Abstract

Density is widely used in mass flow computations for custody transfer of supercritical CO2. Determination of density from densitometers is shown to be more reliable than estimation from temperature and pressure. Three types of vibrating element densitometers are described, and typical installation schemes are outlined. Advantages and disadvantages of each densitometer type are reviewed.

Introduction

The injection of carbon dioxide (CO2) into oil fields as a means of enhanced oil recovery (EOR) has been well publicized. Naturally occurring CO2 is drawn from domes in New Mexico, Colorado Wyoming and Mississippi, processed, and compressed to yield a supercritical fluid. The supercritical CO2 is transmitted in large diameter pipelines to oil producing areas in West Texas, Wyoming and Louisiana, where it is distributed in smaller lines to individual fields. The result is a large number of custody transfer (i.e. billing) points between CO2 producing wells, transmission pipelines, distribution pipelines, producing wells, transmission pipelines, distribution pipelines, and individual oil producing clients.

The flow of CO2 for billing purposes is widely measured by orifice plates or turbine meters. The outputs of these meters are converted to mass flow using densitometers and the following equations:

Turbine Meter: q = q x

Orifice meter: q = (CONSTANT) x [ x P]

In either case: Qv = q /p

Obviously, the respective performances of the densitometer, flowmeter, and associated flow computer must be combined to yield overall system accuracy.

The objectives of this paper are directed solely to the densitometer portion of such systems. It will describe the types of densitometers used, show typical installation schemes, and discuss the relative merits of each.

DENSITY

"Density" is defined as "mass per unit volume". Many types of operational principles have been applied to the estimation of fluid density including the following: continuous weighing, buoyancy, constant head, acoustic, nuclear, capacitance, and vibrating element. Of these, vibrating element densitometry has emerged as the dominant method for CO2 custody transfer. Therefore, this paper will concentrate exclusively on vibrating element density transducers.

For pure CO2 density may be estimated from temperature and pressure as shown in Figure 1 Each line represents the pressure as shown in Figure 1 Each line represents the relationship between pressure and density at a single temperature. The domelike curve is the locus of points for which gas and liquid are in equilibrium at any specific temperature and pressure. The isotherms intersecting the dome on its left pressure. The isotherms intersecting the dome on its left side represent gaseous CO2. Those intersecting the dome on its right side and continuing to the upper right represent liquid CO2.

Note in Figure 1 the isotherms above 304.2K (88F) miss the dome altogether. For these isotherms, CO2 passes from the gas phase to the liquid phase without ever yielding two phases in phase to the liquid phase without ever yielding two phases in equilibrium.

P. 345

This content is only available via PDF.
You can access this article if you purchase or spend a download.