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Abstract: The In Salah CO2 storage project in Algeria, initiated in 2004, provides a unique early-mover case study on CO2 storage. After 5 years of operation, the project has developed a substantial storage-unit characterisation and monitoring dataset. The question we address in this paper is: which reservoir features have proven most critical in controlling the injection performance so far? This is also an important question for future CO2 storage sites, as part of the planned global ramp-up in CCS as a greenhouse gas mitigation option. Our main findings are that structural geological and rock mechanical aspects are most critical in the early injection phase, while characterisation of the pore space, combined with the dynamically created fracture permeability, becomes more important when considering the medium to long-term effects (10–1000 years) including geochemical, fluid dynamical and geomechanical aspects. When considering the quantity and type of data that are needed to sufficiently characterise this CO2 storage site, high quality 3D seismic proves to be highly valuable (despite its relatively high cost). For the pore space characterisation, core samples from both the reservoir and the caprock are vital in order to calibrate and properly interpret static and dynamic well data. Finally, monitoring datasets, including well-head gas and tracer data, time-lapse seismic data, and satellite (InSAR) monitoring data need to be interpreted and only bring true value when utilised with detailed geological descriptions and models of the subsurface. [Copyright &y& Elsevier]

Abstract: This paper presents a structured qualitative approach to analysing the varied kinds of information from a CO2 storage site, so as to produce scenarios that are amenable to numerical analysis. The approach is illustrated by application to an industrial scale CCS project at Krechba, In Salah, in Algeria. A structured approach is needed to support assessments of the likely performance of CCS systems over operational, monitoring and longer term time-frames. Very varied information concerning such systems’ engineered and environmental components must be obtained and evaluated to attain sufficient confidence that performance will be acceptable. Computer simulations and risk assessment models are needed to help understand the behaviour of CO2 and place plausible bounds on the temporal evolution of all aspects of the system. The outcomes will be uncertain, even if underpinning data sets are of good quality The approach included identification of the important the Features, Events and Processes (FEPs) that together describe the Krechba system and its likely evolution. An ‘expected evolution’ scenario was then identified by systematically evaluating existing knowledge. Scenarios describing potential situations that could involve alternative evolution mechanisms were also identified; these included consideration of mechanisms that could in principal lead to containment failure. These scenarios need to be analysed to show that they are either unlikely to occur and/or will be of limited impact and so do not represent threats to adequate performance. After audit against Quintessa’s freely available generic online CO2 FEP database to ensure and demonstrate comprehensiveness, the site-specific scenarios identified and the associated list of remaining uncertainties, were used to prioritise future (e.g. systems modelling) work. The outcomes of this and other data analysis and modelling programmes will be used to update the FEP and scenario descriptions. [Copyright &y& Elsevier]

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]