Surface Processing of Carbon Dioxide in EOR Projects
Authors: Ormiston Robert M. , Luce Mark C. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Summary
This review paper describes surface processing considerations forCO2 EOR projects in light of the unique properties of CO2. The effects of CO2density, water content, hydrate formation conditions, solvent properties, corrosion properties, and acid properties are described, along with the latestadvances in CO2 purification technology. It is intended to give reservoirengineers, geologists, and process engineers working in associated areas abetter understanding of the variety of factors involved in processing CO2 forEOR.

Introduction
During the past few years there has been a rapid increase in the use of CO2to improve the recovery of crude oil from underground reservoirs. Itseffectiveness has been well documented. The use of CO2 for EOR started with theinjection of CO2 at the SACROC Unit in 1972 and by early 1986, there were 66active projects. Several more are being built or planned. Although the recentdrop in oil price has caused some projects to be deferred, in the long term, CO2 is expected to continue to play a significant role in EOR.
CO2 can be obtained from natural underground reservoirs or as a byproductfrom power plants or chemical facilities, In most cases, it must undergoexpensive purification before pipelining or reservoir injection: it must bedried, treated to remove H2S and other contaminants, and stripped ofhydrocarbons. To meet these needs. conventional processes had to be modifiedand new processes developed to handle CO2'S unique physical properties. Itsdensity, phase behavior, corrosion characteristics, and chemical properties arequite different from those of the more usual gas processing feedstocks. Whatfollows is a discussion of the properties of CO2 and how they affect theselection and design of surface processing facilities.

CO2 Properties
The density and phase behavior of CO2 can vary greatly with small changes inprocessing conditions. Fig. 1 shows the variation in CO2 density with pressureat various temperatures. In the region near the critical point (1,071 psi [7385kPa], 88 degrees F [31 degrees C]), the density changes very rapidly with asmall change in pressure or temperature. For instance, at 1,300 psi [8963 kPa], the density doubles when going from 116 to 98 degrees F (47 to 37 degrees C]. Small changes in composition can also have a dramatic effect on fluidproperties in this region, as indicated in Fig.
An addition of 10% methanedecreases the CO2 density over 30% at the conditions shown, Because of thesevariations in volumetric properties, accurate metering of the CO2 fluid can bedifficult. Mass flowmeters and volumetric meters in combination withdensitometers are used to measure flow at pressures near or above thecritical.
Fig. 3 shows the vapor/liquid equilibrium line of CO2, methane. and thephase envelope of a 50/50 CO2/methane mix. CO2 will form a liquid phase atlower pressures and higher temperatures than methane. As CO2 concentrationsrise in the EOR production gas. a significant amount of liquid may form, whichcould create problems in pipelines previously designed to operate in thegas-phase region. Phase envelopes must be generated for the range of conditionsand compositions expected over the duration of a flood.
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