Reservoir characterisation is one of the most important factors in maximising project economics. Cost effective methods for reservoir data acquisition are particularly relevant to subsea field developments, and they are required in all phases of a field development.
Several examples of improved economics of the reservoir development due to data acquisition will be discussed. The recovery factor of the Tordis Subsea Field has increased since the start of drilling and production, and drilling of water injectors have been postponed due to improved reservoir characterisation. Costs of data acquisition are compared to project economics and compared to added value as a result of improved reservoir characterisation.
An innovative data acquisition plan, which covers all types of data for the entire field life, was established for the Tordis Field in 1993 before predrilling was initiated. After the production drilling was completed in 1996, conclusions can be made regarding relative importance of the different types of data and the corresponding impact on key reservoir management issues.
All planned producers have been drilled on the Tordis Field and 40 % of the reserves have been produced. Still, improved reservoir characterisation is required for further project optimisation. A specific target for oil recovery factor has been established, and the measures for obtaining the target have been prioritised. For the Tordis Field, this approach has resulted in an ambitious Long Range Plan for improving project economics through additional data acquisition and specific reservoir characterisation studies for improving sweep and waterflood performance.
There is never enough data to completely characterise the reservoir. This paper illustrates how cost effective data acquisition for a subsea field is related to reservoir management from which plans and strategies for the project is made. The objective relates to reducing risk and hence improve the economical return.
The Tordis Oil Field is located in Block 34/7 on the Norwegian Continental Shelf (Fig. 1). The field is located 11 km north-west of the Gullfaks C platform at a water depth of 200m.
A subsea development was approved in 1991, and the field was developed through a subsea manifold with production and injection satellites tied back to the Gullfaks C platform (Fig. 2) (Ref. 1). The field has been developed with five producing wells, and two water injectors are planned. The processing capacity is 16.000 Sm3 liquid/day. Oil is piped via the Gullfaks C platform and offloaded to tankers, whilst gas is piped via Gullfaks C and Statfjord C platforms into the Statpipe pipeline system via Karsto in Norway to customers in Emden in Germany and Zeebrugge in Belgium.
The Tordis Field was discovered in 1987 with well 34/7–12 drilled in the Western Segment of the field (Fig. 3). With the second well, 34/7–14, drilled in the Southern Segment, close to 80 % of the estimated reserves of the Tordis Field was proven, leaving only the Eastern Segment untested.
The Jurassic Brent sandstones in the Tordis Field are divided into an Upper Brent and a Lower Brent unit, separated by a shale-sand-coal pressure barrier sequence of the Ness Formation. It shares many of the key geological and production characteristics of other Brent tilted fault block fields in the area. (Table 1). The reservoir was deposited as delta-associated Brent Group sandstones which generally have high porosity, but vary considerably in vertical permeability distribution. The geometry of the field, the good to excellent reservoir quality and the favourable water-oil mobility ratio, indicated that a frontal displacement of oil by water will provide an effective recovery mechanism. P. 595^
