Numerical Modeling of Aquifer Disposal of CO2

Abstract

Summary Disposal of CO2 from stationary sources (fossil-fired power plants) into brackish (saline) aquifers has been suggested as a possible means of reducing emissions of greenhouse gases into the atmosphere. Injection of CO2 into such aquifers would be carried out at supercritical conditions and would give rise to the evolution of a two-phase fluid system, in which most of the injected CO2 will reside in a dense supercritical gas phase, while also dissolving partially into the aqueous phase and reacting with native minerals. This paper presents scoping studies of the amounts of CO2 that can be trapped into the various phases (gas, aqueous, and solid) for a range of conditions that may be encountered in typical disposal aquifers. Our analyses employ a realistic fluid property (PVT) description of brine/CO2 mixtures for supercritical conditions, which takes into account real gas density and viscosity effects for CO2, and includes pressure, temperature, and salinity dependence of CO2 dissolution into the aqueous phase. The fluid property description has been incorporated into a multipurpose reservoir simulator, and has been used to evaluate dynamic effects of CO2 injection into aquifers. A survey of minerals commonly encountered in crustal rocks was made to identify possibilities for chemical sequestration of CO2 through the formation of carbonates of low solubility. We also performed batch reaction modeling of the geochemical evolution of representative aquifer mineralogies. Results indicate that under favorable conditions the amount of CO2 that may be sequestered by precipitation of secondary carbonates is comparable to the amount of CO2 dissolved in pore waters. The accumulation of carbonates in the rock matrix and induced rock mineral alteration caused by the presence of dissolved CO2 lead to a considerable decrease in porosity.