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).

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