Summary This paper presents the success story of waterflooding in a geologically complex reservoir that contains high-viscosity (90 cp) oil. This reservoir is part of a large brownfield in south Oman and has been on production for more than 25 years. The reservoir comprises glacial sandstones of Palaeozoic age and is highly heterogeneous, with wide variation in reservoir characteristics at scales smaller than well spacing. The reservoir geology is complicated because of the presence of faults and fractures. The reservoir has been subdivided into different areas that we call "catchments," characterized by their geological setting and dynamic reservoir behavior. Full-field water injection was started in the field after approximately 18 years of primary depletion. Different water-injection philosophies for different areas of the field have been adopted with consideration to factors such as primary drive mechanism operative in the reservoir, presence or absence of vertical-flow barriers; pre-existing field-development pattern; and more importantly, future development beyond waterflood to maximize the value of the reservoir in its entire life cycle. The paper illustrates that the success of a waterflood in a complex reservoir relies on the implementation of a tailor-made development plan with flexibility to be changed on the basis of data from reservoir surveillance. The importance of well and reservoir management (WRM) to improve the value of waterflood is emphasized.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA major reservoir in a large brown field in South Oman has been on production for more than 25 years. The reservoir comprises glacial sandstones of Palaeozoic age and is highly heterogeneous with wide variation in reservoir characteristics at scales smaller than well spacing. These variations include impermeable diamictite and shale beds and multi-Darcy gravel or conglomerate beds. The reservoir geology is further complicated due to the presence of faults and fractures that often provide highly conductive vertical pathways to the aquifer. The reservoir crude has high viscosity (90 cp) and moderate to low API gravity (22 deg API oil). The field has been developed using vertical and horizontal wells in different phases of development. Water-flooding is in operation in the reservoir for more than 6 years. Injectors comprise vertical injectors in five spot pattern, horizontal injectors and inclined aquifer injectors in the flanks. Different water injection philosophy for different areas of the field has been adopted considering factors like primary drive mechanism operative in the reservoir, presence or absence of vertical barriers to flow, pre-existing field development pattern and more importantly future development for maximizing recovery from the reservoir over the entire life cycle. Despite the geological complexity of the field and adverse mobility ratio conditions under which water-flooding is being carried out, the reservoir has responded very favourably to water flooding. The production decline trend from the reservoir has been arrested and reservoir pressure has increased in most areas of the field after initiation of water injection. The paper presents the success story of water-flooding in a geologically complex reservoir containing high viscosity oil. The paper clearly illustrates that the success of a water-flood in a complex reservoir rests on implementation of a tailormade development plan with flexibility to make changes to the same based on data from reservoir surveillance. The importance of well and reservoir management to maximize the value of waterflood is clearly brought out.
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