Ingenious Method for Water Injection Optimization on Mature Carbonate Reservoir with Rapid Pressure Decline Problem, Case Study: XJN Field - South Sumatra, Indonesia

XJN field has implemented water injection as pressure maintenance since 1987, only one year after initial production. XJN is carbonate reservoir with weak aquifer underlying the oil zone. Initial reservoir pressure was 2,700 psi and peak production was 27,000 BOPD. Reservoir pressure was drop to 1,800 psi within 5 years of production. During 1991-2007, better injection management was performed to provide negative voidage. This action has managed to bring reservoir pressure back to its initial pressure, eventually enabling all wells to be converted from gaslift to naturalflow. In 2013, watercut has increased to 97% and several naturally flowing wells began to ceased-to-flow, then production mode was changed gradually from naturalflow to artificial lift using Electric Submersible Pump (ESP). In 2017-2020, there was rapid reservoir pressure decline around 300 psi/year while XJN water injection performance considered flawless. Voidage Replacement Ratio (VRR) was 1.3, but reservoir pressure was kept declining. This situation will cause ESP pump off on producer wells which in turn means big production loss. This paper will elaborate about the simple-uncommon-yet effective methods for problem detection and its solution to revive pressure and production. Analysis was began with observing the deviation of VRR and reservoir pressure, this was to estimate "leak" time of water injection. Next analysis was evaluation of injection rate leak off using material balance with reverse history matching. Reverse here means making reservoir pressure as main constraint rather than history matching goal. After that, it was continued with water injection flow path analysis. This was done by plotting production-injection-pressure data then make several small groups of injector-producer based on visible relationships. The purposes were to find key injector wells and to shut-in all inefficient ones. Furthermore, injection re-distribution was also performed based on VRR calculation on groups from previous step, water distribution priority was focused on key injector wells. These analysis have also paved the way for searching channeling possibility on injector wells. The results, XJN reservoir pressure showed an increasing trend of 100 psi/year after optimization was performed, with current pressure around 2000 psi. The increase in reservoir pressure has also made it possible to optimize ESP, field lifting has increased for 5000 BLPD. This project has also successfully secured XJN remaining oil. This project was racing with rapid pressure decline that will lead to early ESP pump off and production loss. The integrated subsurface analytical methods and actions being taken were simple but effective. Close monitoring on reservoir pressure, water injection and ESP parameters will be needed as field surveillance. Integrated analysis with surface facility engineering should also be carried out in the future in regards to surface network, injection rate and reservoir pressure.