No abstract

The Halfdan Northeast gas accumulation was discovered in 1968 and comprises a low permeability chalk of the Ekofisk formation. The thin reservoir interval has an effective permeability of about 1 mD in the best parts, segmented vertically by cm-thick layers of extreme low permeability and considerable lateral extent. As a result, the productivity is relatively poor. To overcome the challenges presented by the tight chalk reservoir, a stepwise approach to field development has been applied. Maximum reservoir exposure obtained in the most cost effective manner was from the beginning identified as a pre-requisite for a commercially attractive development. The first development phase in 2004 comprised three long reach horizontal wells drilled from the nearby Halfdan field. These wells enabled early development of the gas utilising existing facilities. During this phase it was realised that despite reservoir sections of 15,000 ft production rates were disappointing. Two long dual lateral wells were drilled during the second development phase showing that a doubling of the reservoir section could be achieved at 1.6 times the cost of a single lateral well while productivity increased proportionally with reservoir exposure. An ambitious third development phase was planned with 6 dual and 2 single lateral wells covering the eastern part of the field. The wells were planned in a spiralling pattern which provides a uniform coverage of a large area in comparison with the traditional radiating well pattern. To date, 8 of the planned wells have been successfully drilled. As a result of several novel ideas to maximise reservoir exposure while minimising costs, the development of the thin Ekofisk reservoir in the Halfdan Northeast field is well underway and production in line with expectations. Halfdan Northeast Location and Reservoir Geology A thin gas column was discovered at the crest of the Tyra-Igor ridge by the Igor-1X well drilled in 1968 during the early exploration of the Danish North Sea (Figure 1). The gas zone was contained within chalks of Lower Tertiary Danian age (Ekofisk formation) which in this part of the Central Graben is heterogeneous with a lower reservoir unit (D2) of low porosity (20–30 %), high clay content and very low permeability (< 0.1 mD) and an upper reservoir unit (D1) consisting of cleaner chalk with high porosity (30–40 %) and permeability up to 1 mD (Figure 1). Within these reservoir units and in particularly in the D2 unit, diagenesis of the chalk gives rise to extreme reduction of the permeability. The permeability reduction is most pronounced across laminated smectite rich beds formed by alteration of volcanic ash (Reference 1). The layered nature of the reservoir is illustrated by the core photograph in Figure 2. The Danian reservoir is generally less than 70 ft thick (gross) and is separated from underlying Maastrichtian chalk of varying porosity and fluid contents by a regional hardground. The uppermost porous D1 reservoir unit is commonly only 20–30 ft thick. A more detailed discussion of the reservoir geology is given in Reference 2.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper covers the drilling related optimizations which have been implemented during the first development phase of the Al Shaheen field offshore Qatar. The issues adressed include bit optimization, BHA enhancement and replacement of Class 'G' cement, which have led to an overall cost reduction of some 18% compared with the initial phase.

The aim of the present paper is to supply additional sedimentological observations and to add new details to the existing interpretation of the Atane Formation on eastern Disko. Examples of epsilon cross-bedding reflect formation of point bars and indicate intermittent development of sinuous channels in the coarse-grained braided river. Large tabularsets of planar cross-bedding are interpreted as transverse bars and the coalescerice of two such bars are discussed in detail. The field work was carried out during five days in July 1987 as part of a sedimentological research project supported by GGU and financed by SNF.

The reservoir position of horizontal wells drilled in low permeability chalk reservoirs is of vital importance for the well performance. Accurate placement of horizontal wells drilled in the low permeability chalk fields in the Danish North Sea is being achieved by rigorous evaluation of geological information acquired during drilling. The tools which are being utilized for geological steering of horizontal wells comprise: High resolution biostratigraphy, Logging While Drilling, shows evaluation and analysis of 3D seismic data. Examples from the Dan, Kraka, Skjold and Tyra fields illustrate the current applications of geological steering in chalk reservoirs.