Study of different water alternating carbon dioxide injection methods in various injection patterns in an iranian non fractured carbonate reservoir

In the commonly-used EOR methods such as water flooding and gas injection, high and unsuitable mobility ratio lead to viscose fingering and reduction of sweep efficiency. Early breakthrough of the injected fluid 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 reservoir 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 compared to a single water flooding process. This higher pressure is caused by the gas slug injection which causes 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 compared to the pure gas injection. Therefore, less gas breaks through to the producing wells, reducing gas handling requirements. Furthermore, the lower producing GOR associated with WAG injection results in less erosion of the production equipment. In this work, scenarios of water alternating carbon dioxide injection such as WACO ₂, HWACO₂, WACO₂a.WF, SWACO₂, SSWACO₂ and pure water and carbon dioxide injection are compared to specify the appropriate injection method. Also these scenarios in Four-spot, Dual Four-spot, Five-spot, and Dual Five-spot well patterns were compared. Results indicated that SWACO₂ injection in Dual Five-spot pattern has the highest oil production and lowest residual oil saturation. Five-spot pattern had the higher recovery and lower residual oil saturation than Dual Four-spot pattern, so adding the number of wells could not raise the recovery.