Feasibility study was performed to evaluate the possibility of reinjecting produced gas back to formations in a green field, Iraq. Uncertainties associated with application of EOR in green fields were addressed and conceptual field development plan was developed to optimize oil production from targeted formation.

Formation in which the gas was to be reinjected had to be optimally selected among multiple isolated producing formations, as there is not enough gas to enhance recovery in all oil formations. Selection criteria depended on formation rock/fluid properties, miscibility, gas availability/adequacy in comparison to the in-place of each formation and other general screening criteria for EOR application.

The challenge was to define and optimize the composition of reinjected gas. For this purpose and also to understand the oil recovery mechanism and displacement efficiency, different EOR related lab experiments were performed. Experiment data were used to calibrate the numerical model.

Composition of injected gas was optimized by considering different mixing ratios of produced gas from different stages of separators and gas contribution from different formations. The criteria to define optimum mixing ratio were to achieve miscibility in lowest possible pressure and also to avoid condensing the injected gas to liquid in the reservoir.

Gas injection parameters were optimized for selected formation by using 2D cross section, 3D sector, and full-field simulation models. Full-field production performance under gas reinjection was forecasted based on conceptual development plan.

To correctly optimize the field development plan by gas reinjection, number of possible geomechanics-related issues due to increase in reservoir pressure was considered. Study showed that there is no indication of cap rock fracturing during gas injection period, and risk for significantly reducing the capacity of cap rock seal and integrity is low.