A Technical Review of CO2 for Enhanced Oil Recovery in Unconventional Oil Reservoirs

Unconventional oil reservoirs have become an increasingly essential resource for energy in recent years. Despite the massive oil reserves in these reservoirs, as well as the current developments in horizontal well technology and multi-stage hydraulic fracture treatments, primary oil production in these formations is limited due to low recovery factor (< 10%). Thus, a considerable amount of hydrocarbon remains in the reservoir. Recent studies have demonstrated that the implementation of the carbon dioxide enhanced oil recovery (CO2-EOR) technique could recover the remaining hydrocarbon. In this review, the potential of CO2 huff 'n' puff technique for EOR in unconventional reservoirs is examined thoroughly. The aspects reviewed include oil recovery mechanisms, experimental studies, advanced numerical models, and pilot tests. The most important mechanisms of the CO2-EOR technique in improving oil production in shale oil reservoirs such as, molecular diffusion, viscosity reduction, oil swelling, depressurization and oil extraction, geochemical and aqueous solutions, are discussed in detail. A series of recent studies have indicated that the oil recovery could be either underestimated or overestimated when these mechanisms are poorly understood. Experimental studies on the CO2-EOR technique showed success in unconventional reservoirs and could be applied in formations with porosity as low as 4.1% and reservoir permeabilities as low as 0.0004mD. Numerical models used to simulate CO2-EOR processes were examined in detail. Previous studies demonstrated the reliability of the Embedded Discrete Fracture Model (EDFM) as a potential approach to simulate CO2-EOR processes. Although EDFM has proven to be more reliable than the dual continuum method, its application in field-scale is still limited. This is because the simulation of the CO2-EOR process requires multiple components and compositional numerical models. Additionally, EDFM requires complex gridding and large computational demand. Numerical results on the CO2 huff 'n' puff technique show that it works in reservoirs with matrix and fracture porosity as low as 6% and 0.3%, respectively. In field pilot studies, the injection rate was between 300 and 4000 Mscf/day and the injection pressure was kept from 2000 to 3000 psia. The results indicated a disparity between experimental research conclusions and pilot test performances.