Hollow Rods: Development Of A New Technology For PCP
Authors: Olmos D.E. , Ernst H.A. , Villasante J.A. , Johnson D.H. , AMEGLIO A.F. , Del Pozo L. 
Published: December 2007

Conventional sucker rods (working under torsion loads) are used nowadays to drive PCP pumping systems (Progressing Cavity Pumps). Conventional rods have been developed to work under alternating tensile stress, not under torsion stress. The use of these rods to drive PCP systems has several limitations and problems.

These limitations result in high operative costs for artificial lift systems. Also, in many cases the high capacity pumps commonly used today cannot be driven, because of their high torque requirements.

Thus PCP pump users require a reliable High Torque rod to drive their systems. This rod must work under high torque stress.

The following technical paper refers to the development of the "Hollow Rods" a technical solution tailor made for PCP pumping systems.

Different topics covered by this work are:General description of this product.Design methodology of the connection.Laboratory tests (fatigue, destructive tests, etc.)Pipe technical specifications.

As part of this job, field experience with this product is described. This experience started at October 1999 in Diadema, an oil field operated by CAPSA S. A. Due to the excellent results obtained, CAPSA S. A. decided to extend the installation of "Hollow Rods" to 70 wells at this oilfield. Today started a third step, were will be installed in another 90 wells during this year.

Several advantages of this new technology are:Lower operating due to the elimination of connection failures.Less tubing wear.Increase in pumping rates due to a high torque capacity.Less initial equipment cost for high displacement pumps compared with ESP.Possibility to inject different kind of fluids by the inside of the "Hollow Rod". I.e. viscosity reduction fluids to pump extra heavy oil, chemical corrosion inhibitors, etc.

1. Introduction

In the PCP (Progressing Cavity Pump or Moineau Pump) pumping system, the pumping is done through the rotation of a progressing cavity pump installed at the well bottom, this is different to the traditional beam pumping system where the pump is a piston type that works under alternative movement.

The pump in the PCP system is made of a high strength steel helicoidal rotor moving within an elastomeric stator. Pumping is done by the ascendant displacement of isolated cavities formed by rotor and stator. The rotational movement driving the pump is transmitted from the surface by a sucker rod string. This sucker rod string works under torsion, traction and flexion (in case of slightly deviated wells) stresses.

Sucker rods are manufactured under the Specification API 11B, which defines the product to work just under traction loads and used for other pumping system: beam pumping system.

As time went by, the use of sucker rods became standard for PCP pumping, but with the progress and technological advancement of this kind of pumps, depths and capacities were raised, producing greater demands on the rods. The increase in the working loads meant an increase also in the early failure of sucker rods.

These problems raised the operating costs due to failures and inherent limitations of the system. For example, the evolution of these pumps allowed new developments as monolobe and multilobe equipment for high volumes (between 250–500 cubic meters per day, volumes usually serviced by Electro Submersible Pumps, ESP). In many cases, it is not possible to install these new PCP pumps for the lack of a trust worthy driving device that could transmit the required torque (between 1500/2500 lb-ft).

Mainly two factors increase the PCP pumping costs:Premature failure of sucker rods unions for overloading or in the bodies for flexion-torsion combined stresses.Tubing and sucker rods friction wear.