Abstract

Progressive cavity pumps (PCPs) are a special type of rotary positive displacement pumps. PCP was first introduced in petroleum engineering as an artificial lift method in 1970's, and is becoming a popular lift tool for its advantages of broader applications, less maintenance, and higher efficiency. PCP performances have been studied for many years, but no published paper has discussed the relation of pump performances with well performances systematically.

This paper analyzes the system performances of PCP wells. Using nodal analysis method, the paper presents algorithms and procedures to design pump rotational speed and production rate from well inflow and outflow performances. For the effect of viscosity on pump volumetric slip, unlike traditional methods of trying to set up a general correlation for all PCP types, the paper propose a method to correct the effect of viscosity on each pump's catalog performance curves, and demonstrates how to use the corrected performances to design the pumping of viscous fluid.

By unfolding a PCP cavity and using simplified slot flow, this paper presents correlations to determine the critical pump intake pressures for filling pump cavity completely for Newtonian and non-Newtonian fluids.

The presented algorithms can be used not only to design a PCP well for pump rotational speed and production rate, but also to analyze the performance of a PCP pumping well. The presented correlations of the viscosity effect on slippage and the critical pump intake pressure are useful in PCP design and system analysis.

Introduction

Progressive cavity pumps (PCPs) are a special type of rotary positive displacement pumps, and were first introduced in petroleum engineering as an artificial lift method in 1970's. In a PCP, the flow through the pump is almost axial, while in all other rotary pumps, pumping fluid is forced to travel circumferentially. This gives PCP unique axial flow pattern and low internal velocity, which reduces fluid agitation and churning and therefore reduces fluids emulsion and solids erosion.

PCP has advantages of lower investment, broader applications to fluid mixtures, less maintenance, and higher efficiency to other artificial lift methods. It is becoming a popular lift tool and, for some wells, the best choice in artificial lift methods.

In petroleum industry, the most commonly used progressive cavity pump is a single lobe pump that consists of a single external helical rotor turning eccentrically inside a double internal helical stator. The rotor and the stator have the same minor diameter and are made of metal (steel). The fits between the rotor and the stator may be metal to metal, or metal to elastomer which is set inside the stator. Compression fits are usually used for metal to elastomer contacts, while very small clearance is left for metal to metal fits. There are chambers between the rotor and the stator, which are separated by the fits as cavities in 180° apart. The fits work as seals to prevent fluid communication between adjacent cavities. As the rotor rotates, the seal lines change positions and form fully enclosed cavities moving continuously from pump inlet to outlet.

These cavities trap fluid at the inlet and carry it along to the outlet, thus providing a non-pulsation smooth flow. Unlike centrifugal pump, fluid viscosity will not degrade pump head of a PCP, but increase pumping volumetric efficiency. Since PCP is a positive displacement pump, it doesn't have gas lock problem theoretically, but due to temperature increase from gas compression, PCP can only handle high gas slug in a short time. Due to the feature of moving seal lines, scale does not normally deposit in a PCP. PCPs have relatively low inertia of their rotating parts, and have a reliable working life.

Although PCPs have been used for a few decades as an artificial lift method in petroleum engineering, most studies focused on their working mechanism(Gaymard et al., 1988, Saveth and Klein, 1989, Delpassand, 1997) and pumping behaviors (Gamboa et al., 2003, Bratu, 2005). To author's knowledge, no paper has discussed the design of PCP wells in production system. Fluid production from a PCP pumping well is not only determined by the PCP, but also controlled by reservoir inflow performance, fluid outflow performance and surface condition. This paper fills the gap by using nodal analysis method to design a PCP.

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