Unconventional oil, such as tight oil and shale oil, has become one of the most significant contributors of oil reservoirs and production growth. Due to low porosity and ultra-low permeability, unconventional oil reservoirs require multistage hydraulic fracturing technique to maximize production. However, the primary recovery remains very low to narrow the profit margin heavily. Although CO2 huff-n-puff process holds great potential to increase oil recovery and has a chance to sequester CO2 to reduce environmental footprint, our current knowledge of the performance of this process is very limited.

With numerical simulation, we performed a series of sensitivity work to present the impacts of reservoir properties, fracture properties and operation parameters such as CO2 injection rate, injection time, soaking time, number of cycle of CO2 on enhanced oil recovery in the tight oil formation. What's more, the method of analysis of variance (ANOVA) was used to evaluate the performance of CO2 huff-n-puff process and beneficial result from CO2 EOR technology. Simulation results showed that bottom hold pressure and injection cycles impose more significant impose on oil recovery increment than injection time, injection rate and production time per cycle. Based on the typical reservoir and fracture properties from tight oil reservoir, the numerical models were established to evaluate the performance of four EOR methods: CO2 huff-n-puff, water huff-n-puff, nanofluids huff-n-puff and water alternating gas (WAG). With the comparison of oil recovery and its increment of four EOR methods and depletion method, it is found that CO2 huff-n-puff method would lead to much more incremental oil recovery than other three methods, which reveals its huge potentials of enhancing oil recovery and improving development profit in unconventional reservoirs. The conclusion of this work has the potential to advance our understanding of the role of CO2 in developing unconventional oil reservoirs, which will benefit both energy economy and environment with CO2 geological sequestration.

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