This paper presents the results of a field trial conducted in Saskatchewan, Canada to investigate drive string dynamics between conventional and continuous rod and the impact of interference fit between rotor and stator in progressing cavity pumps (PCP).
In August 2009, a well was completed with a downhole monitoring system and a PCP driven by a continuous rod string. At the same time, a second well was completed with a downhole monitoring system and a PCP driven by a conventional rod string. The pumps were identical with the exception of the interference fit of the rotor in the elastomeric stator.
Thousands of data values were collected over a period of several months and evaluated to better understand rod stretch, torsion, and axial movement during start-up, shut-down and normal operating conditions and how these values change as a function of the interference fit of the pump itself. While the data evaluation did provide answers to help ConocoPhillips improve their PCP operations, there were even more questions generated that will prove thought-provoking to the industry and stir some great discussion.
With more types of rod-driven systems appearing in the industry such as metallic stator PCPs, and others in development such as the geared centrifugal pump [Patterson], ConocoPhillips needed a better understanding of rod string dynamics since the impact of downtime associated with these high volume systems could be significant. In addition, some rod-driven systems do not have the rods directly coupled to the pump, which results in minimal to no axial load on the drive string itself, and since there was little industry data and experience with these systems, the test program was structured to capture this information.
The field that was chosen for these trials was the Marengo field, located in southwest Saskatchewan, Canada. This field trial started in 2008 as a single installation (Well A) with a very high fluid level to collect data on a conventional drive string with low axial load. A downhole monitoring tool was also installed that was to measure downhole revolutions, axial position of the rotor, tubing and annular pressure / temperature, and vibration in the radial (x) and axial (z) directions. A sensor from another supplier was also installed on the polish rod to measure surface revolutions. Unfortunately, this trial ended just after a few months as the downhole monitoring tool began to report erroneous values. The trial was put on hold until the tool could be modified and a workover scheduled to replace the failed system. During that time, ConocoPhillips decided to expand the field trial to two wells to study both continous and conventional drive string dynamics.
Two identical wells in terms of depth, casing size, bottomhole pressure and production rate were selected for this trial; Well A and Well B. In order to evaluate minimal axial load conditions on the drive strings, the wells needed to have high fluid levels to provide as much buoyant force as possible. In the Marengo field, most of the wells are produced with very high fluid levels due to water handling constraints; however since the PCPs are designed to drawdown the wells if needed, the pumps tend to be oversized in terms of differential pressure capability.
The test program was segmented into two parts - 1) understanding the dynamics of start-up and 2) determining changes in the pump and rod string over time due to swell, wear, stretch, etc.
Author: Noonan, S. G., & Skoczylas, P.
Publisher: SPE Progressing Cavity Pumps Conference, 12-14 September, Alberta, Canada
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