R.A. Chadwick

4D seismic quantification of a growing CO2 plume at Sleipner, North Sea

CO 2 produced at the Sleipner natural gas field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data were acquired in 1999 and 2001, with 2.35 and 4.26 million tonnes of CO 2 in the reservoir respectively. The CO 2 plume is imaged as a number of bright sub-horizontal reflections within the reservoir unit, growing with time, and underlain by a prominent velocity pushdown. No leakage has been detected from the repository reservoir. The reflections are interpreted as tuned responses from thin ( 2 trapped beneath thin intra-reservoir mudstones and the reservoir caprock. However, these alone are unable to account for the amount of observed pushdown. A two-component 3D saturation model is therefore developed for the 1999 dataset, with high-saturation CO 2 forming the layers and a lesser component of low-saturation CO 2 between the layers. Saturations are calculated from the observed reflectivity and velocity pushdown and the resulting model contains 85% of the known injected mass of CO 2 . A 2D synthetic seismic section through the saturation model matches the observed seismic response well and the model is considered to provide an acceptable description of the CO 2 distribution. Signal attenuation is more pronounced within the 2001 plume and its effects are likely to become more significant with time, perhaps reducing the efficacy of seismic verification techniques as the plume grows further. Other geophysical methods, such as microgravimetry, may become increasingly useful at this stage.

Flow processes and pressure evolution in aquifers during the injection of supercritical CO2 as a greenhouse gas mitigation measure

ABSTRACT Regional saline aquifers offer the greatest potential for very large-scale underground CO2 storage as a means of mitigating greenhouse gas emissions. Their dynamic storage capacity, in terms of induced increases in formation pressure, will limit the rate at which CO2 can be injected and may ultimately limit the amount of CO2 that can be stored. Generic flow models were generated to examine the effects on pressure evolution of various reservoir parameters (dimensions, permeability, porosity, presence and nature of flow barriers). CO2 injection involves dominantly hydrogeological (single-phase flow) processes in much of the reservoir and surrounding adjacent strata, with additional two-phase flow effects around the CO2 plume itself. Large, thick aquifers with no significant flow barriers can accept high injection rates (c. 10 million tonnes of CO2 per year) without undue pressure effects. However, flow barriers, such as faults, increase induced pressures considerably; for reservoirs with such features, careful site characterization and operational planning will be required for large storage projects. The principles established from the generic modelling were applied to a real aquifer storage operation at Sleipner in the North Sea. Here, CO2 is being injected into the Utsira Sand, a large relatively homogeneous reservoir. Modelling indicates that pressure increase should be negligible. In fact, observed wellhead pressures do show a small rise, but this can be attributed to temperature changes in the fluid column in the wellbore. Pressure changes in the reservoir are likely to be very small.

Review of monitoring issues and technologies associated with the long-term underground storage of carbon dioxide

Abstract Large-scale underground storage of CO2 has the potential to play a key role in reducing global greenhouse gas emissions. Typical underground storage reservoirs would lie at depths of 1000 m or more and contain tens or even hundreds of millions of tonnes of CO2. A likely regulatory requirement is that storage sites would have to be monitored both to prove their efficacy in emissions reduction and to ensure site safety. A diverse portfolio of potential monitoring tools is available, some tried and tested in the oil industry, others as yet unproven. Shallow-focused techniques are likely to be deployed to demonstrate short-term site performance and, in the longer term, to ensure early warning of potential surface leakage. Deeper focused methods, notably time-lapse seismic, will be used to track CO2 migration in the subsurface, to assess reservoir performance and to calibrate/validate site performance simulation models. The duration of a monitoring programme is likely to be highly site specific, but conformance between predicted and observed site performance may form an acceptable basis for site closure.

Geological Characterization of CO2 Storage Sites: Lessons from Sleipner, Northern North Sea

Publisher Summary This chapter draws some generic conclusions on reservoir characterization based on the Sleipner operation where CO 2 is being injected into the Utsira Sand. Regional mapping and petrophysical characterization of the reservoir, based on 2D seismic and well data, enable gross storage potential to be evaluated. Site specific injection studies, however, require precision depth mapping based on 3D seismic data and detailed knowledge of reservoir stratigraphy. Stratigraphical and structural permeability barriers, which are difficult to detect prior to CO 2 injection, can radically affect CO 2 migration within the aquifer. The Sleipner sequestration operation is the focus of the SACS (Saline Aquifer CO 2 Storage) project, whose aims include monitoring and modeling the fate of the injected CO 2 and regional characterization of the Utsira reservoir and its caprock. This chapter describes some of the results of the investigations and draws out some generic aspects of geological reservoir characterization, which are particularly applicable to CO 2 injection into flat-lying aquifers of regional extent.

The case for underground CO 2 sequestration in northern Europe

Abstract In northern Europe numerous industrial point sources of CO2 surround the North Sea Basin, which contains a number of viable underground sequestration opportunities. These include injection into depleted oil and gas fields and into major regional aquifers; the latter probably offering the greatest ultimate storage potential. At the Sleipner gas field, CO2 is being injected into the Utsira Sand, a large saline aquifer. More than 6 Mt of CO2 have currently been injected, with a projected final target of about 20 Mt. Time-lapse seismic reflection data are being used to monitor the operation and have provided clear images of the CO2 plume and its development with time. Moreover, CO2 volumetrics derived from the seismic data are consistent with the well injection figures. In conjunction with reservoir simulation studies, time-lapse seismic monitoring seems, therefore, to offer an effective means of predicting the future growth, migration and dispersion of the CO2 plume. Another important aquifer, the Bunter Sandstone, stretches from Britain to Poland. In the UK sector alone, the pore volume in structural closures is equivalent to about 350 years’ worth of current CO2 emissions from UK power generation. Industrial CO2 sources in northern Europe are well placed to exploit these major subsurface reservoirs and European countries are technically very well equipped to use and develop this emerging technology.