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.
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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The Mariner field - license 9/11 in the UK – was discovered in 1981 and is situated on the East Shetland platform. Mariner consists of two tertiary reservoir intervals of unconsolidated sand at depths between 1200 and 1500 mMSL. The deeper Maureen reservoir consists of stacked non-channelized sheet-like sand lobes deposited in a shelf to slope setting. The Maureen oil water contact (OWC) is stepping considerably and is shallower to the west. The Heimdal reservoir consists of deep-marine slope-channel sands within the mudstone-rich Lista interval, ranging in thickness from a few meters up to 40 m. The reservoir is heavily remobilised. The Heimdal OWC is uncertain with variable ODT (Oil Down To) observations and anticipated perched water. The total Mariner oil reserves are estimated to app. 400 MMBO (Million Barrels). The Maureen reservoir has 67 cp oil (14°API) in a 0–40 m oil column and a large bottom aquifer. The Heimdal reservoir contains two thirds of the reserves and has 508 cp oil (12 API). Steady State SCAL studies indicate that Krw may be viscosity dependent. The Heimdal reservoir is planned to be developed using an inverted 9-spot well pattern due to uncertainty in mapping of the reservoir. The Maureen reservoir will be developed with horizontal wells. As the recovery factors are only 22%, Enhanced Oil Recovery using polymer is investigated as an upside to be matured towards production start. IOR through use of down-hole inflow control devises is progressed in a technical qualification program. OBC (Ocean Bottom Cable) data will be acquired to improve the mapping of the Heimdal reservoir.
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractThe Gullfaks Field has been in production for eleven years and 2/3 of the estimated reserves have been recovered, The Tarbert and Ness Formations, belonging to the upper part of the Brent Group, contain 44 % of the total STOOIP, Several 3D reservoir simulation models have been made. Due to the structural complexity and horizontal barriers it has been very difficult to history match these models, especially for the Ness Formation, Repeated cased hole saturation logs have been collected in two wells located in the same fault block with initially oil filled upper Brent reservoirs, Standard saturation evaluation was not sufficient to understand the flooding pattern between the wells. A method making it possible to distinguish between formation and injection water even though the salinities are fairly equal has been used in this study. This has improved the understanding of the water flooding in the area. As a result of this, one of the wells (B-3) was successfully re-perforated in a thin undrained zone in the Tarbert Formation.In 1995 a new 3D seismic survey for reservoir monitoring (4D-pilot study) was acquired over the studied area. The most pronounced 4D effects are observed at top reservoir level and at the initial oil/water contact.One well -drilled in 1997 -confirmed an undrained area as shown by our monitoring tools and our general knowledge. In some areas there is less agreement between the 4D data and other data such as in the area around well B-3, were saturation monitoring was performed simultaneous with the acquisition of the seismic data set. Here the 4D data is not in accordance with the saturation profile in the well.In 1996 a new seismic data set was acquired over the whole field. This data set is also interpreted for reservoir monitoring Color printing paid for by Statoil 315 purposes. These new data together with earlier collected data will help to locate the remaining oil in the Gullfaks Field.
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