Geological carbon sequestration in the context of two-phase flow in porous media: a review

In this review, various aspects of geological carbon sequestration are discussed in relation to the principles of two-phase flow in porous media. Literature reports on geological sequestration of CO2 show that the aquifer storage capacity, sealing integrity of the caprock and the in situ processes, e.g., the displacement of brine by supercritical CO2 (scCO2), convection-diffusion-dissolution processes involving scCO2 and brine, geochemical reactions, and mineral precipitation depend on the fluid-fluid-rock characteristics as well as the prevailing subsurface conditions. Considering the complexity of the interrelationships among various processes, experimental investigations and network of mathematical functions are required for the ideal choice of geological site with predictable fluid-fluid-rock behaviours that enhance effective monitoring. From a thorough appraisal of the existing publications, recommendations are made for improvement in the existing simulators to fully couple the entire processes involved in the sequestration operations and in situ mechanisms which include injection rate and pressure, brine displacement, simultaneous flow of free and buoyant phases of CO2, various trapping mechanisms, convection-diffusion-dissolution processes, scCO2-brine-rock reactions, precipitation of the rock minerals and the consequences on the hydraulic and hydrogeological properties in the course of time as well as the quantity of injected CO2. Suggestion is made for the inclusion of leakage parameters on site-specific basis to quantify the risks posed by the prevailing fluid-fluid-rock characteristics as well as their immediate and future tendencies. Calls are also made for thorough investigations of factors that cause non-uniqueness of the two-phase flow behaviour with suggestions for the use of appropriate experimental techniques. The review comprehensively synthesizes the available knowledge in the geological carbon sequestration in a logical sequence.