There's No Free Lunch, Pumping Two Phase Fluids with ESP

Abstract

SPE ESP Workshop, April 30 - May 2, 2003, Houston, Texas This paper examines the conjecture that a properly designed centrifugal pumpwill have no problem with free gas. The centrifugal pump is a dynamic pumpingdevice. One of the traditional limitations has been their inability to handlesignificant quantities of gas. This doesn't mean that it can't handle gas butthere is a cost. The user must recognize the operation boundaries that dependon the situation and the equipment selected. This presentation reviews the nature of gas in its relation to wellproduction with Electrical Submersible Pumps and examines some gas handlingmethods and devices more recently introduced to the industry. It also attemptsto 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 alarge variety of elements and short, medium and long chain hydrocarbons. TheAPI gravity of an oil can be interpreted as a bulk average of the substancespresent at standard conditions. Under sufficient pressure, the natural gas andthe oil exist together in a liquid phase. When the pressure is decreased, themixture expands. As pressure is continued to decreased, the elemental gassesand lighter hydrocarbons molecules have sufficient energy to liberatethemselves from the liquid to form a separate gas phase. The point at which thegas phase first appears is the bubble point. The composition of the gas at thispoint is a function of the solubility of the individual constituents in theremaining liquid. The lighter gasses tend to come off first. The gas phase isin a complex equilibrium state between the evolved substances and thesubstances 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 crudeoils may have larger volumes of gas in solution, and/or greater percentages ofthe light substances in the gas phase. Removing a portion of the dissolved gaschanges the properties of the remaining crude. For a given sample of crude witha 5,000 psi bubble point, if the pressure is lowered to 3,000 psi and the gascap 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 bythe gas oil ratio (GOR). It represents the volume of liberated gas in contrastto the volume of liquid at standard conditions of temperature and pressure.This can be expressed as a direct ratio, (barrels per barrel, or cubic meterper cubic meter) or in Oil Field Units as standard cubic feet of gas per barrelof 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 productionof oil from a well and it is accomplished by drawing the well pressure down asfar as practical.