Application of Permanent-Magnet Motors in Oil Production
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 117386, "Application of Permanent-Magnet Motors in Oil Production," by Igor Azanov and Arthur Shamigulov, TNK-BP, originally prepared for the 2008 SPE Russian Oil and Gas Technical Conference and Exhibition, Moscow, 28-30 October. The paper has not been peer reviewed.

Currently, asynchronous submersible electric motors are used to drive most electrical submersible pumps (ESPs). At the same time, the possibility of improving their performance has been practically exhausted. Permanent-magnet motors, which already occupy a leading position in a number of industries, have better performance characteristics than asynchronous submersible electric motors. ESP-system drives based on permanent-magnet motors were not used in the oil industry until recently, but they currently are achieving wider application in the fields of various oil-producing companies.

Introduction

Compared to traditional asynchronous submersible electric motors, permanent-magnet motors have a number of characteristics that make their application economically attractive. Furthermore, introduction of permanent-magnet motors will increase ESP run life and improve health and safety indicators.

It is necessary to regulate and adjust pump-operation parameters during well startup, rate stabilization, and operation. Use of a permanent-magnet motor as a drive for an ESP system makes it possible to change the pump speed in an expanded range and, thereby, to react to changes in well flow rate and dynamic level as promptly as possible without stopping production and performing tripping operations. In such situations, wells equipped with conventional alternating-current (AC) motors require choking or change of pump running depth, which leads to equipment wear, excessive power consumption, and increased accident probability. The fundamental difference between permanent-magnet motors and asynchronous electric motors is that the former are capable of regulating the revolution speed by varying the current intensity, while the revolution speed of an asynchronous motor can be regulated only by changing the current frequency by use of frequency converters. Thus, the simpler construction of a permanent-magnet motor makes it possible to regulate the rotation speed without implementing additional, expensive equipment such as variable-speed drives.

One of the substantial weaknesses of traditional asynchronous submersible electric motors is their low efficiency—no more than 85%. The efficiency of permanent-magnet motors is much higher—more than 90%—and it barely changes from fluctuations in voltage from the power source and changes in load on the drive. Low idling and operating-current values make it possible to measure load on the drive more accurately, optimize ESP operation, and set up minimal current protection in the best way possible.

An important issue in well stabilization is the problem of submersible electric motors overheating. Therefore, it is necessary to stop the unit periodically for cooling when using AC motors. Use of a permanent-magnet motor as the drive makes it possible to stabilize a well without stopping for cooling. Because of their higher efficiency, permanent-magnet motors overheat less often, which increases the service life of insulation materials and makes it possible to reduce the number of failures caused by cable melting at cable input.