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Abstract: In the paper we share our operational experience gained from three sites: Sleipner (14 years of injection), In Salah (6 years) and Snøhvit (2 years). Together, these three sites have disposed 16 Mt of CO2 by 2010. In highly variable reservoirs, with permeability ranging from a few milliDarcy to more than one Darcy, single wells have injected several hundred Kt of CO2 per year. In the reservoirs, the actual CO2 plume development has been strongly controlled by geological factors that we learned about during injection. Geophysical monitoring methods (especially seismic, gravity, and satellite data) have, at each site, revealed some of these unpredicted geological factors. Thus monitoring methods are as valuable for reservoir characterisation as they are for monitoring fluid saturation and pressure changes. Current scientific debates that address CO2 storage capacity mainly focus on the utilization of the pore space (efficiency) and the rate of pressure dissipation in response to injection (pressure limits). We add to this that detailed CO2 site characterisation and monitoring is needed to prove significant practical CO2 storage capacity–on a case by case basis. As this specific site experience and knowledge develops more general conclusions on storage capacity, injectivity and efficiency may be possible. [Copyright &y& Elsevier]

Abstract: The Krechba field is one of several gas fields located in the Algerian Sahara desert, and was set in operation in August 2004 as part of a joint venture with BP, Sonatrach and StatoilHydro. The natural gas in the fields contains up to 10% CO2, which has to be reduced to 0.3% before the gas is sold, resulting in the production of around 1 million tonnes/year CO2. Rather than vent the CO2 to the atmosphere (business as usual), it is re-injected into the water leg of the Krechba Carboniferous Sandstone gas producing reservoir (20 m thick) via three horizontal wells at a depth of around 1,900 metres. CO2 injection started in August 2004 and to date nearly 2.5 million tonnes of CO2 have been injected, amounting to approximately 25% of the gas extracted from the Krechba field over the same period. A number of key technologies to monitor the injection, and the subsurface movement and storage of CO2 have been, and will continue to be, deployed to provide long term assurance of sequestration. Time lapse satellite images (using PSInSARTM Technology) which measure ground deformation to assess the movement of CO2 in the subsurface have proven to be much more successful than initially thought, despite the depth of injection and the low voidage replacement rate (25%). Satellite images collected since start of injection show clear increases in ground elevation of up to 30mm around the three injectors while subsidence is also apparent in the area of maximum gas production. The images have also confirmed the CO2 is moving in the direction of preferred fracture orientation at reservoir level. Recent downhole pressure measurements in one of the injectors also indicates that the CO2 is being contained within the injection horizon. Work is ongoing to integrate the satellite images with geomechanical and seismic data to better understand how these images can be used for monitoring of CO2 movement in the subsurface. A key part of the forward monitoring programme will be the acquistion of time-lapse 3D seismic, deployment of tiltmeters and GPS to confirm and calibrate the satellite imagery data, microseismic detectors in shallow boreholes to assess the degree of rock strain and use of shallow wells to monitor the water chemistry in the vadose zone. These will supplement the ongoing tracer, wellhead sampling and satellite imagery data acquistion. This paper shares the latest results of the ongoing CO2 monitoring project. [Copyright &y& Elsevier]