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Geological storage of CO 2 that has been captured at large, point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions. However, this technology will only be viable if it can be guaranteed that injected CO 2 will remain trapped in the subsurface for thousands of years or more. A significant issue for storage security is the geomechanical response of the reservoir. Concerns have been raised that geomechanical deformation induced by CO 2 injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO 2 leakage. In this paper, we examine three large-scale sites where CO 2 is injected at rates of ∼1 megatonne/y or more: Sleipner, Weyburn, and In Salah. We compare and contrast the observed geomechanical behavior of each site, with particular focus on the risks to storage security posed by geomechanical deformation. At Sleipner, the large, high-permeability storage aquifer has experienced little pore pressure increase over 15 y of injection, implying little possibility of geomechanical deformation. At Weyburn, 45 y of oil production has depleted pore pressures before increases associated with CO 2 injection. The long history of the field has led to complicated, sometimes nonintuitive geomechanical deformation. At In Salah, injection into the water leg of a gas reservoir has increased pore pressures, leading to uplift and substantial microseismic activity. The differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site.carbon sequestration | geomechanics | InSAR | microseismic monitoring C arbon capture and storage (CCS)-where CO 2 is captured at large point source emitters (such as coal-fired power stations) and stored in suitable geological repositories-has been touted as a technology with the potential to achieve dramatic reductions in anthropogenic greenhouse gas emissions (1, 2). However, its success is dependent on the ability of reservoirs to retain CO 2 over long timescales (a minimum of several thousand years). If CCS is to make a significant impact on global emissions, more than 3.5 billion tons of CO 2 per year must be stored (3), which at reservoir conditions will have a volume of ∼30 billion barrels (4).Secure storage of such large volumes of CO 2 requires more than just the availability of the appropriate volumes of pore space. CO 2 is buoyant in comparison with the saline brines that fill the majority of putative storage sites. Therefore, injected CO 2 will rise through porous rocks and return to the surface, unless trapped by impermeable sealing layers (such as shales and evaporites). Preliminary estimates have tended to indicate that, from a volumetric perspective at least, sufficient storage capacity exists for many decades of CO 2 emissions in deep-lying saline aquifers that have suitable sealing capability (5).It is equally important that the integrity of the seal is not compromised by injection activitie...

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