To date, field wide CO2 flooding is continuous or WAG injection processes. In individual wells a huff’n’puff scheme has been used where CO2 is injected into a well followed by a shut-in period; after a predetermined time, the well is produced. Each of these methods have practical, technical, and economical limitations. This paper describes a new CO2 injection method that combines all three of these methods but excludes water injection.

Continuous CO2 may be optimal in reservoirs that are not conducive to water injection and do not have CO2 mobility control problems such as early breakthrough. However, continuous CO2 requires a large initial CO2 volume that may be unavailable and relatively expensive. WAG is very common with variations of different ratios of water to CO2 volumes and tapered ratios. WAG reduces mobility problems and thereby improves areal sweep efficiency; CO2 purchase expenses may be lower due to the requirement of lower CO2 volumes. Unfortunately, water injectivity following CO2 injection may decrease, and the water increases lift and water handling expenses. Huff’n’puff is effective similar to a near wellbore stimulation, but may not realize the benefits of a full field injection program. Also, this adds the complication of injection and production capabilities required in the same well. This may be impossible for some wells on artificial lift.

The proposed method is to inject CO2 and shut in the well similar to the huff and puff, but instead of producing the well inject CO2 again. This process is a cyclic process like WAG, and shares the variables of the duration and volume of CO2 injection and the duration of the shut-in period. This method eliminates injectivity and production expenses associated with water. Mobility is controlled by near wellbore achievement of miscibility by the increase in mass transfer between the CO2 and oil during the shut-in portion of the cycle; i.e. a smaller portion of the reservoir has relatively low viscosity, injected CO2 due to miscibility between oil and gas occurring nearer the injection well.

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