The Field in this paper is located in the northern part of South Sumatra Block, Indonesia. It is a brown field which was discovered in 1985. The reservoir driving mechanism was identified as weak water drive, so water injection and gas lift system were chosen as part of field development strategy. During the middle of production phase, water injection was able to provide negative voidage, followed withan increasing trend of reservoir pressure that eventually allowed the wells to flow naturally again in 2007. Reservoir simulation study of the Field began in 2009 and was completed in 2010. In 2014 there were additional data for simulation derived from drilling result, production and water injection data. Evaluation result based on these new data concluded that thesimulation model need revalidation due to deviation between actual and simulation data. At the same time in 2012, there was an indication of rapid production decline because many naturally flowing wells began to cease to flow. This was caused by several factors: increasing trend of average field water cut, decreasing trend of reservoir pressure and water injection problems. To prevent further production decline, quick action needed to be taken. The scope of work in this paper coversrevalidation process of thesimulation model and optimization steps to deal with theproduction decline.

Revalidation by performinganother history matching showed that thestatic model needed to be reconstructed because the static model was truncated. It was estimated that the reconstruction process will take a long time, then it was decided to use several traditional analyses with analytical approach to prevent further production decline, such as VRR analysis with areal/zonal distribution, hall plot analysis, nodal analysis, and analytical oil decline curve, parallel with the reconstruction of thestatic model. Inconclusion, four projects were needed for field optimization:installation of artificial lift, optimization of water injection systems, search for potential by-passed oil and capacity upgrade of supporting facilities.

The result was satisfactory; water injection optimization was successfullymaintained oil production by focusing injection on a specific area in natural flow wells, restoring injection capacity in low rate injection wellsand adding water supply to the system. Installation of ESP and workover to search for by-passed oil weremanaged to give significant oil gain. Finally, capacity upgrade of supporting facilities was also successful, thus allowing that all above actions and tasks can be accomplished.

Reservoir simulation is a powerful tool that can be very helpful in the process of field optimization, but if the model used was no longer represent actual conditions, the model must be corrected before the use and analytical analysis should be use instead. The four projects that were carried out based on analytical analysis successfully led to a 750 BOPD initial oil gain and able to arrest rapid production decline.

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