A Review of the Mechanisms of Miscible and Immiscible Carbon Dioxide Injection in Shale Reservoirs for Storage and Increased Oil Recovery Applications

Unconventional shale reservoirs are now considered a huge source of energy for the hydrocarbon industry. One of the currently proposed methods of increasing oil recovery from these reservoirs is carbon dioxide (CO₂) injection. This method of enhanced oil recovery has two main advantages including the ability to increase oil recovery significantly, and the ability to store a large portion of the CO₂ injected downhole. This research aims to perform a detailed review of the difference in mechanism between immiscible and miscible CO₂ injection in shale reservoirs to illustrate the mechanisms, benefits of each method, and the optimum conditions at which each should be applied to reach the maximum oil recovery possible, and the largest storage potential. The main difference in mechanism between miscible and immiscible CO₂ injection comes in the interfacial tension (IFT). In miscible CO₂, the IFT is reduced to zero, however, unlike common belief, immiscible CO₂ injection will also have an impact on IFT reduction due to CO₂ solubility in the oil. Miscible CO₂ usually is applied best with light and intermediate hydrocarbons due to the vaporization-condensation mechanism, while immiscible CO₂ injection is applied best with heavy oils due to the oil swelling mechanism. In terms of CO₂ storage, it was found that it is a function of both the fluid injected, and the rock itself. Increasing the injection pressure will result in a higher adsorption which in turn will result in a larger storage potential. If there is a large concentration of minerals that can interact with the CO₂ however, the storage potential is then impacted, either negatively or positively depending on the situation. This research aims to provide a detailed review of miscible and immiscible CO₂ injection in shale reservoirs and determine the best application conditions for each to reach the highest recovery and storage potentials.