Lawrence Berkeley National Laboratory

This report presents numerical simulations of isothermal reactive flows which might be induced in the caprock of an Italian depleted gas reservoir by the geological sequestration of carbon dioxide. Our objective is to verify that CO2 geological disposal activities already planned for the study area are safe and do not induce any undesired environmental impact.Gas-water-rock interactions have been modelled under two different intial conditions, i.e., assuming that i) caprock is perfectly sealed, or ii) partially fractured. Field conditions are better approximated in terms of the "sealed caprock model". The fractured caprock model has been implemented because it permits to explore the geochemical beahvior of the system under particularly severe conditions which are not currently encountered in the field, and then to delineate a sort of hypothetical maximum risk scenario.Major evidences supporting the assumption of a sealed caprock stem from the fact that no gas leakages have been detected during the exploitation phase, subsequent reservoir repressurization due to the ingression of a lateral aquifer, and during several cycles of gas storage in the latest life of reservoir management.An extensive program of multidisciplinary laboratory tests on rock properties, geochemical and microseismic monitoring, and reservoir simulation studies is underway to better characterize the reservoir and cap-rock behavior before the performance of a planned CO2 sequestration pilot test.In our models, fluid flow and mineral alteration are induced in the caprock by penetration of high CO2 concentrations from the underlying reservoir, i.e., it was assumed that large amounts of CO2 have been already injected at depth. The main focus is on the potential effect of these geochemical transformations on the sealing efficiency of caprock formations. Batch and multi-dimensional 1D and 2D modeling has been used to investigate multicomponent geochemical processes. Our simulations account for fracture-matrix interactions, gas phase participation in multiphase fluid flow and geochemical reactions, and kinetics of fluid-rock interactions.The main objectives of the modeling are to recognize the geochemical processes or parameters to which the advancement of high CO2 concentrations in the caprock is most sensitive, and to describe the most relevant mineralogical transformations occurring in the caprock as a consequence of such CO2 storage in the underlying reservoir. We also examine the feedback of these geochemical processes on physical properties such as porosity, and evaluate how the sealing capacity of the caprock evolves in time.