The block H-59 in the Daqingzijing region was selected as a pilot site for the first stage of the CCS project in Jilin oilfield after an extensive assessment. This block is a light oil reservoir with a low permeability. The performance of water flooding after the primary oil recovery was very poor. Therefore, CO2 injection has been started since April 2008 for EOR associated with CO2 storage for environmental benefits. This paper is aimed at assessing the current CO2 storage capacity and distribution at different states in the oil reservoir after 6-year injection until April 2014. Based on various CO2 trapping mechanisms, an evaluation method of CO2 storage potential is established to calculate the theoretical and effective CO2 storage capacities in target oil reservoir the current amount of CO2 buried in the H-59 block was calculated according to the field data. The reservoir numerical simulation was used to analyze the distribution and existing state of CO2 underground. The assessment results show that the theoretical capacity of CO2 storage in the H-59 block is 72.32×104 t, and the effective capacity of CO2 storage is 26.37×104 t. The calculation of effective CO2 storage capacity in oil reservoir considers the engineering practice of field operation during project life. The coverage factor of well pattern (k1) and the sweep coefficient of CO2 within the well pattern (k2) have been introduced in the method. Meanwhile, the mineral trapping was neglected for short-term storage of CO2 based on a preliminary geochemical simulation analysis. There are 17.45×104 t CO2 which has been buried in the block until April 2014. The distribution of buried CO2 between the injection and production wells is mainly determined by the reservoir physical properties and the total amount of CO2 injected in each well. Reservoir simulations indicate that 61.0% of CO2 buried in the oil reservoir has been trapped at supercritical state, and the amounts of CO2 dissolved in oil and water account for 24.4% and 14.6% respectively. These proportions of CO2 at different states are very close to the calculation results of effective CO2 storage capacity. In comparison to the effective CO2 storage capacity, it is thought that the block H-59 still has a certain storage potential of 8.92×104 t at present. For the assessment methods, the parameters k1 and k2 for calculation of effective CO2 storage capacity deserve for further discussion. It should be also noted that the accuracy of CO2 distribution predicted by reservoir simulation greatly depends on the accuracy of geological model. It needs more efforts to improve the understanding of the target reservoir properties.

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