Reservoir Aspects of Ekofisk Subsidence

Abstract

Summary In Nov. 1984. Phillips Petroleum Co. discovered subsidence of the seabed overlying the Ekofisk oil reservoirs offshore Norway. This phenomenon is the result of the compaction of the porous chalk reservoirs and the transmission of this compaction through the overburden to the seafloor. This paper describes the geologic- and reservoir-related aspects of subsidence, including the mechanism leading to reservoir compaction and its effect on reservoir performance. The compaction of the Ekofisk reservoirs is shown to be a result of pore-pressure depletion. Although some of the compaction is elastic, pore-pressure depletion. Although some of the compaction is elastic, the bulk results from plastic deformation (pore collapse) of high-porosity chalk. Reservoir compaction is shown to cause subsidence of the seabed through deformation of the overlying sediments, Reservoir compaction has also had a pressure-maintenance effect on the reservoir. Thus far, no loss in reservoir productivity has been observed. On the basis of analogy with the neighboring West Ekofisk field, no loss in reservoir productivity is anticipated, at least for the near future. Introduction The Ekofisk field is located in the central graben in the southern part of the Norwegian sector of the North Sea (Fig. 1). It is the part of the Norwegian sector of the North Sea (Fig. 1). It is the largest of six fractured chalk fields operated by Phillips Petroleum Co. Norway on behalf of the Phillips Norway Group. Water depth in the area is about 235 ft [72 m], and frequent storms during the fall and winter make the environment rather hostile. In 1963, the Phillips Norway Group started seismic surveys in the Norwegian sector of the North Sea. Exploration drilling beginning in 1967 led to the discovery of the Cod gas-condensate sandstone reservoir in 1968 and the Ekofisk chalk field late in 1969. Over the next 3 years, five additional chalk fields were discovered in what is now referred to as the greater Ekofisk area. Early production started in July 1971, and since then field development has production started in July 1971, and since then field development has evolved through various phases with a peak production rate of 349,000 B/D [55 500 m/d] reached in 1976. Today, the Ekofisk complex is the processing center for all production from the Ekofisk-area fields. Figs. 2 and 3 show the size and location of all the fields in the area. To ensure continuous safe operation during severe weather conditions, it became necessary to modify the platforms on the Ekofisk complex. Most of the planned modifications were implemented in Summer 1987, when seven platform decks were elevated 19.7 ft [6.0 M). After the modification program is completed with the installation of a protective concrete barrier around the storage tank, the Ekofisk complex will tolerate subsidence far beyond predicted levels. Reservoir Description The Ekofisk field is an elongated anticlinal structure with the long axis in the north/south direction (Fig. 4). Areal extent, is about 4.2 × 5.8 miles [6.8 × 9.3 km], and the thickness of the overlying sediments is 9,300 ft [2840 m] at the crest (Fig. 5). The producing horizons in the Ekofisk field are the Ekofisk and Tor formations. These formations are chalky limestones of Danian and Maastrichtian Age, respectively (see Ref. 1). Separating the producing intervals is a low-porosity layer referred to as the Ekofisk producing intervals is a low-porosity layer referred to as the Ekofisk Tight Zone. Except for a limited number of fractured areas, this layer prevents fluid movement between the formations. Mineralogy. The reservoir rock is a fine-grained limestone composed of skeletal debris of pelagic unicellular algae (Coccolithophorids). These algae produced spherical calcareous exoskeletons, called coccospheres, consisting of a number of wheel-shaped elements called coccoliths (Fig. 6). The diameter of the coccospheres ranges from 10 to 30 Am-, the size of the coccoliths is between 2 and 20 Am. Coccospheres are seldom preserved in the sediments. Complete coccoliths are relatively common, but the majority are broken up into their basic calcite crystal constituents called coccolith platelets. Fig. 3a in Ref. 1 is a scanning electron micrograph showing a complete coccolith and a number of platelets. The dark areas represent the pore space. Although porosity is high, the individual pores are extremely small (1 to 5 phi m). Within the matrix of coccoliths and platelets are also smaller amounts of coarser carbonate debris, such as foraminiferal remains, mostly of planktonic origin. The chalk also contains a variable amount of noncarbonate material, generally less than 5 % for the Tor and Lower Ekofisk formations and as high as 20% in the upper intervals in the Ekofisk formation. Most of this material is silica and clay, with minor concentrations of pyrite, marcasite, dolomite, feldspar, and siderite. In some zones, secondary chert forms nodules and beds of variable size and thickness. Porosity and Thickness. Matrix porosity and permeability are Porosity and Thickness. Matrix porosity and permeability are related to the packing of coccolith platelets. The platelets are held together by cementation in the form of secondary calcite overgrowth and spot welding of grain contact points. Fig. 7 is a typical crestal well porosity and water saturation log. In the Ekofisk formation, local porosities get as high as 48%. The lower part of the formation is typically more porous than the upper intervals. The thickness of the Ekofisk formation excluding the Tight Zone varies between 330 and 500 ft [100 and 150 m]. The porosity of the Ekofisk Tight Zone ranges from less than 10% to - 20 %. Except for local areas, this zone, which typically is about 50 ft [15 m] thick, prevents fluid movement between the Tor and Ekofisk formations. The Upper Tor, also very porous in high structural elevations, has a porosity ranging from 30 to >40%. The Tor formation thickness varies between 250 and 500 ft [75 and 150 m]. For both formations, porosity varies significantly both laterally and vertically and generally decreases toward the flanks and into the water zone. Fracturing. Even high-porosity chalk intervals exhibit rather low matrix permeability, typically 1 to 5 md. As a result of extensive natural fracturing, the effective permeability is substantially higher than the matrix permeability. Fracturing in the Ekofisk field is of various origins and generally can be classified as healed, tectonic, and stylolite-associated fractures. Natural fractures enhance the matrix permeability by up to a factor of 50. Effective permeability ranges from 1 to 100 md. Overburden. The sediments overlying the reservoir consist mainly of clays and shales interbedded with silty streaks. From a depth of 4,000 to 5,000 ft [1220 to 1520 m] and below, the sediments are overpressured, and mud weights of 13 to 15 Ibm/gal [1558 to 1797 kg/m3] (1.56 to 1.80 specific gravity) are used for drilling these intervals. Permeability is extremely low (10 to 10 darcies), and there is no indication of pressure communication between the reservoir and the overlying sediments. Field Development Development planning for the Ekofisk field began in 1970. Geological interpretation and data from the four appraisal wells indicated more than I billion bbl [159 × 106 M] recoverable oil and flow potentials in excess of 10,000 BOPD [1590 m/d oil] per well. The effects that pressure depletion would have on reservoir performance were uncertain. performance were uncertain. JPT P. 709