1887
Volume 34, Issue 2
  • ISSN: 0263-5046
  • E-ISSN: 1365-2397

Abstract

L arge-scale underground storage of industrially produced carbon dioxide is the most effective way of keeping cumulative man-made emissions of greenhouse gases within safe limits (IPCC, 2005). The CO2 injection operation at Sleipner, in the central North Sea between the UK and Norway, commenced in 1996 and is the world’s longest-running industrial-scale storage project. It is also the first example of underground CO2 storage arising as a direct response to environmental legislation (Baklid et al., 1996). CO2 separated from natural gas produced at the Sleipner field is being injected into the Utsira Sand, a regional saline aquifer of late Cenozoic age, in excess of 200 m thick in the Sleipner area (Figure 1a). The aquifer comprises mostly clean unconsolidated sand of high porosity (> 0.3) and high permeability (> 1 Darcy). A number of thin intra-reservoir mudstones, typically 1-2 m thick, are evident from geophysical logs acquired in wells around Sleipner (Figure 1b). The CO2 is injected in a dense phase via a deviated well at a depth of 1012 m below sea level, approximately 200 m beneath the top of the reservoir. Injection commenced in 1996 at a roughly constant rate, with around 16 million tonnes of CO2 stored by 2015. A comprehensive deepfocused monitoring programme has been deployed of which time-lapse seismic has proven to be the key tool (Arts et al., 2008). A baseline 3D survey was acquired in 1994, with repeat surveys in 1999, 2001, 2004, 2006, 2008, 2010 and 2012. The plume is imaged on the seismic data as a tiered structure some 200 m high comprising a number of bright sub-horizontal reflections (Figure 2a). These are interpreted as reflections from thin layers of CO2 trapped beneath the intra-reservoir mudstones which are partially but not wholly sealing. The reflective layering had formed by 1999 with each individual reflection traceable on all of the subsequent surveys. As a general rule the middle and upper reflections in the plume have increased in amplitude and lateral extent on successive time-lapse surveys, whereas the lower layers have ceased growing, in some cases shrinking and dimming. A key objective of the monitoring at Sleipner is to demonstrate that geological storage of CO2 is a safe and viable technology. One aspect of this is to quantitatively verify or constrain predictive flow simulations of plume development. However, because the injection well is near-horizontal, no wellbore penetrates either the CO2 plume or the exact stratigraphy that the plume now occupies, and quantitative analysis is challenging.

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2016-02-01
2024-03-29
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  • Article Type: Research Article
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