The fundamental distinction between a CO2-EOR project and a CO2 Storage (CCS) project is that the former aims to maximize oil production while minimizing CO2 retention until the oil production becomes uneconomic and the project is abandoned, while the objective of the latter is to store as much CO2 as possible during and after cessation of oil production. A new transition scheme between these two main phases is proposed and described here. The objective of this transition hybrid scheme is to maximize CO2 storage while continuing oil production which otherwise would have been uneconomic. This will create additional storage space in the reservoir by producing incremental oil, with the reservoir produced water being disposed of in another formation.

A hypothetical reservoir simulation model is used to illustrate the shift over time from a conventional CO2-EOR project to a hybrid CO2-EOR/CO2 storage scheme and then to a pure CO2 storage scheme. The most common case of primary-water flood-water alternating CO2 gas (WAG) injection was simulated to mimic a typical oil pool development history. Insertion of the distinct hybrid transition phase between the end of the CO2-EOR operation and the initiation of a pure CO2 storage operation leads to an incremental oil recovery factor of approximately 9% OOIP (a function of the residual oil saturation after the end of the CO2-EOR operation). In other words, cases with the lowest recovery factor (highest residual oil) after CO2-EOR operations and just before the start of the hybrid CO2-EOR/CCS phase could potentially deliver the highest incremental oil recovery factor during this hybrid phase. More importantly, it is shown that the implementation of a hybrid scheme significantly increases the amount of CO2 storage in the reservoir, by up to 67%.

In conclusion, the advantages of having a hybrid CO2-EOR/CCS phase between the end the commercial CO2-EOR operation and pure CO2 storage phase are twofold: 1) generating oil sales revenue to offset some of the costs associated with CO2 capture, transportation and storage by delivering some incremental oil production, and 2) storing significantly more CO2 in the reservoir by creating additional voidage by producing incremental oil and water.

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