RIASSUNTO
Abstract
Six horizontal wells have been drilled in the V Fields reservoirs with a further two wells planned to be drilled in 1992. Several objectives pursued by these wells include,
1) Increasing the probability of penetrating a high permeability sand body in a heterogeneous formation.
2) Increasing deliverability by extending the length of contact of the well with the formation.
3) Draining low relief, flank areas of the field.
4) Accessing multiple, potentially isolated fault compartments.
The degree of success has been mixed. Some wells surpassed their objectives while others have been disappointing. Regardless of the commercial results, every well has provided a major insight into the reservoir dynamics and description of these complex fields.
The results show that,
1) Accurate reservoir description is important to a successful well.
2) Horizontal wells appear to improve the chances of attaining commercial gas production rates from heterogeneous formations.
3) Deliverability can be enhanced by extended formation contact in both good quality sands and individual high permeability features,
4) Compartmentalisation may affect V Fields reservoir performance but faulting is less important than sedimentary heterogeneity.
Introduction
The V Fields are four separate gas accumulations (Vanguard, North Valiant, South Valiant and Vulcan) located in blocks 49/16, 49/21, 48/20a and 48/25b in the Southern North Sea. These have been developed together through the Lincolnshire Offshore Gas Gathering Station (LOGGS) transportation system. The fields produce from the Rotliegendes Group, Leman Sandstone Formation at depths of between 7200 ft and 8500 ft ss (Ref 1, Fig 1).
The fields have been on production since 1988 with 47 development wells drilled to date. Six of these wells have been drilled horizontally to pursue a range of objectives. The history and results of each horizontal well will be addressed individually and the impact of each well on the evolution of the V Fields reservoir description discussed.
RESERVOIR GEOLOGY
The key to interpreting both vertical and horizontal well results is understanding the complex sedimentology of the Leman Sandstone Formation. In the V-Fields area, the Leman Sandstone comprises between 700 ft and 1000 ft of aeolian and fluvial sandstones. The fluvial deposits are much the subordinate, and are restricted in their development to the basal 100 ft to 150 ft. The remaining 600 ft to 850 ft of the formation are aeolian in origin and were deposited in either desert (erg) or desert margin (sabkha) environments.
The salient features of aeolian environments are conceptually summarized in Fig 2 (Refs 2-5). Fig 2a shows a schematic regional paleogeography for a desert. A proximal fluvial flood plain passes via a mixed wadi/erg system into the erg proper, which then passes distally into an erg margin sabkha adjacent to the contemporary coastline. The main sedimentary bedforms in the erg are draas, large compound dune bedforms that migrate (climb) in response to the dominant wind vector (Fig 2b).
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