Since October 1996, Statoil and its Sleipner partners have injected CO 2 into a saline aquifer, the Utsira Sand, at a depth of approximately 1000 m. The aquifer has a thickness of more than 200 m near the injection site and is sealed by thick shales. A multi-institutional research project SACS (Saline Aquifer CO 2 Storage) was formed to predict and monitor the migration of the injected CO 2 . To this end two time-lapse seismic surveys over the injection area have been acquired, one in October 1999 after 2.3 million tonnes of CO 2 had been injected and the second in October 2001 after approximately 4.4 million tonnes of CO 2 had been injected. Comparison with the base seismic survey of 1994 prior to injection provides insights into the development of the CO 2 plume. In this paper some selected results of the seismic interpretation of the CO 2 plume at the two different time-steps will be shown.
The release from a point source of relatively heavy fluid into a saturated porous medium above an impermeable boundary is considered. A theoretical relationship is compared with experimental data for the rate of propagation of the front of the resulting gravity current and its shape. A motivation of the study, the problem of carbon dioxide sequestration, is briefly discussed.
The CO 2 storage operation at Sleipner in the Norwegian North Sea provides an excellent demonstration of the application of time-lapse surface seismic methods to CO 2 plume monitoring under favourable conditions. Injection commenced at Sleipner in 1996 with CO 2 separated from natural gas being injected into the Utsira Sand, a major saline aquifer of late Cenozoic age. CO 2 injection is via a near-horizontal well, at a depth of about 1012 m bsl, some 200 m below the reservoir top, at a rate approaching 1 million tonnes (Mt) per year, with more than 11 Mt currently stored.A comprehensive time-lapse surface seismic programme has been carried out, with 3D surveys in 1994, 1999, 2001, 2002, 2004, 2006 and 2008. Key aims of the seismic monitoring are to track plume migration, demonstrate containment within the storage reservoir and provide quantitative information as a means to better understand detailed flow processes controlling development of the plume in the reservoir.The CO 2 plume is imaged as a number of bright sub-horizontal reflections within the reservoir, growing with time ( Figure 1). The reflections mostly comprise tuned wavelets arising from thin (mostly < 8 m thick) layers of CO 2 trapped beneath very thin intra-reservoir mudstones and the reservoir caprock. The plume is roughly 200 m high and elliptical in plan, with a major axis increasing to over 3000 m by 2008. As well as its prominent reflectivity, the plume also produces a large velocity pushdown caused by the seismic waves travelling more slowly through CO 2 -saturated rock than through the virgin aquifer. This paper summarises some of the quantitative methods that have been applied to the Sleipner seismic datasets.
a b s t r a c tIn the ten years since publication of the IPCC Special Report on CCS, there has been considerable progress in monitoring and verification (M&V). Numerous injection projects, ranging from small injection pilots to much larger longer-term commercial operations, have been successfully monitored to the satisfaction of regulatory agencies, and technologies have been adapted and implemented to demonstrate containment, conformance, and no environmental impact. In this review we consider M&V chiefly from the perspective of its ability to satisfy stakeholders that these three key requirements are being met. From selected project examples, we show how this was done, and reflect particularly on the nature of the verification process. It is clear that deep-focussed monitoring will deliver the primary requirement to demonstrate conformance and containment and to provide early warning of any deviations from predicted storage behaviour. Progress in seismic imaging, especially offshore, and the remarkable results with InSAR from In Salah are highlights of the past decade. A wide range of shallow monitoring techniques has been tested at many sites, focussing especially on the monitoring of soil gas and groundwater. Quantification of any detected emissions would be required in some jurisdictions to satisfy carbon mitigation targets in the event of leakage to surface: however, given the likely high security of foreseeable storage sites, we suggest that shallow monitoring should focus mainly on assuring against environmental impacts. This reflects the low risk profile of well selected and well operated storage sites and recognizes the over-arching need for monitoring to be directed to specific, measureable risks. In particular, regulatory compliance might usefully involve clearer articulation of leakage scenarios, with this specificity making it possible to demonstrate "no leakage" in a more objective way than is currently the case. We also consider the monitoring issues for CO 2 -EOR, and argue that there are few technical problems in providing assurance that EOR sites are successfully sequestering CO 2 ; the issues lie largely in linking existing oil and gas regulations to new greenhouse gas policy. We foresee that, overall, monitoring technologies will continue to benefit from synergies with oil and gas operations, but that the distinctive regulatory and certification environments for CCS may pose new questions. Overall, while there is clearly scope for technical improvements, more clearly posed requirements, and better communication of monitoring results, we reiterate that this has been a decade of significant achievement that leaves monitoring and verification well placed to serve the wider CCS enterprise.Crown
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