Citation: Gugl, R.; Kharrat, R.; Shariat, A.; Ott, H. Evaluation of Gas-Based EOR Methods in Gas- Invaded Zones of Fractured Carbonate Reservoir. Energies 2022, 15, 4921. https://doi.org/10.3390/ en15134921 Academic Editors: Shu Tao and Min Wang Received: 1 June 2022 Accepted: 30 June 2022 Published: 5 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Evaluation of Gas-Based EOR Methods in Gas-Invaded Zones of Fractured Carbonate Reservoir Ronald Gugl 1 , Riyaz Kharrat 1, * , Ali Shariat 2 and Holger Ott 1 1 Department Petroleum Engineering, Montanuniversität Leoben, 8700 Leoben, Austria; ronald.gugl@hotmail.com (R.G.); holger.ott@unileoben.ac.at (H.O.) 2 Computer Modeling Group Ltd., Calgary, AB T2L 2M1, Canada; ali.shariat@cmgl.ca * Correspondence: riyaz.kharrat@unileoben.ac.at Abstract: More than half of all recoverable oil reserves are found in carbonate rocks. Most of these fields are highly fractured and develop different zonations during primary and secondary recovery stages; therefore, they require a different developmental approach than conventional reservoirs. Experimental results for water-alternating gas injection [WAG] and foam-assisted water-alternating gas [FAWAG] injection under secondary and tertiary recovery conditions were used to investigate these enhanced oil recovery [EOR] methods in gas-invaded reservoirs. The relative permeability curves of the cores and the fitting foam parameters were derived from these experiments through history matching. These findings were then used in a quarter five-spot, cross-sectional, and a sector model of a carbonate reservoir where a double five-spot setup was implemented. The fracture and matrix properties’ impact on the recovery was illustrated through the cross-sectional model. The gas mobility reduction effect of the FAWAG was more noticeable than that of WAG. The apparent viscosity of the gas was increased due to the foam presence, which caused a diversion of the gas from the fractures into the matrix blocks. This greatly enhanced the sweep efficiency and led to higher oil recovery. The gas front was much sharper, and gravity overrides by the gas were much less of a concern. The properties of the fracture network also had a significant effect on the recovery. Oil recovery was found to be most sensitive to fracture permeability. At the same time, sweep efficiency increased substantially, improving the recovery rate in the early injection stages, and differed slightly at the ultimate recovery. However, a lower fracture permeability facilitated gas entry into the matrix blocks. The results of the reservoir sector model were similar to the core and pilot. However, the WAG injection recovered more of the uppermost layers, whereas significant portions of the lowest layer were not effectively recovered. In contrast, FAWAG was more effective in the lowest layer of the reservoir. The FAWAG was a beneficial aid in the recovery of gas-invaded fractured reservoirs, increasing the oil recovery factor with respect to WAG. Keywords: fractured reservoir; EOR; FAWAG; WAG; gas invaded zone 1. Introduction Naturally fractured reservoirs contribute considerably to the world’s hydrocarbon reserves; however, they usually have a lower oil primary recovery. This is related to different wetting conditions in reservoirs. Conventional reservoirs and fractured reservoirs are very different in nature. The distribution of the fluids is not the same for both. In a conventional reservoir, the two-phase contacts of water–oil and gas–oil are smooth, and the transition zone may be significant in static conditions and, thus, will remain so in dynamic conditions. During production, gas-invaded zones and water-invaded zones may form as a result of oil production. On the contrary, the transition zone in a fractured reservoir is sharp and abrupt; hence, the transition zone is horizontal in static and dynamic conditions (Figure 1). Given that the transmissivity in a fracture network is high due to its large permeability, any change in level is rapidly re-equilibrated [1]. These fractures Energies 2022, 15, 4921. https://doi.org/10.3390/en15134921 https://www.mdpi.com/journal/energies