A newly discovered offshore oilfield in QHD 29-2 block is facing the problem of selecting the appropriate developing method. It covers approximately 2300 km2 and with an average water depth of 27.6m. The depth of the exploration well is over 3600 m with thick sand layer and oil zone. Conventional waterflood cannot be implemented due to the reservoir characteristics of small pores and throats, complex lithology, strong heterogeneity and low water injectivity. Near-miscible flooding is proposed considering the wide range of CO2 content (24–90 mol%) in the production gas.

Slim tube test and slim tube simulation are conducted successively to determine the minimum miscible pressure (MMP) of the production gas and oil samples from the targeted reservoir. The relationship of displacement efficiency (DE), interfacial tension (IFT) and displacement pressure are provided and chosen as the basis for the division of the pressure interval of near-miscible flooding. The lower limit of the CO2 content in the production gas to achieve near-miscible flooding are determined for the well 29-2E-5 with the well depth of 3308-3330 m. On this basis, an adjustment measures of adding intermediate components of (C2-C6) is proposed and assessed. The amount of the adding components is calculated and provided correspondingly.

Boundaries of the pressure region of near-miscible flooding are obtained for different CO2 contents. Considering the reservoir conditions (112.1 °C, 31.96 MPa), the lower CO2 content of 64% is estimated to be able to achieve near-miscible flooding for the targeted well. Accordingly, 2.3, 6.5, and 10.3 mol% of (C2-C6) are determined to be the lowest amounts for the adding components to achieve near-miscible injection for the CO2 contents of 55%, 40%, and 24%, respectively.

Thus, the evaluation of the feasibility and optimization measures of near-miscible flooding by production gas re-injection with varying CO2 content in a newly discovered offshore reservoir was conducted. Specific regions in the vicinity of MMP for impure CO2 near-miscible flooding on the basis of comprehensive analysis of displacement efficiency and IFT from the views of both engineering and physicochemical were determined.

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