Reservoir characterization by push–pull tests employing kinetic interface sensitive tracers – Quantification of residual trapping in geological storage of carbon dioxide

Abstract

Kinetic interface-sensitive (KIS) tracing is a recently developed methodology to monitor the change in fluid–fluid interfacial area (FIFA) in the context of geological storage of supercritical CO₂ in saline aquifers. Previous studies have demonstrated the applicability of KIS tracers at the scCO₂ injection stage, while its application at the storage stage when the tracked scCO₂ is residually trapped has not yet been studied. The main challenges that impede tracer application at the storage stage are the unknown concentration of the reacted tracer accumulated in the measurement region, resulting from the persistent interfacial reaction that occurs at the scCO₂ / brine interface with the KIS tracer injected by the scCO₂, as well as the effects of the spatial distribution and shape of residual scCO₂ blobs. A “water push–pull” method is proposed here, enabling the application of the KIS tracer at the storage stage. This study applies pore-scale and field-scale numerical simulations to study the KIS tracer reactive transport during the “water push–pull” experiments. It is found that the measurement is not affected by the residual scCO₂ architecture, when the Péclet number is smaller than 5. The measurement relies on the steady-state concentration distribution formed in the measurement region, resulting from the equilibrium between the reaction production and the tracer removal due to the freshwater injection. The tracer breakthrough curves (BTCs) are not affected by the initial concentration nor a change in flow rate, and correlation functions between the FIFA and the slope of the BTCs are found.