than rotary power, but without power at greater speeds than rotary power, but without the complexity and cost of turbodrills or electrodrills. This motor does not require that the drillpipe rotate as in conventional rotary drilling. Since drilling torque is directly proportional to the pressure drop across the fluid-powered motor, the fluid-pressure gauge at the surface accurately indicates drilling-bit weight.

The tool was tested originally for straight-hole drilling, but was considered uneconomical because of short bearing life and no bits that could operate at high rotational speeds. However, the tool later was such an economic and technical success in directional drilling that, until 1968, almost all research and development was directed toward that end. Since then, further research and development has produced a tool that now is used in straight-hole drilling.

Developments that have increased the life and performance of down-hole motors include improvements in the performance of down-hole motors include improvements in the dump-valve assembly, the stator and rotor, and the connecting rod and bearings. These improvements have resulted in motors that have consistently performed more than 100 hours, with drilling weights ranging from 2,300 to 3,300 lb/in. of bit diameter and a rotational speed of 720 rpm. Auxiliary tools, such as the adjustable bent sub, the bent connecting-rod housing, and the alignment sub also have been developed. These tools simplify rig operations and increase the effectiveness of directional drilling with the motors. In the future, further improvements will be realized with the development of improved elastomers for the stators, improved bearing and seal packages, and the introduction of multicavity motors.

New Developments

Dump Valve

Because fluid will not flow readily through the tool without motor rotation, a dump (bypass) valve assembly is provided at the upper end of the tool. provided at the upper end of the tool. JPT

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