Feasibility Study of Gas Reinjection in Giant Carbonate Reservoir in the Middle East
EOR application in a greenfield
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
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.