The oil and gas industry has invested billions of dollars on research and development of the CO2 EOR process, which continues today.
Miscibility in CO2 EOR Processes
From a fundamental point of view, CO2 EOR works on a very simple principle, namely, that given the right physical conditions, CO2 (Carbon Dioxide) will mix miscibly with oil, acting much like a thinning agent, much the same way that gasoline does with motor oil. After miscible mixing, the fluid is displaced by a chase phase, typically water.
In more scientific terminology, Holm 19 describes miscibility as: “the ability of two or more substances to form a single homogeneous phase when mixed in all proportions. For petroleum reservoirs, miscibility is defined as that physical condition between two or more fluids that will permit them to mix in all proportions without the existence of an interface. If two fluid phases form after some amount of one fluid is added to others, the fluids are considered immiscible.”
Technically, the critical consideration is that in miscible displacements the residual oil saturation, that is, the oil left after being miscibly contacted with CO2, is reduced nearly to zero. This leads to high oil recoveries and favorable project
economics. This is in distinction to immiscible displacements where considerable residual oil saturations can remain, often leading to unfavorable project economics.
Diagram of CO2 injection in an oil well
Flooding a reservoir with CO2 can occur either miscibly or immiscibly. Miscible CO2 displacement is only achieved under a specific combination of conditions, which are set by four variables: reservoir temperature, reservoir pressure, injected gas composition, and oil chemical composition. The test to determine whether a miscible or immiscible CO2 displacement is described below.
Minimum Miscibility Pressure (MMP)
“The most common method used to determine the conditions at which miscible displacement is achieved is known as a slim tube experiment. A long (40-80 ft), small diameter (1/4 in), high-pressure tube is packed with clean sand (or glass beads) to achieve a fluid permeability of 3 to 5 Darcies. It is then saturated with the reservoir oil of interest and the apparatus is maintained at reservoir temperature. A series of floods are conducted at different pressures, while the exact composition of the displacing CO2, (it may be either highly purified, >96% CO2, or mixed with other hydrocarbon gases such as methane CH4, ethane C2H6, propane C3H8, etc.), is injected.A correlation between oil recovery versus pressure is developed.
Miscible displacement is achieved at the flooding pressure or minimum miscibility pressure (MMP) where about 95% of the oil in the tube is recovered after about 1.3 pore volumes of fluid have been injected. Below this pressure, oil recovery decreases dramatically.”