Integrated modeling and experimental approach for caprock integrity, risk analysis, and long term safety assessment

A global safety assessment of geological storage of CO2 involves looking both at the direct protection of the populations with respect to health and sanitary risks, and at the mitigation of CO2 with respect to global warming. A set of safety criteria can be defined in order to assess the impact on human and environment of CO2 leaking from the reservoir. These criteria have to take into account the fact that CO2 can either take the form of gas and cause a threat in case of inhalation or affect the global climate; or it can be dissolved in the water and cause a direct threat to health measurable according to drinking water criteria (pH, dissolved elements). The nature and amount of potential leakage are calculated by modeling the evolution of the reservoir-caprock system which has to include the most important physico chemical phenomena and which can be organized according to (1) a reference case scenario for the performance assessment, and (2) to different scenarios in altered conditions for the safety assessment. In this exercise, exhaustiveness is often not possible because of the complexity of the system, the coupling between th e different phenomena, or the lack of value for some parameters. Dedicated experiments are needed to determine the most critical parameters but simplifying hypotheses may still be required. In this case, they have to be justified in terms of their conservative character, especially with respect to uncertainties and heterogeneities. Such an approach is illustrated by looking at a crucial element of the performance of CO2 storage: the integrity of the caprock overlying the reservoir. The safety function of the caprock is to oppose the migration of CO2 by stopping, limiting, or delaying the fluxes towards the geosphere, relying on adequate petrophysical, geochemical, and geomechanical properties. Even if the caprock initially presents the expected confinement properties of a continuous barrier, it may also contain natural heterogeneity to some degree (in composition and/or in properties, i.e. the presence of fractures) and will also evolve in time due to perturbations such as mechanical stress and aggressive fluids. The different leakage scenarios that are identified result from a phenomenological analysis of the situations encountered in the evolution of the global system. They involve the diffusion of dissolved CO2, the migration of a CO2 gas bubble through the matrix of the caprock or through fractures. A first series of calculations is shown which intends to assess the flux of CO2 out of the storage system in the nominal, or reference situation. These results are then compared with calculations in degraded conditions including data from geochemical experiments in order to investigate the effect of the CO2 reactivity with the caprock on transport and geomechanical properties.