QHD 29-2 block is a newly discovered offshore oilfield of China with an average water depth of 27.6m, covering approximately 2300 km2. Waterflood is not suitable due to the characteristics of complex lithology, strong heterogeneity, small pores and throats, and low water injectivity. In addition, a wide range of CO2 content (24-90%) in the production gas that presents a difficulty for the mixed gas handling. Thus, near-miscible flooding of the production gas re-injection is proposed for the development of this reservoir. It is concluded that the loss of displacement efficiency can be compensated by the mobility improvement with the pressure decrease. Thus, oil recovery is comparative with that of miscible flooding. Also, the economic and operational costs declined to some extent because smaller gas volumes and lower compression are needed with the reduction of pressure.

Slim tube test, slim tube simulation, and selected empirical formulas are used together to determine the near-miscible pressure interval under the condition of different CO2 contents (40%, 55%, 70%, 85%, 100%). On this basis, a new empirical formula was established to determine the lower and upper boundaries of near-miscible flooding.

Two new points are determined as minimum near-miscible pressure (MNMP) and minimum miscible pressure (MMP), respectively. A higher MMP can be expected by this process which is consistent with the argument that some successful application of CO2 miscible flooding are actually partial miscible flooding or near-miscible flooding. Results showed that the ratio of MNMP to the newly defined MMP is about 0.8 to 0.86. In addition, for the wells QHD 29-2E-4 and QHD 29-2E-5, the lower limit of the CO2 content in the production gas to achieve near-miscible flooding are 34% and 64%, respectively. Moreover, the relative error of the new empirical correlation is less than 10% for the prediction of the pressure interval of near-miscible flooding in the targeted wells.

To the best of our knowledge, this is the first time to study the potential of implementing near-miscible flooding by the production gas re-injection with varying CO2 content in an offshore reservoir of China. The results in the paper can provide a new screening criteria for both enhancing oil recovery and reducing greenhouse gas emissions for similar offshore reservoirs.

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