Due to increased hydrocarbon demand and technological advances, production from heavy oil fields in the United Kingdom Continental Shelf (UKCS) has become possible over the past 10 years. Despite substantial reserves in the UKCS with crudes less than 20° API, most of the activity has been confined to exploration and appraisal drilling. The main reason for the restricted activity has been the high uncertainty of the reservoir and fluid properties. Operational complexities inherent to heavy oil also limit the use of conventional appraisal-well testing technology.A method was developed to determine the most suitable technology for testing wells with heavy oil using an electrical submersible pump (ESP). The solution was applied in the Bentley field located in the UK sector of the North Sea in block 9/3b, on which final appraisal well 9/3b-6Z was flow tested in December 2010.The technical challenges included a short weather window, maintaining fluid mobility through the surface-testing equipment, oil and gas separation for metering, obtaining accurate flow measurements, and designing the most appropriate ESP system. A combination of technologies-dual-energy gamma ray venturi multiphase flowmeter, real-time monitoring, and a novel ESP completion-provided a solution that enabled successful well test execution. A multirate test reaching a final stabilized rate of 2900 bpd, with a subsequent period of pressure buildup was accomplished in less than 2.5 days with 10 to 12° API crude. A key lesson was how to obtain the quality of data that would enable reservoir engineers to extract with confidence a productivity index and perform pressure transient analysis for reservoir characterization. This success paves the way for development drilling to commence on the Bentley field at the end of 2011, but also demonstrates potential that can enable new heavy oil field developments.
In recent years, polymer flooding of heavy oil has been extensively studied in laboratories and successfully applied in several fields. This paper reports the laboratory corefloods, development of mechanistic models, and simulation studies of polymer flooding in a heavy oil reservoir with active aquifer influxes. Bentley Field, operated by Xcite Energy Resources, is located on the UK Continental Shelf. Flow tests confirmed the existence of a large, active bottom aquifer which may cause polymer loss and decrease the economic attractiveness of polymer flooding. To analyze the impact of the aquifer on oil recovery efficiency, a reservoir simulation model was set up. Several development scenarios have been simulated for the optimization of development strategy. Conventional thinking, based on previously accepted EOR screening criteria, would be that the oil viscosity (approximately 1500 cp) exceeds the economic and technical limit of oil viscosity for polymer flooding. However, this paper demonstrates that the limit for effective polymer flooding can be extended to considerably higher viscosity oils. To validate the applicability of polymer flooding, two laboratory experiments were conducted with two different high permeability sandpacks. Due to the unconsolidated nature of the Bentley formation no cores were available. Waterflooding was stopped when water cut reached 90% and up to that point, less than 25% of oil in place had been recovered. However, the remaining oil saturations after both tertiary polymer corefloods achieved around a 5% level. We investigated the recovery mechanisms and developed a mechanistic model to match the laboratory observations. Simulation results show that for this heavy oil field with an active aquifer, polymer flooding economics can be improved by optimizing well locations, number of horizontal wells, polymer injection time, etc. In history matching coreflood experiments, two oil saturation reduction mechanisms were considered: (1) viscous polymer solution reduces viscous fingering and channeling effects especially in the heavy oil displacement process and also reduces remaining oil saturation after waterflooding; (2) remaining oil can be mobilized by viscoelastic properties of synthetic polymer solutions. Both mechanisms were considered in the simulation study where a favorable match of oil recovery and pressure drop was obtained. In this paper, polymer flooding is shown as a viable technology in a heavy oil reservoir, despite the highly unfavorable mobility ratio and strong aquifer influxes. Considering the diminishing conventional oil reserves, polymer flooding provides a non-thermal approach for producing heavy oil reserves that may be particularly attractive in an offshore environment, compared to thermal techniques.
The Bentley Field, located on the UK continental shelf in block 9/3b in 110 m of water, contains approximately 900 MMstb in-place of heavy (10 to 12 o API) viscous (1500 cP) crude. The field is four-way dip closed at uppermost Palaeocene, lowermost Eocene, Dornoch sandstone level, and covers an area of about 16 Km by 5 Km.An appraisal programme culminating in the 2012, 9/03b-7, 7Z extended well test (EWT), has addressed the key technical concerns associated with developing the viscous crude in an offshore environment. The programme demonstrated how sustainable commercial flow-rates can be achieved through the selection of a suitable completion design, including a downhole electrical submersible pump (ESP), a downhole diluent injection strategy, and through keeping within an appropriate operating pressure and temperature envelope. It further demonstrated that the movement of water from the underlying aquifer into the production bore proceeds in a predictable and manageable way, that produced water and oil can be separated even though emulsions are created in the ESP, and that water can act as a carrier fluid within the export pipeline.The information from the EWT has been used to define and de-risk the field development design which is currently estimated to deliver 257 MMstb of 2P Reserves over a 35 year production period. In addition it is estimated that 48 MMstb of 2C Contingent Resources could be commercially extracted beyond the end of the currently planned facilities life. Investigations are underway to accelerate this tail-end production, with initial studies indicating that a polymer flood, enhanced oil recovery scheme, could be attractive.This paper describes the appraisal programme, the lessons learnt and how these have been applied in designing the field development plan.
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