In the commonly-used EOR methods such as waterflooding and gas injection, high and unsuitable mobility ratio leads to viscose fingering and reduction of sweep efficiency. Early breakthrough of the injected fluid caused in the production wells is the major problem associated with these processes. A solution to this problem is an alternative process called water-alternating-gas (WAG).
WAG injection could increase the recovery by contacting the upswept zones, especially recovery of attic or cellar oil by exploiting the segregation of gas to the top or accumulating of water towards the bottom. The WAG process has been proved beneficial in re-pressurizing the reservoir when compared to a waterflood only process. This higher pressure is caused by the gas slug being injected at an extremely high voidage replacement rate because of its high mobility. WAG injection increases the efficiency of the plain gas injection, too. By alternating the gas injection with water injection, the gas relative mobility in the reservoir is reduced over gas injection only. Therefore, less gas breaks through to producing wells, reducing gas handling requirements. Furthermore, the lower producing GOR associated with WAG injection over straight gas injection results in less erosion of the production equipment.
In this work, scenarios of water alternating carbon dioxide injection such as (WACO2), (HWACO2), (WACO2a.WF),(SWACO2),(SSWACO2) and water and carbon dioxide injection are compared to specify the appropriate injection method. Also these scenarios in four patterns such as 4-spot, Dual 4-spot,5-spot, Dual 5-spot were compared.
Results indicated that, SWACO2 injection has the higher oil production and lowest residual oil saturation. In addition, amoung these scenarios SWACO2 in Dual 5-spot pattern had the highest recovery and lower residual oil saturation. 5-spot pattern had the higher recovery and lower residual oil saturation than Dual 4-spot pattern, so adding the number of wells could not raise the recovery.
