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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 , HWACO 2 , WACO 2 a.WF, SWACO 2 , SSWACO 2 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 SWACO 2 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.

Primary sweep efficiency in carbonate reservoirs is low due to the low contribution of the matrix in production. Also, due to unfavourable mobility ratio, secondary recovery is not also effective. Hence, design and application of an effective enhanced oil recovery method is essential to improve recovery. Different methods such as water injection, gas injection, chemical injection, thermal, and a combination of these methods such as water alternating gas (WAG), simultaneous WAG (SWAG), selective simultaneous WAG (SSWAG), foam injection, and foam-assisted WAG (FAWAG) are applied to improve the recovery efficiency in naturally fractured reservoirs. In this paper, selection of the best EOR method and well pattern for a naturally fractured reservoir is studied to reach the highest net present value (NPV) and field oil recovery efficiency (FOE). Results showed that the FAWAG process in a five-spot well pattern with 22.7% oil recovery efficiency is the best technical and economical method.Keywords: carbonate reservoirs; water injection; gas injection; foam injection; well pattern.Reference to this paper should be made as follows: Bagrezaie, M.A. and Pourafshary, P. (2016) 'Screening and optimisation of water/foam/gas injection EOR scenarios in a fractured reservoir', Int.