Abstract

This presentation examines the phenomena of ESP run life reduction that can occur when rotary gas separators are employed. This reduction can be severe enough that some producers have stopped using them all together. This paper presents information for the need for rotary gas separators and some alternate production strategies that could be attempted. The ESP manufactures now offer equipment that can extend the run life of these units in the problem wells. Familiarity with some of the alternate strategies and the improved equipment available will aid the user in selecting the strategy and equipment to optimize the run-life/production in his particular situation.

Introduction

One of the limitations of centrifugal pumps is their inability to handle significant quantities of gas. Through the history of the ESP, a variety of devices and techniques have been attempted in order to allow production with greater quantities of gas. The rotary gas separator was introduced in the late 1970's. It was purported to out perform all other methods with ESP's in gassy wells. At that time the price of oil was up and drilling activity was high. Anything that could be done to increase production looked to be a good move.

The rotary separator was new and the possible problems that it might create were poorly understood. Wells were being drawn down as far as practical. Any well equipped with an ESP that was not having gas problems could be drawn down further. Rotary separators were sometimes installed without regard to actual need or downside risks. This practice lead to many problems and failures. In the intervening years, manufactures made numerous improvement in the ruggedness and the reliability of the rotary separators. Problems still arise in the understanding the function and the limitations of these devices.

Gas And The Centrifugal Pump
The Centrifugal Pump

The centrifugal pump is a dynamic device that uses the fluid velocity to produce the energy to lift the fluid column. The equations that govern the device relates the produced head to the flow, geometry, and rotational speed. The density of the fluid does not effect the head produced. This means that a centrifugal pump will produce the same head regardless of the density of the fluid. Simply put, a pump that oar lift a column of water 6,000 feet could also lift 6,000 feet of air at the same flow rate. The difference is that the required pressure for water is 2,600 psi verses 3 psi for air. The ESP is an inefficient compressor and should never purposely be selected for this.

Multi Phase Mixtures

The produced fluid from an oil well usually has more than one phase. It is most common to have three phases, water, oil and gas, flowing together. It is conceivable to have four phases, if you are willing to count sand, silt, or rocks as an integral part of the flow.

If the phases can be evenly dispersed in one another, the pump may be able to handle the fluid. In the pump, multiphase fluid may not be able to remain homogeneous. The density difference may cause the phases to segregate more quickly than the turbulence can mix them back in.

The size of the bubble is important in maintaining a homogeneous mixture. The drag force encourages particles to move with the fluid, and the buoyant force encourages segregation. The drag is a function of the cross sectional area of the particle and the buoyant force is a function of the volume. As the size of the particle decreases, it is more likely to flow with the fluid than to separate. A very finely dispersed phase can be difficult to separate and in the case of water oil emulsions, the fluid can develop some unusual bulk properties.

Gas GOR & GLR

Crude oil is not a pure substance. It is a mixture of a variety of elements and short, medium and long chain hydrocarbons. The API gravity of an oil reflects a bulk average of the densities of the constituents. Under sufficient pressure, all of these exist together as a liquid. If the pressure is decreased to the bubble point, these atoms and molecules start to liberate themselves and form the gas phase (Fig. 1). The lighter gasses come off first. As pressure is lowered, the gas phase expands and the liquid phase shrinks.

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