In our previous publications (Tovar 2017; Tovar et al. 2018a, 2018b; Tovar et al. 2014), we presented a philosophy for the operation of gas injection processes in unconventional liquid reservoirs (ULR) that consisted in using a huff-and-puff scheme at the maximum possible pressure, regardless of the MMP. We also postulated a kinetically slow peripheral vaporizing gas drive as the main recovery mechanism underlying the rationale for such operational philosophy. We based all of our findings in a collection of 21 experiments performed using crude oil and core plugs from the Wolfcamp. The main focus of this paper is that the fundamentally different production mechanisms taking place in ULR cause the recovery factor to continue increasing when pressure is increased beyond the MMP. We do this using core plugs and crude oil from a different field, the Eagle Ford. Confirmation of this finding is necessary, since it directly contradicts the behavior in conventional reservoirs. We also demonstrate the addition of a dopant, into the crude oil, has little effect in the phase behavior, which widens the validity of all our work so far; and provides additional insights into the gas transport in the porous media. The production of oil from unconventional liquid reservoirs (ULR) has seen a significant increase in the last decade due to the implementation of horizontal drilling and hydraulic fracturing technologies. However, these reservoirs have mainly been exploited through primary production, which exhibits fast production decline and low ultimate recovery. Therefore, the need to understand different transport mechanisms and to develop enhanced oil recovery (EOR) techniques to improve ultimate oil recovery and extend the life of the asset is critical. This work investigates the effects of miscibility on enhancing recovery and the implementation benefits we can obtain from it.

We performed five additional core-flooding experiments. The cores were cleaned using an extended Dean-Stark extraction and re-saturated to known initial oil in place in the laboratory. Gas injection through a hydraulic fracture was simulated using high permeability glass beads surrounding the cores that were then packed in a core holder. The high permeability media was then saturated with CO2 at constant pressure and reservoir temperature. The production was monitored using a CT scanning technology throughout the length of the experiments to track changes in composition and saturation as a function of time and space. Soak time was maintained constant and the experimental pressures were selected above and below the slim-tube MMP to show the effect of MMP on recovery.

Our results are consistent with a kinetically slow, peripheral vaporizing gas drive production mechanism. Recovery factor was 50% at the highest pressure of 3,500 psig. This is higher than the maximum of 40% we previously observed in the Wolfcamp, possibly due to the higher concentration of intermediate hydrocarbon components in the Eagle Ford, and the higher experimental pressure. Recovery factor increases with pressure, even above the MMP. The addition of 5% Iodobenzene in the Wolfcamp oil, increased the MMP by only 136 psig, or 7 %, indicating our conclusions are valid.

This work confirms our previous findings, which challenge the paradigm that establishing miscibility is enough to achieve the highest recovery factors during CO2 flooding, as is the case in conventional reservoirs. This finding has a significant impact on field operations and should be considered during the design of gas injection EOR processes in ULR.

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