Impacts of Methane on Carbon Dioxide Storage in Brine Formations.

In the context of geological carbon sequestration (GCS), carbon dioxide (CO ) is often injected into deep formations saturated with a brine that may contain dissolved light hydrocarbons, such as methane (CH ). In this multicomponent multiphase displacement process, CO competes with CH in terms of dissolution, and CH tends to exsolve from the aqueous into a gaseous phase. Because CH has a lower viscosity than injected CO , CH is swept up into a 'bank' of CH -rich gas ahead of the CO displacement front. On the one hand, this may provide a useful tracer signal of an approaching CO front. On the other hand, the emergence of gaseous CH is undesirable because it poses a leakage risk of a far more potent greenhouse gas than CO if the cap rock is compromised. Open fractures or faults and wells could result in CH contamination of overlying groundwater aquifers as well as surface emissions. We investigate this process through detailed numerical simulations for a large-scale GCS pilot project (near Cranfield, Mississippi) for which a rich set of field data is available. An accurate cubic-plus-association equation-of-state is used to describe the non-linear phase behavior of multiphase brine-CH -CO mixtures, and breakthrough curves in two observation wells are used to constrain transport processes. Both field data and simulations indeed show the development of an extensive plume of CH -rich (up to 90 mol%) gas as a consequence of CO injection, with important implications for the risk assessment of future GCS projects.