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The Study on Characteristics of Low Permeability TZ/ROZ via CO2 Displacement

Abstract

Geological reserves of the oil-water Transition Zone (TZ) and Residual Oil Zone (ROZ) in the low permeability reservoir has been considered as uneconomical for further development because of the high water cut and low abundance of resource. Though the TZ may account for 30 %-50 % of the whole reservoir, it has not got more attention yet. The average oil saturation of TZ is about 35 %, which is equal to that of a reservoir after water flooding. Currently, the CO2 flooding is an effective technique for residual oil recovery after water flooding, which could reduce the residual oil saturation greatly. Therefore, it will be significance for the Enhanced Oil Recovery (EOR) of reservoir with low permeability if the technique of CO2 flooding could be applied in the TZ successfully. In the present study, a method based long core to simulate the distribution of oil saturation in the TZ has been set up in laboratory by using bidirectional saturating. In order to investigate the CO2 flooding characteristics of TZ, experiments have been carried out with 3 different initial oil saturations. The experiment data demonstrate that the residual oil saturation after CO2 flooding relies in the initial oil saturation. The residual oil saturation after CO2 flooding is 23.77 % with oil saturation of 64.6 %, while the residual oil saturation after CO2 flooding is 17.1 % with 38.1 %, whose EOR rate is of 59.3 %. It was believed that the residual oil saturation was as same as the upper main pay zone. But the experiments data shows that the value of residual oil saturation changes vertically according to the initial oil saturation, which means the profit of TZ has been underestimated greatly. In the end of the paper, the recoverable reserve of TZ and its contribution to EOR were calculated based the reservoir WJ in the Jilin Oilfield, China.