**1. Introduction**

Current dilemma faced by the United States in lowering the dependency on foreign energy source and curbing emissions of greenhouse gases has brought light to carbon dioxide (CO2)-enhanced oil recovery (EOR) method [1]. Oil recoveries using natural (primary) mechanisms rarely exceed 20% of the original oil-in-place (OOIP) [2]. Secondary methods of recovery often add few percentages to the above figure [2]. Hence, companies would welcome any methods that could lead to an increase in the production of the postwater/gas injection trapped oil in the reservoir. Injecting CO2 into oil reservoirs to improve the recovery of oil on commercial (field) scales has been practiced since nearly half a century ago. The idea of CO2 flooding first emerged in 1930s, and more laboratory and field studies were conducted between 1950s and 1970s. Most of these implementations have been in North America. However, some small-to-large scale CO2 injection projects have been reported in other parts of the world [3, 4].

In this method, CO2 is usually injected as a supercritical fluid. When injected at or above the critical point of pressure and temperature, supercritical CO2 can

maintain the properties of a gas while having the density of a liquid. In this state, oil could be more efficiently mobilized from the depleted reservoir due to the improved volumetric efficiency. Conventionally, CO2 injection method is usually applied to the reservoirs with oil gravity less than 25 [5]. As CO2 is injected into the reservoir, the miscible CO2 will blend thoroughly with the oil in a manner that the interfacial tension between these two fluids becomes zero. The other mechanisms of CO2 by which the oil recovery is improved are the dissolution of CO2 in oil, swelling of oil, and eventually reduction of the viscosity of oil.

Since 2002, as a consequence of Kyoto protocol and imposing of the carbon tax, CO2 sequestration as a method to mitigate the high concentration of CO2 in the atmosphere has received a lot of attention [6, 7]. However, the lack of economic incentives has been the biggest hindrance to industrial field-scale application of CO2 sequestration. Emerging in the last decade, CO2-EOR was proposed as a method to add economic benefits of CO2 injection to mature oil fields to the environmental merits of CO2 sequestration [6]. Therefore, considering the large amounts of research dedicated to CO2-EOR, it is expected that in the near future, more field applications of this technology will be implemented globally. In this chapter, the phase behavior and hydrocarbon miscibility of CO2 is discussed in detail.
