TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractVeslefrikk is a North Sea oil field in its tail-end production period where optimal well placement is critical for the drainage of the remaining reserves. This paper presents two case studies representing different challenges with respect to geosteering. In both cases a newly developed Directional Electromagnetic logging while drilling tool (D-EM) was used together with a fully rotated point-the-bit 3D rotary steerable system (RSS) to achieve proactive geosteering. The LWD tool was able to detect resistivity contrasts in any direction up to 5 m from the wellbore. In the first case the objective was to position a 570 m long horizontal well section 1-3 m below the top of the reservoir sand, thereby attaining maximum distance from the water level and ensuring that no attic oil was left behind. In the second case the challenge was to optimize the amount of oil filled sand along the 1100 m horizontal trajectory, while drilling perpendicular to the depositional direction in a fluvial channel system.The early detection of the sand to shale boundaries resulted in an increase of 10-15 % in the recoverable reserves for each well compared with conventional geosteering.The workflow setup for both cases included the use of a Web-based system for communication and data transfer. This ensured efficient decision-making involving geosteering specialists, wellsite geologists, and onshore company personnel.
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This paper discusses the most important field development challenges of Mariner and Bressay with focus on subsurface related issues on Bressay. The fields have 11-14 o API oil. The reservoir viscosity is from 65 cp to 540 cp. No other offshore fields have yet been developed with 540 cp oil viscosity or more. Challenges include achieving high oil rate and recovery. Unconsolidated sand means limited availability of high quality exploration data and limitation to production well design. Lack of seismic resolution between the sands and shales limits the mapping potential in some of the reservoirs. Old well data means lack of high quality data. Significance of relative permeability and PVT uncertainties has been assessed and a reduced uncertainty span has been suggested by reinterpreting old data in combination with new laboratory experiments. Cuttings and log data is used primarly to establish permability distribution. Reservoir development strategy has been significantly revised compared to the previous Operator. The Mariner and Bressay field development sanction work is ongoing. This includes an updated flow assurance strategy using diluent to reduced challenges relegated to production and processing of heavy oil. IntroductionThe Bressay field, Figure 1, was discovered in 1981 by well 3/28-1. In total 5 exploration wells have drilled trough the oil column. The field is situated on the Shetland platform just west of the Viking graben. The reservoir sands are of late Palaeocene (Teal Mbr.) / Early Eocene (U.Dornoch Fm.) age. The Bressay structure is an elongated 10km. long feature, contained within, and including, the sand rich Dornoch delta. The trapping mechanism is most likely a simple structural closure, with possible a weak stratigraphic element to the west were the reservoir sands are terminating. A 3D Seismic shot in 1997 was successfully reprocessed by Chevron in 2008 and furthermore by Shell in 2010. 5 wells + 1 sidetrack have been drilled on Bressay. All wells on Bressay have been tested (DST's) and test interpretations provide important data as core data are limited due to the unconsolidated nature of the reservoir rock. All wells have been fluid sampled. Only 3/28a-4 (1997) and, -6 (2008) are "recent" with high quality measurements and tests. The reservoir is at approximately 1000 to 1116 m mMSL depth (OWC). The reservoir is up to more than 230 m thick. The oil reserves are estimated to 250 MMBO. The oil is heavy, with 540 cp oil (12 API). The reservoir consist of a combination of deep marine incised valley and deltaic sediments. Bressay is currently in the concept selection stage.
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