Chemical Stability of Cap Rock Clay Minerals in Acid Gasses Geological Storage Contexts—Method for Reactivity Appraisal by Geochemical Simulations

Caroli, Emmanuel¹, Marc Lescanne¹, Stéphane Teinturier¹, Nicolas Flichy², Harald Johansen³ (1) Total, Pau, France (2) Total, Paris La Défense, France (3) IFE, Kjeller, Norway

Geological storage of acid gases increasingly becomes an option for the reduction of greenhouse gases and the production of high H2S content fields. One challenge is to pre­dict the evolution of the cap rocks sealing efficiency through time. This integrity assessment is partly dependant on the mineralogical stability of the cap rocks. We discuss in this paper a method that enables to evaluate the mineralogical sensitivity of different clayey cap rocks submitted to various ranges of CO2 and H2S partial pressures.

This simulation based method uses simple cap rock data: an average quantitative min­eralogy (from cores or cuttings), the water composition of the underlying reservoir and a Residual Salt Analyses (RSA) of some cap rock plugs. The first step is to build a simplified chemical model of the cap rock to identify the saturated minerals. Only these minerals will be considered as reactive. Thus, the fugacity of the acid components in the model can be modified to quantify the sensitivity of the studied cap rock and the behaviour of some key clay minerals.

Three cap rocks have been evaluated with this method: a non carbonated and a carbon­ated clayey cap rock from two sour hydrocarbon reservoirs, and a pure silico-clastic cap rock from a non sour hydrocarbon reservoir. The main conclusions derived from the geo­chemical models are: - Reactivity of the cap rocks minerals is very low; - The reactivity is focused on specific minerals such as Illite. These findings are now being validated by H2S experiments at reservoir conditions.