Carbon dioxide (CO2) is being injected into geological storage sites around the world at both pilot and commercial\ud scales. A variety of monitoring technologies are employed at these sites ultimately with the aim of demonstrating\ud storage integrity both at the current time and also into the future. This paper reviews selected monitoring technologies\ud employed at two pilot-scale and two commercial sites. At the pilot-scale CO2 injection site at Ketzin (Germany),\ud preliminary analysis of electrical resistivity measurements indicate the presence of injected CO2 between the injection\ud and monitoring wells. At Nagaoka, a pilot-scale CO2 injection site in Japan, a suite of time-lapse wireline logs (including\ud resistivity, neutron and sonic) were used to monitor injected CO2. Resistivity readings increased in the injected\ud formation and later decreased particularly in the layers below. This decrease was attributed to CO2 dissolving in the\ud formation water implying that the site is moving towards increased storage stability. Satellite-based monitoring is being\ud used at the onshore commercial-scale CO2 injection site at In Salah, Algeria, to monitor millimetre-scale ground\ud displacement as a result of CO2 injection. The displacement rates show a NW-SE trend, mirroring the known structure\ud of the area, suggesting a structural control on the subsurface distribution of the CO2. Sleipner is a commercial CO2\ud injection site in the North Sea which has been in operation since 1996. Results of time-lapse (4D) seismic surveys over\ud the site clearly show the migration of the CO2 plume in the subsurface. The results add to the portfolio of successful ‘fit\ud for purpose’ CO2 storage monitoring techniques
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