A Holistic Approach of Development in Thin Heterogeneous Carbonate Oil Reservoirs: Challenges and Mitigation

Abstract

The work discusses the unique challenge of developing deep and thin oil reservoirs spread across onshore and offshore area of Abu Dhabi. The field is being developed with a cluster development approach utilizing available Natural/artificial Islands in offshore areas. The reservoirs under considerations are thin heterogeneous carbonate reservoirs with moderate permeability (avg ~<10-15 mD) and containing volatile to critical oil. The reservoirs were discovered quite early; limited data is gathered in old wells and have associated uncertainties. Some wells were deepened for sake of collecting additional data; very few completed lately under early production scheme (EPS) to evaluate the well potential, performance sustainability, reservoir drive etc. Their production behaviors also carry an overprint of reservoir diagenesis. The available data, their associated uncertainty and EPS performance are combined to build a holistic reservoir understanding and field development plan, under implementation with phased drilling. An early water-alternate-gas injection (WAG) is planned to support declining reservoir pressure in volatile oil reservoirs in absence of aquifer support. These reservoirs comprise of thin parasequences (<10 ft) separated by dense intervals associated with stylolites; reservoir thickness falling below seismic separation limit. The structural setting is complex due to undulating anticlines and extensive faulting. Diagenesis has heavily influenced reservoir properties, making significant reservoir saturation profile variation both laterally and vertically. This has been confirmed with production performance of EPS wells, behaving differently due to their areal location. The current development plan considers producers with 4000’ horizontal lateral in high oil saturation along with multiple sub-zone coverage to achieve an effective depletion strategy. The limited data availability, structural uncertainty and reservoir heterogeneity in combination with limitation of cluster drilling rig capacity has made well placement a challenging task. Placement of horizontal laterals in good reservoir properties, away from gas cap or O/W transition zones is achieved by utilizing unified understanding of structure, carbonate lithology, diagenesic imprints, logs, analogue saturation-height function, openhole tests and production data. The learning from each new well is incorporated to optimize further development plan. The reservoir quality of completion interval is critical in terms of saturation considering water production, well lifting and long term sustainability. The learning from successful implementation of WAG in another reservoir of the field is incorporated. Understanding from production behavior during EPS has provided a broad guideline for reservoir development; this paper discusses the challenges of implementation and their mitigation approach.