8 Case study - The Sleipner Gas field
8.1 Background
8.1.1 Offshore geology
The geology of the Norwegian continental shelf is varied; both with respect to age and rock/sediment type (Sigmond 1992). The areas of the present continental shelf were strongly influenced by the Caledonian Orogeny 500-400 million years ago. The Devonian, ca. 400-350 million years ago, was a period of collapse, erosion and molasse sedimentation of the orogen.
Thick sedimentary units were deposited in Carboniferous (~345-300 million years) and Permian (~280-250 million years) times on Svalbard and in the Barents Sea (Figure 17a). The Permian was a period of extensive stretching of the continental crust, widespread faulting and deposition of thick sedimentary successions, especially in the Skagerrak, in the North Sea and off Mid-Norway (Figure 17a). Skagerrak experienced significant volcanic activity associated with rifting. In the Triassic (~240-185 million years), thick sedimentary units were deposited in the Barents Sea, on the Trøndelag Platform and in the North Sea (Figure 17a). In the North Sea and the Norwegian Sea this was accompanied by extensive normal faulting.
Extensive rifting and normal faulting occurred in the North Sea and the Norwegian Sea in the Jurassic (~180-135 million years), and source rocks and reservoir rocks very important for the Norwegian hydrocarbon production were deposited. Other phases of rifting and normal faulting, in the Cretaceous (~135-65 million years) and Tertiary (~65-3 million years), were associated with extension leading to opening of the North Atlantic Ocean. Especially during Cretaceous times sedimentary successions approaching 10 kilometers in thickness were deposited in the Møre and Vøring Basins. Cretaceous rocks are widespread on the Norwegian continental shelf.
In the Pliocene (~14 million years) and Pleistocene (~2 - 0.5 million years), the continental shelf was strongly influenced by glacial processes. Major uplift and erosion took place on the Norwegian mainland, in the Barents Sea, and in the Skagerrak area. The erosional products occur as large slope aprons along the continental margin, especially off the Svalbard-Barents Sea margin (thickness of several kilometers), and in the Norwegian Sea off Mid-Norway and in the Møre Basin (Figure 17b).
Figure 17a: Map of Norway showing names of places and hydrocarbon fields, modified after NGU 2002.
Figure 17b: Structural nomenclature offshore Norway south of 62_N. (Source NGU)
8.1.2 Utsira Formation
The Utsira Formation was deposited during the late Middle Miocene (~20 million years) to Early Pliocene (~14 million years), Eidvin et al. 2002. The formation belongs to the Nordland Group present in the Viking Graben (Gregersen and Michelsen 1997), area from ca. 58ºN to 62ºN (Figure 18). The Utsira formation is a highly elongated sand reservoir, extending for more than 400 km from north to south and between 50 and 100 km from east to west, with an
area of some 26 100 km2. The top Utsira formation and surface generally varies relatively smoothly, mainly in the range 550 to 1500 m, but mostly from 700 to 1000 m. There are two main depocentres. One is in the south, around Sleipner, where thicknesses range up to more than 300 m. The second depocentre lies some 200 km to the north of Sleipner. There the Utsira formation is locally 200 m thick, with an underlying sandy unit adding further to the total reservoir thickness (Chadwick et al., 2000). At the nearest the formation, lies some 60- 70 km, from the Norwegian coast.
From well logs in Eidvin et al. (2002) it is estimated that 70% of the Utsira Formation is made of sand/sandstone. The Utsira Formation is overlain by Pliocene marine claystones of the upper part of the Nordland Group. The cap rock succession overlying the Utsira formation is rather variable, and can be divided into three main units, the lower, the middle and the upper seal (Torp and Gale, 2003). The lower seal extends well beyond the area currently occupied by the CO2injected at Sleipner and seems to be providing an effective seal at the present time (Figure 19). Empirically, therefore, the caprock samples suggest the presence of an effective seal at Sleipner, with capillary leakage of CO2unlikely to occur (Chadwick et al., 2000). The claystones are grey, sometimes greenish-grey and grey-brown, soft, sometimes silty and micaceous. The uppermost part of the Nordland Group consists of Pleistocene unconsolidated clays and sands, with glacial deposits uppermost (Isaksen and Tonstad 1989). The thickness of the seal is 500-1500 m. The seal on top of the Utsira Formation is assumed to be continuous across the area. In the east, the rocks are inclined such that stored CO2 would migrate eastwards and up towards the Pleistocene boundary.
