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Abstract
The safe long-term storage of gas/CO2 in spatially limited underground volumes requires the combination of a structural trap with intact structural integrity and a suitable low permeability caprock (seal). The occurrence of natural gas reservoirs proves that certain lithotypes do provide efficient seals which can prevent leakage of gas to the atmosphere over long geological time periods (millions of years). In order to assess the risk of CO2 leakage through caprocks on top of potential storage sites to the surface one has to consider both, the present sealing capacity of the rock and its likelihood to alter in contact with CO2.

The aim of this paper is to provide an overview of mechanisms affecting sealing integrity of intact (non-fractured/faulted) caprocks. Although certain caprocks can be suitable for hydrocarbons over geological time periods, CO2 in contact with the seal may pose additional risks. Depending on the lithofacies and the amount of reactive mineral species, CO2/water/rock interactions might alter the caprock, physical adsorption on organic matter or mineral surfaces will affect sealing integrity and interfacial properties will affect capillary entry and fluid transport behaviour.

Introduction
Geological storage of CO2 from fossil-fired power plants is intensely examined as an option for reducing anthropogenic emissions of greenhouse gases. Deep saline aquifers, depleted oil and gas fields and unminable coal seams are the primary targets for the underground disposal of supercritical CO2. One major concern of all CO2 storage options is the sealing efficiency of low-permeable sequences overlying potential storage reservoirs. The long-term integrity of these sealing layers (caprocks) is one prerequisite to keep the CO2 in place and avoid dissipative loss towards the atmosphere. The assessment of leakage risks and leakage rates, considering different potential mechanisms, is therefore an important issue for site approval and public acceptance [1].

Subsurface storage of CO2 is always associated with an excess pressure resulting from buoyancy forces and injection-related overpressure which constitutes the driving force for different transport processes. Pressure-driven volume flow (Darcy flow) through the caprock is commonly considered as the main risk scenario and can be envisaged to range in intensity over several orders of magnitude:
(i)   rapid leakage by seal-breaching (mechanical failure) or damage of wells (corrosion of pipes and cements),
(ii)  seepage along existing open fault systems and fracture networks,
(iii)   leakage through the pore-space controlled by capillary forces and permeability (after capillary breakthrough pressure is exceeded).

Another important topic that is still to be investigated thoroughly is the effect of CO2 and its reactivity on the mechanical stability of caprocks. Up to now only a few experimental results have been published [2, 3].

This paper addresses aspects related to fluid transport through caprocks; capillary threshold pressure, effective permeability after threshold pressure is exceeded as well as diffusion through the water-filled pore space and adsorption. Since CO2 in aqueous solution reacts with the mineral phases it is important to consider rock/pore network alterations, especially for the sealing lithologies.