Tunu is one of the biggest gas fields in Indonesia with 1400 km2 area in Mahakam Delta, East Kalimantan. This field has been producing since 1990 with cumulative production of more than 9.5 tcf and 190 mbbl condensate by the end of 2020 from over 1000 operating wells. Today, Tunu field contributes for approximately 40% of Mahakam production. After 30 years of production, Tunu production level is currently in declining phase, shown by its yearly production profile which exhibits a declining trend since 2008. Furthermore, Tunu well development project was considered marginally economical due to depleting reserve per well. Thus, an integrated study was conducted in order to reduce surface expenditure cost of Tunu pipeline based on current operating parameters. The study consisted of WHSIP history matching to determine new pipeline design pressure, evaluation of future wells production lifetime, and adjustment of pipeline corrosion allowance based on actual corrosion rate observed in Tunu field. Results show that most of future Tunu wells are predicted to have WHSIP below 200 barg and 1.5 to 3 years’ production lifetime. Corrosion rate in Tunu field as measured using corrosion coupon in piping with corrosion inhibitor injection is found to be less than 1 mm/20 years. Therefore, corrosion allowance for Tunu pipeline is optimized from 5 to 3 mm for 10-years design lifetime. For exceptional circumstances where actual well WHSIP > 200 barg, other method of producing the well will be implemented. Hence, by integrating recent subsurface behavior (WHSIP and well lifetime) with surface understanding (corrosion rate), it was then proposed new pipeline design for Tunu development. This study has generated USD 13 million cost saving for pipeline procurement in 2020. Moreover, implementation of the new pipeline design reduces 40% of pipeline unit cost for future pipeline procurement. This study has become the basis for future well development projects in Tunu field which significantly prolong Mahakam's production sustainability.
Mahakam Contract Area is located in East Kalimantan Province, Indonesia. It covers an operating area of 3,266 km2, and consists of 7 producing fields. Most of Mahakam hydrocarbon accumulations are located below body of water, with wellhead production facilities installed in the estuary of Mahakam river (referred as swamp area, 0 to 5m water depth) and the western part of Makassar Strait (referred as offshore area, 30 to 70 m water depth). Mahakam production history goes as far back as mid 1970s with production of Handil and Bekapai oil fields. Gas production started by the decade of 1990s along with emergence of LNG trading, supplying Bontang LNG plant, through production of 2 giant gas fields: Tunu and Peciko, and smaller Tambora field. In the mid 2000s, Mahakam attained its peak gas production in the level of 2,600 MMscfd and was Indonesia's biggest gas producer. Two remaining gas discoveries, Sisi Nubi and South Mahakam, were put in production respectively in 2007 and 2012. Due to absence of new discoveries and new fields brought into production, Mahakam production has entered decline phase since 2010, and by end of 2020, after 46 years of production, the production is in the level of 600 MMscfd. In 2018, along with the expiration of Mahakam production sharing contract, Pertamina Hulu Mahakam (PHM), a subsidiary of Indonesian national energy company, Pertamina, was awarded operatorship of Mahakam Block. This paper describes the efforts undertaken by PHM to fight production decline and rejuvenate development portfolio, with focus on expanding subsurface development portfolio and reserves renewal by optimizing development concept and cost through fit-for-purpose design, innovation, and full cycle value engineering.
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