Abstract

The elastomer and its bonding system has always been the one of the most critical areas in downhole motor design. The elastomer's physical properties change due to temperature or from reactions with certain chemicals commonly used in drilling applications. This limits both the motor's operational temperature range and the chemicals that can be used in the mud system. While drilling with a conventional motor, heat is concentrated in a regular elastomer power section's lobes forming hot spots which contribute to a premature heat aging of the elastomer. Today, however, a new manufacturing process for stator tubes permits the forging of a pre-contoured lobe configuration - significantly reducing the elastomer content in the power section. This new design broadens motor capabilities by allowing heat to radiate faster from the thin elastomer and significantly improving the motor's mechanical and volumetric efficiency.

The power output of all sizes of positive displacement downhole motors has more than tripled during the past fifteen years. For example, today's 4–3/4" motors are significantly stronger than the 9–1/2" motors built in the 1980's. The latest step change in motor power was achieved in the manufacturing processes that eliminated 60%–80% of the elastomer used in the power section's stator.

These latest generation downhole motors are used as performance motors - delivering over 50% more power output compared to the previous generation motors of the same length and diameter and permitting the operator to select more aggressive bit designs. As a result, operators have experienced Rates of Penetration (ROP) improvements of 100%–300%. In addition, high speed configurations with bit speeds up to 1200 RPM are used in hard and abrasive formations with temperatures up to 160°C (320°F). These configurations outperform previously used turbines where the motors have greater torque capabilities while maintaining full steerability throughout the entire run. Typical performance drilling applications cover re-entry, deepwater and extended reach wells in all parts of the world.

This paper describes the technical features of the latest generation downhole motors in detail and documents their capabilities with case histories from various worldwide applications.

Introduction

Since the commercial introduction of hydrostatic downhole motors to the oil and gas industry in the 1970's, development engineers have worked on the increase of reliability and power output of these systems. Today downhole motors are among the most reliable components in the BHA - providing rotary power to the bit and keeping the well on the desired wellpath. Due to the development of improved manufacturing processes, better materials, and the application of latest design and simulation software, the power output of these downhole motors has increased steadily. Figure 1 shows the development in power output for three typical tool sizes during the past 20 years. Power sections are available as

  • High-speed power sections for applications with impregnated bits,

  • High-torque power sections for PDC bit applications

  • Low-speed power sections for roller cone bit applications

The latest generation of high-performance motors delivers up to twice the power of previous generation motors and was developed specifically for challenging performance drilling applications. The rugged design reduces the number of thread connections and includes strengthened thread connections to prevent tool failures, high load axial and radial bearings for the use of greater weight-on-bit and faster penetration rates and an improved steering head for more precise directional control.

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