Over the last decade, Unconventional Liquids Rich Reservoirs (ULR) have become the main target for oil and gas investors as conventional formations started to deplete and diminish in numbers. These unconventional plays have a huge oil reserve; however, the primary oil recovery factor is predicted to be less than 10%. Unconventional Improved Oil Recovery (UIOR) techniques are still a new concept in the oil industry since there is no commercial project reported for any IOR technique yet. Miscible gas based EOR technique might be the most potential strategy to improve oil recovery in such complex plays.

In this study, a comprehensive and critical review has been conducted to evaluate the feasibility of miscible gas based EOR technique in ULR. The reports and studies from three different approaches (lab, simulation and pilot tests) were summarized and combined to provide in-depth insights and lessons learned from the applicability of miscible gas based EOR in ULR. Firstly, the main problems in the previous lab and simulation approaches, which were used to investigate the viability of different EOR methods, have been diagnosed. Secondly, the performance of injecting different miscible gases to enhance oil recovery in the pilot tests conducted in ULR has been extensively discussed. Thirdly, the physical and chemical reasoning behind the performance gap for the injected gases in the lab scale versus the field scale of ULR been diagnosed.

This study reported that most of the previous lab and simulation approaches suffered from significant lacks and drawbacks, which created a clear gap in the performance of the injected gases in the lab scale versus the field scale. This research clearly found that the performance of Natural Gas (NG) injection is significantly better than the performance of CO2 injection in terms of enhancing oil recovery in the field pilots. This study also found that the production response of unconventional reservoirs to the injected NGs is much faster than that for the injected CO2. Combining the pilot tests data and simulation studies showed that the number of cycles in huff-n-puff operations has a negative impact on CO2-EOR while it has a positive impact on NGs-EOR. Finally, this research provided deep insights on what the operators can expect from the EOR performance by injecting different miscible gases in the lab scale versus the field scale of ULR.

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