Abstract

This paper examines the conjecture that a properly designed centrifugal pump will have no problem with free gas. The centrifugal pump is a dynamic pumping device. One of the traditional limitations has been their inability to handle significant quantities of gas. This doesn't mean that it can't handle gas but there is a cost. The user must recognize the operation boundaries that depend on the situation and the equipment selected.

This presentation reviews the nature of gas in its relation to well production with Electrical Submersible Pumps and examines some gas handling methods and devices more recently introduced to the industry. It also attempts to quantify limitations to two phase production with ESPs.

What is Gas?

Crude oil is not by nature a homogeneous substance. It is a mixture of a large variety of elements and short, medium and long chain hydrocarbons. The API gravity of an oil can be interpreted as a bulk average of the substances present at standard conditions. Under sufficient pressure, the natural gas and the oil exist together in a liquid phase. When the pressure is decreased, the mixture expands. As pressure is continued to decreased, the elemental gases and lighter hydrocarbons molecules have sufficient energy to liberate themselves from the liquid to form a separate gas phase. The point at which the gas phase first appears is the bubble point. The composition of the gas at this point is a function of the solubility of the individual constituents in the remaining liquid. The lighter gasses tend to come off first. The gas phase is in a complex equilibrium state between the evolved substances and the substances still left behind in solution. As pressure continues to decrease, the gas phase expands and the liquid phase shrinks.

The Bubble Point is a property of the bulk solution. High bubble point crude oils may have larger volumes of gas in solution, and/or greater percentages of the light substances in the gas phase. Removing a portion of the dissolved gas changes the properties of the remaining crude. For a given sample of crude with a 5,000 psi bubble point, if the pressure is lowered to 3,000 psi and the gas cap is bled off, the remaining crude will have an increased API gravity, viscosity and a 3,000 psi bubble point.

The relation between the liquid phase and the gas phase is represented by the gas oil ratio (GOR). It represents the volume of liberated gas in contrast to the volume of liquid at standard conditions of temperature and pressure. This can be expressed as a direct ratio, (barrels per barrel, or cubic meter per cubic meter) or in Oil Field Units as standard cubic feet of gas per barrel of oil (SCF/BBL). GOR values of several hundred are common and values exceeding1000 are not unknown. The goal of artificial lift is to maximize the production of oil from a well and it is accomplished by drawing the well pressure down as far as practical.

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