Determining of reservoir surface subsidence is a crucial problem especially for soft and unconsolidated formations. For long time production with water injection and/or water influx, the pore pressure decreases with increasing in effective stress leading to reservoir compaction and vertical surface subsidence. In this paper, a Tank Capacitance-Resistive Model (TCRM) has been done to estimate the history of vertical subsidence in the upper sandstone member/main pay of South Rumaila oil field in Iraq. It is a mature oil field with around 58 years of production with 40 producing wells and it has also 20 injection wells located only at the east flank. The average surface area for this reservoir is 142 km2. The reservoir is modeled considering an infinite acting aquifer at the east and west flanks. A commercial reservoir simulator has been adopted parallel with the Tank Capacitance-Resistive Model to estimate the subsidence in very active aquifer. The Carter-Tracy water influx model has been adopted to calculate the water influx rate. The TCRM depends on the concept of continuity equation that considers the difference between the water injection and oil production as reservoir input and output, respectively. So, the simulator calculates the total water injection rate and water influx to be treated as water injection in the continuity equation.
The results demonstrated that reservoir formation has plastic deformation because it has recurrence subsidence in the reservoir thickness. The current subsidence for this reservoir in the current time after 58 years of predict is 1.5 ft at the crest of the reservoir and it is approximately close to the local subsidence at each grid in the reservoir crest.