Tanjung field in Kalimantan at Barito Basin has a long story. It was discovered in 1898 and initially developed few decades later in 1939. Primary recovery had been the main production mechanism until 1995 where peripheric and line-drive water injection begun. By latest cumulative production in 2021, Field Recovery Factor (RF) is still below 25%. This fact of low RF has become attractive subject of study to unlock the remaining oil in place, particularly in Layer A, B, C, and D as the main hydrocarbon accumulation. Therefore, those layers were selected as focus of study in revolutionarily changing peripheric and line drive injection into waterflood patterns.

The unlocking initiative was manifested officially in original Plan of Further Development Plan (POFD) issuance in 2015 and revisited later in 2017 as Revised POFD to develop full-scale waterflood implementation with following scope of work: drilling new wells of producers and injector, work-over on existing producer and/or injector wells to establish waterflood pattern, and Water Treatment Injection Plant (WTIP) upgrading. The objective of this paper is to deliver several focuses: (1) Front-to-end workflow of waterflood project from reservoir studies to facilities design, (2) Gap of actual vs design encountered during project execution, and (3) Overcoming the challenges and/or gap in order to achieve target.

According to POFD, injection rate was required at 68 kbwipd vs current condition of 40 kbwipd. WTIP upgrade to provide injected water quality on-spec in required quantity is the key driver. In parallel with WTIP upgrade, various workover activities (zone change, perforation, Sucker Rod Pump or ESP replacement, and conversion-to-injector) to establish waterflood pattern while maintaining the field existing production. The main challenge lies on getting the pattern ready as latest as WTIP upgrade completed without disregarding anticipated production loss due to workover on existing producers either shutting-off commingling layers or converted-to-injector (CTI). Moreover, the established patterns were to put in optimization efforts through detailed production and injection performance evaluation to maintain field production target despite not having designated injection rate. Improvised actions also taken as discrepancy between reservoir simulation forecast vs actual performance at pattern level had been encountered. Despite the challenges, all the efforts and actions involving reprioritizing budget, changing well materials procurement, rescheduling injection line replacement timing, rescheduling SRP or ERP delivery, and reallocating injected water from out-pattern into established patterns succeeded contributing incremental production.

This paper summarizes lesson learned from actual full field waterflood project: (1) What can go differ from designed requirement in executing a waterflood project, (2) What to anticipate, especially on forecast vs actual pattern performance, (3) How we improvise to overcome gap of reality vs forecast. All the efforts by Pertamina EP successfully slow the decline and boost oil production of Tanjung Field back to level of 2018's in this year.

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