Exploratory Simulation Studies of Caprock Alteration Induced byStorage of CO2 in Depleted Gas Reservoirs
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 CO 2 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
... 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 CO 2 sequestration pilot test. where Δt Δx are the time and the space discretization, and D is the effective diffusivity. In contrast, when the advection term dominates, the stability criterion is shown to be: where v is the effective velocity of phase (liquid or gas) considered. Calculations have been carried out under the local equilibrium assumption for aqueous complexation, acid-base, redox and gas dissolution/exolution reactions. Aqueous activity coefficients are computed using an extended Debye-Hückel equation, according to Helgeson et al. (1981) and Tanger and Helgeson (1988) . Fugacity coefficients are computed for H 2 O(g) and CO 2 (g) species by means of the virial equation of Spycher and Reed (1988) . Precipitation and dissolution of minerals are kinetically-controlled. The kinetic laws incorporated in the code are derived from transition state theory (Lasaga, 1984) . Effective reaction rates can be expressed through the following general equation: where A m is the specific surface area, k m is the kinetic rate constant, Q is the ion activity product, K the equilibrium constant for the specific mineral-water reaction, µ and η two constants which depend on experimental data; they are usually but not always taken equal to 1.