The quest for carbon dioxide sequestration – North Sea potential
The dichotomy of the North Sea being a source of oil and gas and a possible repository for CO2 was explored at an event held at The Geological Society in London, UK, on 24-25 March. Michael Forrest reports
The North Sea could be the centre of low-carbon technology. The sub-surface geology is well known, through numerous seismic surveys and well logs that delineate oil and gas horizons. This Mesozoic basin hosts extensive gas and oil reserves that have powered the UK economy for the past 30 years. As a result, some of the 100-plus fields are drained of their hydrocarbons and now offer a storage solution for sequestering CO2. The surveys have also identified many other basins that do not contain hydrocarbons but host large saturated saline reservoirs.
The ultimate capacity of the North Sea is not known but significant depleted reservoirs will be the first target, already in use in the Sleipner (west of Stavangar, Norway) and Snørvit (northwest of Hammerfest, Norway) natural field projects. Of greater significance are the saline aquifers, as by volume these outweigh the hydrocarbon traps. These horizons can occur above and below the hydrocarbons.
The two-day conference on Carbon Storage Opportunities in the North Sea at The Geological Society, London, UK, discussed the technical implications of a substantial injection of gas, the ability of the reservoir to permanently contain sequestered gas, and the legal and climate change obligations of the process.
The North Sea undoubtedly has the right geology in a series of porous rocks capped by clay and salt in a number of basins and fault-bounded grabens. Dr Mike Stephenson from the British Geological Survey illustrated the regional geology, noting the clustering of power generators in and around the North Sea, principally in the UK, The Netherlands and Germany. Within these countries there are a series of hubs that will provide the infrastructure for sequestration, related to the locations of power stations and the on- and offshore infrastructure for oil and gas.
Security of sequestration is the prime objective and a number of presentations focused on the seals and faults between permeable sands and clay caps. Some faulting juxtaposes clays against sands or links permeable layers through displacements, seen in well logs as clay smears (clay gouge) that may seal the horizons.
Much research has concentrated on identifying possible ‘overflow’ points within a succession. These points are assessed by seismic identification of geological profiles that map faulting and strata within a basin. Logs of wells are also used to determine the lithology at each level and an analysis is made as to the fluid/gas security of the proposed sequestration site. Those sites where there is a possibility of leakage along faults or due to porous to non-porous rock juxtaposition are to be avoided.
Other features giving cause for concern are naturally occuring injected sand pipes that may connect hundreds of sediment succession metres caused by flow and pressure. They pose a problem in linking gas and oil reservoirs with other parts of the succession that may be permeable and allow leakage.
An empirical view, however, can be taken of oil and gas basins that presumably held their pressures over millions of years until drilled. Using these depleted reservoirs is a first step where flow characteristics are known, and represent the ‘low hanging fruit’ according to several speakers. However, others, notably Professor Joe Cartwright at the University of Cardiff, UK, contended that in his examination of over 1,000 fields, all had seepages of varying degrees, and this was more prevalent in shallower basins, which are less compressed and hence more likely to have seepages than a deeper one with similar configuration. Seepages are estimated by pressure differences.
Measuring the amount of CO2 that can be contained in a North Sea reservoir was also considered. A broad rule of thumb given by Bert van der Meer at the University of Utrecht, The Netherlands, who has over 30 years’ experience working as an oil and gas engineer, is that between two per cent for a closed reservoir and six per cent for an open system by volume is appropriate. Gas pressure would modify, but, in the interests of long-term security, maximum pressures of 5-10bar have been recommended.
Monitoring the migration of CO2 within a reservoir is difficult as geophysics cannot
distinguish between methane and injected CO2. However, the Sleipner field has been charged with CO2 since 1996, building up to a rate of about one million tonnes per year. Monitoring has shown that a diatreme plume has developed, confining CO2 in the reservoir.
A component of the Sleipner field is the Utsire sands, an ideal geology with 35-40% porosity. Within this horizon are sub-metre clays that act as impediments to the upward movement of injected gas via an extraction well. Sleipner is proving to be a laboratory for future sequestration in the North Sea.
However, as noted by several speakers, the technology for ensuring zero seepages is some years away.
Nevertheless, the North Sea basins could be the future for carbon sequestration and a new industry for the UK. In a keynote presentation by Dr Stuart Haszeldine, of the Scottish Centre for Carbon Storage, values for mid-range off-shore storage were identified as 60Gt CO2, comparing well with annual UK power plant emissions of 200Mt.