Soak alternating gas: A new approach to carbon dioxide flooding
Murray, Malcolm David
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Carbon dioxide (CO2) flooding as a method of enhanced oil recovery (EOR) has been used successfully for many years to increase the recovery of the original oil in place (OOIP) of a reservoir. The two most common methods of using CO2 to accomplish this are the continuous CO2 flood and the cyclic CO2 flood, or CO2 hufTn'puff. In the continuous CO2 flood, CO2 is injected continuously into a wellbore while fluids are recovered continuously at the adjacent wellbores. A frequently used variation of this process is the water alternating-gas (WAG) flood where CO2 and water are injected alternately to combine the solvent properties of CO2 with the mobility properties of water and further optimize the recovery. However, one limitation to the WAG process is that of water shielding where the relatively high water saturation in the pore spaces prevents much of the potential contact between the crude oil and the CO2 from occurring and causes a significant portion of the oil to be bypassed. In the cyclic CO2 flood, CO2 is injected into the reservoir, shut in for a soak period, then produced from the same wellbore until the economic limits of production are reached and the cycle must be repeated. During the injection phase of a CO2 hufTn'puff, CO2 is distributed over as great an area as possible throughout the reservoir. Next, during the soak period CO2 is allowed to disperse through the water to contact as much oil as possible and make full use of the CO2 recovery mechanisms, which allows production to be optimized in the production phase of the process. In this work, a new concept in CO2 flooding is introduced as "soakalternating-gas," or SAG, which incorporates the soak period of a CO2 huffn'puff into the continuous CO2 flood to provide additional mobility control and a viable alternative to a WAG process in cases where water injectivity is too low to allow WAG to be feasible. Since SAG does not depend on water injectivity the prospect of greater recovery in such cases could be quite significant. In addition, the mobility control provided by SAG may offer advantages over those of WAG, even where water injectivity is adequate. Thus, the integration of continuous CO2 flooding techniques with those of the CO2 hufTn'puff appears to offer greater recovery potential than those of either method used separately. The concepts behind SAG appear to be supported by previous literature on the research, testing, and implementation of the continuous CO2, the CO2 hufTn'puff, and the WAG processes. In order to ascertain its potential, it is recommended that the SAG process be investigated with respect to miscibility conditions as well as the parameters of the injection stage, soak period and production stage, then verified experimentally using slim tube experiments, coreflood experiments and pilot tests prior to full-scale field implementation.