The effects of CO2-brine rheology on leakage processes in geologic carbon sequestration

Abstract

Leakage from geologic carbon sequestration (GCS) sites is inherently challenging to study because CO2, driven by buoyant forces, travels over long distances, undergoing phase changes and encountering numerous connate brine and formation chemistries as it rises to the surface. This work explores the effect that CO2 has on the rheological properties of brine solutions over a range of GCS-relevant temperature, pressure, ionic strength, and shear conditions. Under the fluid-liquid equilibrium conditions that prevail in the deep subsurface, viscosity of CO2-brine mixtures was found to be a function of temperature and pressure alone. Once leakage conditions ensue, discrete CO2 bubbles form in brine, resulting in the vapor-liquid equilibrium (VLE), and these mixtures exhibit complex linear viscoelastic, time dependent, and thixotropic behavior. The presence of CO2(g) bubbles on the flow of the bulk fluid could have important impacts on impeding (via shear drag force) leakage depending on the geometrical, geochemical and geophysical characteristics of a storage site. Under VLE conditions, the effective viscosity of CO2-brine mixtures was found to be up to five times higher than brine alone but the microstructure was easily destroyed, and not readily regained, under high shear conditions. At higher temperatures and higher ionic strength, the effect is less pronounced. These results were considered in the context of flow through porous media, and the effect on buoyancy-driven flow is significant. Understanding this effect is important for developing an accurate constitutive relationship for leaking CO2, which will lead to better capacity to select and monitor GCS sites. © 2012. American Geophysical Union. All Rights Reserved.