Figure 18: Location map showing areal extent of the Utsira Formation and the Sleipner licence.
Figure 19: The Sleipner CO2 injection scheme. The Utsira formation is a 200 -250 meters thick and very permeable sandstone overlaid with mudstone. The CO2 capture takes place at the Sleipner T (Treatment) platform where it is also compressed. The highly deviated injection well has been drilled from the nearby Sleipner A concrete platform. (Source Statoil)
Macroscopic and microscopic analysis of core and cuttings samples of the Utsira formation show that it consists of largely uncemented fine-grained sand, with medium and occasional coarse grains. Porosity estimates of the Utsira formation core based on microscopy range generally from 27% to 31%, locally up to 42%. Laboratory experiments on the core give porosities between 35 and 42.5% (Chadwick et al., 2000).
8.1.3 Saline Aquifer Carbon dioxide Storage (SACS) Project
The SACS project was a research and demonstration project which is monitoring and forward modelling the underground CO2 sequestration operation taking place at the Sleipner West gas field, offshore Norway.
The offshore gas field Sleipner, in the middle of the North Sea, has been injecting 1 Mt CO2
per year since September 1996 (Baklid et al., 1996). The CO2is injected into salt water containing sand layer, called the Utsira formation, which lies 1000 meter below sea bottom.
During 1998, a group of energy companies together with scientific institutes and environmental authorities in Norway, Denmark, the Netherlands, France and the UK formed the Saline Aquifer CO2Storage (SACS) Project Consortium and started to collect relevant information about the injection of CO2 into the Utsira formation and similar underground structures around the North Sea. The SACS project involves a multidisciplinary approach.
The different scientific disciplines involved in the project include: geology, geochemistry, geophysics and reservoir engineering/simulation.
In 1999 the SACS (Phase 1) project (supported under the European Commission’s Thermie Programme) started monitoring the CO2behaviour and established a baseline by shooting a first 3D seismic survey (Gale et al, 2001), . The Phase 1 Project was extended to SACS2 in 2000 again with European Commission (EC) support. The SACS2 project, which terminated in 2003, continued the work undertaken in Phase 1 with further repeat 3D seismic surveys
completed to track the fate of the injected CO2. In addition, it is using the seismic data to verify available models and tools originally developed for hydrocarbons and water that have been applied to a CO2 and water system (Section 5.2.2). The major difference being that CO2
is soluble in water and methane is not.
The goal of the SACS2 project was to develop a consensus about the monitoring results and validity of available models and tools. To develop such a consensus involves close co- ordination between the scientific institutes involved in the project. The cumulative experiences of the SACS projects are presented in a Best Practice Manual to assist other organisations planning CO2 injection projects to take advantage of the learning processes undertaken and to assist in facilitating new projects of this type. The document Best Practice Manual (Best Practice Manual, 2004) outlines the main findings of the SACS projects and this report reviews the document in this chapter.
8.1.4 CO
2storage quality and capacity
During the SACS-project, it has been shown that the Utsira Formation has good storage quality with respect to porosity, permeability, mineralogy, bedding, depth, pressure and temperature (e.g. Zweigel and Lindeberg 2000). It is a very large aquifer with a thick and extensive claystone top seal. The aquifer is, however, unconfined along its margins, and the time before migrating CO2 might reach the margins of the aquifer is unknown. The Utsira Formation is regarded as one of the most promising aquifers for CO2 storage in Europe. It has both such a considerable thickness and extent that it alone could store the CO2 emissions from all of the north European power stations and other large industrial plants for several hundred years (Torp & Christensen 1998). It is estimated that the Utsira Formation, below 800 m depth, has a pore volume of 918 km3, a storage capacity in traps of 847 Mt (mega tonnes) CO2, and that the storage capacity of the entire aquifer is 42 356 Mt CO2 with an assumption that storage volume representing 3 % the pore volume (See details in Bøe et al. 2002, Table 6). The total pore volume of the aquifer is, however, estimated differently by other workers, 6.05 x 1011m3(Kirby et al. 2001) and 5.5 x 1011 m3 (Chadwick et al. 2000).