Gas injection is the most widely applied recovery method in light, condensate, and volatile oil carbonate reservoirs. Gas has high displacement efficiency and usually results in a low residual oil saturation in the part of the reservoirs that is contacted with gas. The displacement efficiency increases when the injected gas is near-miscible or miscible with the oil. In addition to nitrogen and hydrocarbon gas projects, CO2-EOR has been the dominant gas EOR process. Gas-based EOR has been implemented in both mature and waterflooded carbonate reservoirs.

In this paper, we present the results of a detailed experimental study aimed at understanding the performance and efficiency of gas injection in carbonate reservoirs. A series of immiscible and miscible gas injection coreflood experiments were performed using limestone reservoir cores under different injection strategies. To minimize laboratory artefacts, long cores were used in the experiments and to observe the effect of gravity both 2-inch diameter and 4-inch diameter (whole core) were used. The experiments were performed under reservoir conditions using live crude oil. The core wettability was restored by ageing the core in crude oil for several weeks under reservoir temperature. Hydrocarbon gas (methane) was used as the immiscible injectant and both CO2 and a mixture of 50% C1 and 50% CO2 were used as miscible injectant. All gas injection experiments were performed using vertically oriented cores and the gas was injected from the top unless it is stated otherwise.

The main parameters investigated in this study are: 1- The effect of miscibility on oil recovery for both continuous gas injection and WAG, 2- The effect of gravity on gas sweep efficiency compared to water flooding, 3- the effect of gas-oil IFT on oil recovery when using the same oil, 4- the effect of oil type on oil recovery using the same injected gas at miscible and immiscible conditions, 5- the effect of immiscible gas injection on subsequent miscible gas injection performance and 6- Impact of CO2 cycle length on ultimate oil recovery. In addition, this work investigated the impact of the order of fluid injection where multiple WAG injection cycles were performed in separate experiments after water or gas injection.

The main conclusions of this study are: 1- As expected miscibility has a significant impact on displacement efficiency and oil recovery where miscible gas recovered more than 20% extra oil compared to immiscible gas.2- A significant variation in oil recovery is observed for miscible gas injection, i.e., more than 10 saturation units difference, depending on the MMP between the injected gas and crude oil even when both experiments are performed at miscible conditions using the same injected gas. 3- The performance of tertiary CO2 flood was adversely affected by the slug of immiscible gas injected. Therefore, it is not recommended to have immiscible gas injection before miscible gas injection. 4- Regardless of injected gas type, gas injection with similar IFTs achieved similar oil recovery. 5- During WAG experiments, starting the injection cycles with water or gas did not have any impact on the ultimate oil recovery for both miscible or immiscible cases for one of the reservoirs while WAG_G (WAG starting with gas injection) recovered more oil for another reservoir. 6- Gravity has significant impact on oil recovery for both miscible or immiscible gas injection. Significant difference is observed in oil recovery when comparing CO2 injection on 2-inch and 4-inch diameter core sample or when comparing horizontal vs vertical immiscible gas injection and WAG experiment. 7- The longer the CO2 slug size the higher the oil recovery observed in gas injection experiments.

The results of this study provide a rich and rarely available set of experimental data that can help improve and optimize gas and WAG injection in oil-wet carbonates.

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