The strategy of development with simultaneous downdip water injection adopted in early 90's has enabled efficient pressure maintenance and sustained production from multi stacked reservoirs of an onshore oil field in UAE. Present study addresses bottommost twin reservoirs in the stack sitting one over other. Full field simulation model for these reservoirs built in 2006 encountered great difficulty in matching observed pressure trends in individual wells. The model predicted much higher pressures than observed in the wells. The mismatch was attributed to possible compartmentalization by numerous faults. Accordingly, further improvement in Geological model was affected in 2011 by updating it with more number of faults and modified structural configurations based on new 3D seismic interpretation. The simulation model was updated by incorporating improved geological model and the pressure-production-injection data till 2011. However, the simulation model still suffered from predicting much higher well pressures, as earlier. An MBAL analysis was then performed to investigate the average pressure trend of the twin reservoirs which indicated the possibility of irrational allocation of injected water at perforation level. This finding from MBAL study was later confirmed by the evidences from surveillance data.
The paper discusses how the finding from MBAL study has been confirmed by simulation study by successful history matching of well pressures after rational distribution/allocation of injected water. Evidences for irrational allocation of injected water to individual injectors based on available surveillance data have also been presented. Based on the findings, the management is embarking on the improvement in the surveillance of water injection using continuous and reliable monitoring techniques as accurate accounting of injected water is crucial for the success of ongoing development to ensure uniform sweep and avoid premature water breakthrough in the wells thereby maximizing oil recovery.