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The future of large very mature oilfields relies upon the successful application of EOR techniques. This paper discusses from the Handil field case, the more useful concept and practices that are applicable to optimize oil recovery in a multi-layer, multi reservoir waterflooded field. The Handil field, discovered in 1974, is a giant mature oilfield located in the Mahakam Delta, Indonesia. The field was developed using conventional oil recovery methods: natural depletion and peripheral water injection and produced a maximum of 200,000 BOPD in the late seventies. Currently, the oil recovery is around 50% with a 15,000 BOPD production at high watercut. The Handil EOR lean gas injection project was started in November 1995 on five reservoirs and following its positive result was extended with six reservoirs in 2000. In 2003, an integrated study on the largest EOR reservoir was performed to assess the projects' performance. This included 3D geomodeling, reservoir simulation and chemical tracer injection. The study permitted to track the main effects of the gas injection, to identify "bypassed oil" areas and to define reservoir management guidelines for the other lean gas injection reservoirs. As field scale application of the gas injection technique is not sustainable since the gas is exportable, two other techniques are currently being evaluated as an alternative. Second Contact Water Displacement (SCWD) is planned to be introduced on the maturest lean gas injection reservoirs. Simulation studies forecast potential for this technique which also allows decreasing the volumes of injected gas. An Air injection pilot on a waterflooded reservoir was started in 2001. First results are encouraging but the injection was temporary suspended due to injector well problems after 6 months of injection. The project was comprehensively studied by compositional and thermal simulation and intends to restart early 2005. Integrated studies of the Handil reservoirs provided in-depth understanding of EOR mechanisms. This resulted in the ability to better model field performance and to forecast tertiary oil production more accurately. Introduction The Handil field, discovered in 1974, is a giant mature oilfield located in the Mahakam Delta of East Kalimantan, Indonesia. The field comprises over 500 hydrocarbon accumulations in structurally stacked and compartmentalized deltaic sands. Hydrocarbon accumulations are found at depths as shallow as 300 mSS and continue down to 3000 mSS. The structure of the field is a simple anticline, 10 km long and 4 km wide and is divided in a northern and southern compartment by an east-west fault (Fig. 1).

Fairview field is located in the Bowen Basin in South East Queensland and covers an area of approximately 1600 km2. It has been in production since 1994, from the 6 main coal seams of the Bandanna Formation. Presently, about 150 wells are on-stream feeding the domestic gas market. Further development is in progress to supply the Gladstone LNG plants on Curtis Island in 2015. This paper focuses on a developed zone in the North of the field where extensive production and pressure data; more than 15 years, are available. The majority of the wells in the area are commingled and target the sweet spots with permeability ranging from tens to a few hundreds mDs. Material balance analysis observed a typical layered reservoir behavior which is consistent with permeability discrepancies measured between seams. It also provided a comprehensive large scale calibration of the gas accumulation split between well connected and poorly connected areas. This split tightly controlled the gas production performance and recovery. A strong permeability increase over reservoir pressure depletion was also identified after the two-phase flow production period. The magnitude was a 10 fold increase over 300 psi of depletion which is likely related to the well known coal shrinkage effect. This observation is consistent with previously published data from the San Juan Basin in the US. Within the high quality seams, this phenomenon had limited impact on long-term production recovery; however the impact on expected-ultimate-recovery (EUR) was more significant in the poorer seams. These two phenomena show that permeability characterization, including its relationship with reservoir pressure depletion, is a key element for better production forecast.

Maintaining mature fields provides major challenges in which production and resources are decreasing, while the operations complexity is increasing. This condition leads to increasing project risk, rising cost, and eventually decreasing company’s profit. New development projects may become sensitive to cost. Low oil price environment has made the situation even more challenging. This paper will discuss the strategy being implemented in Mahakam to overcome the challenges as a mature asset. Having produced for more than 40 years, Mahakam fits perfectly in the category of mature, thus fundamental changes are needed for ensuring its sustainability. One of the main focus areas is cost effectiveness. Cost structure was investigated in order to identify the main cost drivers. Then, multiple workshops were held to discuss any possible solutions to improve the cost effectiveness. For capturing ideas from bigger audience, an online platform was created where employees can submit ideas. Lastly, a communication media is regularly distributed to maintain employee’s engagement to the program. Seven pillars in cost effectiveness have been identified for Mahakam to respond to the current situation; Budget accuracy, technical standardization and innovation, change of working philosophy, operations optimization, supply chain optimization and renegotiation, cooperation with other companies, and organizational right-sizing. Several initiatives under those pillars have been successfully implemented resulting in 20% reduction of operational cost in 2019 and will be sustained for the years to come without compromising safety. As the challenge facing mature fields is typically similar, the aforementioned approach will likely not only be applicable in Mahakam but also in other mature fields. Some adaptations might be needed to adjust with the specific characteristic of each individual field. Cost effectiveness will generate capability to continue future investment for sustaining the life of mature fields which is one of the key elements for securing energy supply in Indonesia.

Bekapai is a mature oil and gas field located in the offshore Mahakam Delta, Indonesia. The hydrocarbons are accumulated in complex multilayered reservoirs. Ten platforms were constructed to accommodate the oil production coming from 74 wells drilled between 1974 and 1996. The production started in 1974 with peak production at~60,000 BOPD in 1978, followed by a period of decline until it reached its lowest point at 1,000 BOPD in 2007.A field re-development project, so-called Phase 1 initiated in 2008, had increased the production to 10,000 BOPD and 46 MMSCFD by the end of 2013. It consisted of 9 development wells. Following this successful project, a new 3D OBC seismic was acquired and further development plan, Phase 2, initiated, consisting of 10 development wells and gas production facility debottlenecking. Two wells have been drilled in 2014 and contribute in stabilizing the production of 2014 at 11,500 BOPD and 38 MMSCFD, the highest oil production of the past 25 years. The gas production decreases as a consequence to field production facility limitation. It was shelved due to priority of oil development. However, it is expected to increase to 100 MMSCFD in 2015 after the debottlenecking. In parallel, idle wells reactivation to access the shallow gas resources is also reviewed. Besides contributing to gas export, the gas is also used as artificial lift to revitalize the oil wells.This paper demonstrates the main elements of the redevelopment: evolution of the geological model achieved through seismic and petrophysical data, production management in a complex production system and various optimizations at the production network in order to unlock the production capacity limit through debottlenecking projects and chemical injection at the export line. A continuous effort in even further redeveloping the field is also shown through Phase 2, the objective of which will be chasing the attic oil, blowing down the gas-cap, and double the gas production.