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How safely can we store CO 2 ?

How safely can we store CO 2 ?. Sam Holloway British Geological Survey Erik Lindeberg Sintef Petroleum Research. Natural analogues. We know that natural gas and other buoyant fluids have been trapped underground beneath the North Sea for millions of years

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How safely can we store CO 2 ?

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  1. How safely can we store CO2? Sam Holloway British Geological Survey Erik Lindeberg Sintef Petroleum Research

  2. Natural analogues • We know that natural gas and other buoyant fluids have been trapped underground beneath the North Sea for millions of years • Natural CO2 fields are also quite widespread throughout the world • the Pisgah Anticline, Mississippi, contains about 200 Mt of CO2 • this was generated by thermal metamorphism of limestones when the Jackson Dome intrusion was emplaced - in Late Cretaceous times, more than 65 million years ago (Studlick et al.1990) • cap rock is evaporites and mudstone • Natural analogues give confidence that under the right conditions, CO2 can be stored underground for millions of years, i.e. until well after any greenhouse crisis has passed

  3. Other lines of reasoning • Observations in the North Atlantic region • Distribution of Palaeocene oil and gas fields • Longevity of gas storage in the Rotliegend Sandstone • Monitoring and modelling • Sleipner • Risk assessment • Enhanced oil recovery and CO2 storage at the Forties oilfield

  4. Paleocene sandstones: the Horda and Lark formations as a seal • In the centre of the North Sea Basin there are several superimposed submarine fan sandstones of Palaeocene and early Eocene age • These are widely distributed and highly porous and permeable • They are overlain by sequences of mudstones commonly many tens to hundreds of metres thick • They contain major accumulations of oil and gas

  5. With only one exception - a heavy oil accumulation at shallow depth on the East Shetland Platform - all the oil and gas fields in Palaeocene and early Eocene sandstones are in the highest sandstone – the intra-Palaeocene shales are not good long term seals (Ahmadi et al, 2003, The Millenium Atlas, Geol Soc)

  6. Palaeocene sandstones >1000 m deep store oil and natural gas very well Map redrawn from Ahmadi et al 2003. Chapter 14, Palaeocene in: Evans D, Graham C, Armour A & Bathurst P (Eds) The Millenium Atlas, The Geological Society, London

  7. Conclusions: Palaeocene/ early Eocene reservoirs • The presence of many major oil and gas fields indicates that the very thick mudstones overlying the Palaeocene sandstone reservoirs form a good seal in the basin centre • The absence of light oil or gas accumulations in the shallow section near the Scottish coast suggests: • Biodegradation – geochemical fingerprint indicates that microorganisms have preferentially eaten the paraffinic compounds in the oil • Leakage to surface – e.g. gas chimney above Bressay

  8. Risk assessment - NGCAS • Part of the CCP project – funded by the EU and CCP • Enhanced oil recovery and CO2 storage at the Forties field • Concluded that the risk of CO2 leakage from the natural system (the geology) was negligible • There was not enough information to draw conclusions about the risks of leakage through the wells in the field

  9. Sleipner experience • Parallels between Utsira and Palaeocene sandstones? • Thin, intra-reservoir shales do not prevent upwards migration of the injected CO2 • Thick overlying shales are preventing upwards migration in basin centre

  10. Distribution of the Utsira Sand Contains thin shales but these don’t significantly impede migration of CO2 to top of reservoir

  11. Reservoir and caprock log correlation

  12. Imaging CO2 in the subsurface with time-lapse seismic Brightening of the blue reflection above the top Utsira Sand is an artefact of the seismic response Seismic response before CO2 injection (Arts et al. 2004) Seismic response after CO2 injection (increased impedance contrast produces stronger reflection)

  13. Caprock structure

  14. Lessons learned • The thick North Sea mudstones such as those overlying the Palaeocene and Eocene sandstones in the centre of the basin are proven good seals. They will also seal CO2 unless CO2/water/rock reactions adversely affect them • The same seems to apply to the Nordland group mudstones – Sleipner is not leaking • The thick evaporites above the Rotliegend and Carboniferous gas fields in the southern North Sea have retained methane for tens of millions of years

  15. Issues • Injection may cause pressure rise in the reservoir • Damage to seal, opening of pre-existing fractures, hydrofracturing • CO2/water/rock reactions • Wells – pre-existing wells cased with carbon steel and cemented with Portland cement • North Sea wells have been remediated – e.g. underground blowout

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