Miscible gas injection enhanced oil recovery (huff-n-puff gas injection) has received increased attention especially in the unconventional plays like the Eagle Ford, where oil recovery is as low as 5 – 10% (Sheng, 2015). An increase in 1% of recovery in the Eagle Ford could realize a potential of 2.3 billion barrels of oil, which has an enormous economic value. Laboratory investigation of huff-n-puff gas injection can help in the systematic evaluation of different factors affecting the recovery performance of huff-n-puff gas injection operations. The focus of this study is to evaluate the efficacy of huff-n-puff gas injection in the Eagle Ford..

Eagle Ford shale samples were placed inside the a high-pressure vessel and different types of gas: carbon dioxide (CO2,) methane (C1), ethane (C2), C1:C2 (72:28) mixture, C1:C2 (95:5) mixture, and field gas were injected at various pressures (1000 psi below MMP, MMP, and 1000 psi above MMP) with various soaking time of (1 hr, 3 hr and 6 hr). Nuclear magnetic resonance (NMR), HAWK source rock analysis, and gas chromatography (GC), were performed to quantify measurable changes in produced and residual hydrocarbons in preserved Eagle Ford shale samples.

Various controlling factors such as minimum miscibility pressure (MMP), surface area, soaking time, injection pressure, injection gas rate, and type of injection gas on huff-n-puff gas injection performance were evaluated.

Vanishing Interfacial Tension technique (VIT) was used to measure MMP for the Eagle Ford oil. MMP values with different types of gas: carbon dioxide (CO2,) methane (C1), ethane (C2), C1:C2 (72:28) mixture, and field gas were measured to be 2500 psi, 6000 psi, 1000 psi, 3500 psi, and xx

3000 psi, respectively. Methane concentration plays a major role in MMP. As methane concentration increased, MMP also increased.

Surface area studies showed that after 5 huff-n-puff cycles, the recovery from samples with 7-8 mm and 0.9-2 mm sample sizes were 61% and 42%, respectively. Smaller sample size yields a higher recovery due to more surface area and better access to the small pores, which indicates the importance of stimulated reservoir volume (SRV).

When soaking time is compared per cycle, 6 hr soaking time yields the highest recovery compared to 1 hr or 3 hr soaking time. Longer soaking time also produced slightly heavier hydrocarbons. However, when residence time (soaking time + production time) is considered, there is no significant difference in ultimate recovery. This result suggested that longer soaking time seems to be a better economical choice due to the need for fewer injection cycles.

Injection pressure above MMP yields a higher recovery compared to pressure below MMP. Injection pressure also determines the fraction of hydrocarbons mobilized. When injection pressure was 1000 psi above MMP, mobilized hydrocarbon included up to C25. However, when injection pressure was 1000 psi below MMP, mobilized hydrocarbon was limited to C19 and below. Excessive pressure above MMP did not yield additional recovery in 7-8mm size samples. In addition, the effect of injection rate was investigated. High injection gas rate lead to better recovery (36%) than low injection rate (23%).

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Our study indicates that gas composition has strong effect on the recovery factor. At cycle 3, with 7-8 mm sized particles, P=1000 psi above MMP and T=150°F, one hour soaking, and one hour production time, ethane showed the best performance of all the gases (40% recovery). CO2 performed the second best (32%). C1:C2(72:28) mixture and field gas exhibit the similar recovery (24% and 21%, respectively). C1:C2(95:5) mixture showed the worst recovery (11%). This highlights the potential benefits of enriching injection gas.