Selection of hydrocarbon gas for huff-n-puff IOR in shale oil reservoirs

In shale oil reservoirs, cyclic CO2 injection has proven to be effective at recovering significant amounts of oil, however, certain downsides to the use of CO2 exist owing to its corrosive nature and its higher miscibility pressure compared to hydrocarbon gases such as ethane and propane. This paper presents simulation results focused on the impact of phase behavior on the injection of single component gases such as; N-2, CO2, C-1 and C-2, and a combination of hydrocarbon gases with varying C-1, C-2 and C-3 compositions, as the basis for selecting an injectant, that is more effective than CO2 for cyclic gas injection. The simulations presented were carried out with rock and fluid properties from the Eagle Ford reservoir using Multi-Frac-Comp, an integrated Equation-of-State geomechanical compositional hydraulic fracturing and reservoir simulator (Zheng et al., 2019a, 2019b). Oil recovery results obtained for single component gas injection shows that C-2 injection yields comparable oil recovery to CO2 and lower gas-oil ratio. Of all the injected gases, N-2 yielded the lowest oil recovery and highest GOR because of its poor miscibility with the reservoir fluid. Simulations carried out using hydrocarbon gases with varying compositions of C-1, C-2 and C-3 show recovery factors that are higher than C-2 injection. The results show that hydrocarbon gases with a higher C-1 content over C-2, results in faster oil recovery during the initial cycles of huff-n-puff because the high compressibility of C-1 enhances the volume expansion obtained upon depressurization during the production phase. In contrast, HC gases with higher C-2 and C-3 content yield higher recoveries during the later cycles because the high density of these gases improves miscibility with the reservoir fluid during the soak phase. The results from this study provide operators with information required to select a hydrocarbon gas with an optimum composition of C-1, C-2, C-3 and C-4. This strategy of tuning the injected fluid composition during different cycles of gas injection can play an important role in improving oil recovery from huff-n-puff field pilots.