Performance evaluation and mechanisms study of near-miscible CO2 flooding in a tight oil reservoir of Jilin Oilfield China

Abstract Jilin Oilfield has been conducting a large-scale demonstration project on CO 2 EOR and storage in China. CO 2 separated from a nearby natural gas reservoir (15–30 vol% CO 2 ) has been injected into the northern part of H-59 oil block with the permeability of 3.0 mD and porosity of 12.7%. After six years of operation, nearly 0.26 million ton of CO 2 (0.32 Hydrocarbon Pore Volume) has been injected under a miscible or near-miscible flooding mode with CO 2 utilization efficiency of 6.3 MScf/bbl, and an expected enhanced oil recovery of over 10% would be achieved. A systematic and thorough reservoir surveillance program has been conducted to facilitate efficient operation and evaluation. Casing annulus gas composition was analyzed to monitor gas breakthrough in production wells as well as the gas tracer to detect CO 2 flow across the reservoir. Bottom hole pressure (BHP) survey of producers and fluid sampling were combined to evaluate the miscibility effect in conjunction with the injection-production data analysis. It is revealed that the designed miscible flooding is best described as near-miscible due to the large pressure difference between injector and producer, and the gas channeling and fractures/heterogeneities cause the miscibility instability. Gravity-stabilized displacement may develop in the downdip part of the reservoir during the flooding process. The vaporization and condensation effects have been observed and confirmed through laboratory core flooding and field produced oil compositions analysis. A key insight from the evaluation study is that CO 2 near-miscible flooding is more flexible and easily realizable relative to miscible, particularly in the tight oil reservoir. Further study would be necessary to optimize the reservoir development program under the near-miscible flooding design.

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