D block is a heavy oil reservoir that was first developed in 1997 with the method of Cyclic Steam Stimulation (CSS). After nearly 20 years’ development, several technical challenges have shown: the reservoir pressure decreased from 7.4MPa to 2.9MPa, and the Steam Oil Ratio (SOR) increase from 2.86 to 3.56. As a result, the traditional CSS method seems to be uneconomic.

In view of the problems shown above, CO2 assisted CSS was tested in this oilfield and has shown great success as follow: The steam injection pressure increased from 5.7 MPa to 6.9 MPa and the SOR decreased from 3.45 to 2.86. Besides the underground success, CO2 was separated from the associated gas and then reinjected into the reservoir. This process can reduce the emission of CO2 so that it is both a economic and green method. In addition, according to the field performance, this paper seeks to establish a better understanding of the possible mechanisms involved in the CO2 assisted CSS.

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