A Coupled Geochemical-Transport-Geomechanical Model to Address Caprock Integrity During Long-Term CO₂ Storage

Abstract

Underground storage of CO₂ will lead to chemical fluid-rock interactions which may potentially alter the porosity and the flow patterns in faults. In this study, we present a coupled numerical model combining chemical fluid-rock interactions, aqueous diffusion, fluid flow, and mechanical processes, aiming at a better understanding of mineral reactions leading to changes of the rock properties, the associated flow patterns and the mechanical stability of faulted caprock on the long-term (500 yrs). In the examples that we studied the mechanical stability of the caprock system was not affected, but significant porosity changes were predicted for low-consolidated fault gouges. Coupled chemical-flow modelling is preferred over chemical modelling as this showed significantly higher porosity increases.

1. INTRODUCTION

Global emissions of carbon dioxide (CO₂ ) into the atmosphere, mainly as a result of fossil fuel burning and cement production, is believed to significantly influence global warming [1]. Carbon capture and storage (CCS) is potentially able to reduce these emissions on a short to medium time scale by storing CO₂ in geological structures of the deep subsurface. The CO₂ is trapped below a sealing caprock in porous media like sandstones or carbonate rocks. One of the main risks associated with storage activities is the leakage of CO₂ into shallower regions of continental groundwater systems [2]. The release of CO₂ results preferentially in water acidification and potential mobilization of trace elements, which may have a toxic effect on the environment [3]. Uncertainties are mainly related to potential leakage mechanisms, through faults and fractures, along wellbores and sealing caprock material, by convection and diffusion.