This paper presents the ongoing development planning, by semi-vertical short-radius extensions (a.k.a fishbones) and infill drilling of a mature waterflooded carbonate reservoir in Oman. Given 35 years of production and water injection into the reservoir, a key focus in the development philosophy is maximizing reserves recovery from existing wells. Many of these wells have reservoir zones with significant oil saturation behind pipe. Two development options were considered attractive for the existing horizontal well stock: adding perforations and creating vertical fishbones targeting the by-passed and attic oil. A total of 52 wells have been identified for additional perforations and 25 wells as fishbone candidates. Additionally, by-passed oil targets have been identified for infill drilling. Infill drilling of the by-passed oil targets proved very successful. Ten out of the eleven opportunity areas where infill drilling was completed encountered economic by-passed oil. The single failure still highlights the remaining challenges in tapping the by-passed oil targets in this giant waterflood oil field. In particular, this paper discusses the results of the past three years of tapping undeveloped reserves using fishbone extensions and infill drilling. Sector modeling and analytical analysis methods supported by surveillance data high-graded 8 opportunities targeting attic oil. Following the initial poor success, mainly due to operations related problems, subsequent improvement in setting and retrieving whipstock has significantly improved the success rate. Liner integrity has been found to be the main setback of this development option. Background The field was discovered in 1963 and is situated in north Oman ca. 450 km WSW of Muscat. Production is mainly from the Lower Cretaceous Shuaiba formation. The Shuaiba formation is heavily faulted and consists of intra-shelf basin floor carbonate muds. Production commenced in 1969 and gradually built up to peak production in 1997. The historic production review showed that production gradually decreased in the first 10 years of production due to a lack of water injection support. When sufficient injection support became available in the early eighties, production slowly increased to peak production. Water production sharply increased in 1997 at the onset of high density, horizontal infill drilling. Since 1997, production decreased rapidly, to ensure that the remaining value of the field is maximized; a study has been initiated to design the next phase of field development. The study was kicked off by a "volume to value" (V2V) peer assist in November 2000, and in 2004 a new development plan for the field was issued. The Shuaiba field Development Plan document is aimed at capturing the uncertainty related to STOIIP and reserves with the objective to maximize the future recovery of the Shuaiba reservoir. An extensive amount of effort in the last 5 years focused on increasing knowledge, data gathering, integrated modeling and field applications has improved the understanding of the Shuaiba reservoir and resulted in improved wells reservoir management and successful placement of infill producers and injectors. The integrated modeling has gone through several generations using models at various scales including single well models, conceptual 3D models, sector models, coarse full field model, detailed of full field model and large sector model extracted from the detail full field model. The data gathering that has taken place in order to build these models and the cumulative knowledge base established from the results have led to the field development plan of the field.

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