Evaluation of Subsurface Gypsum-rich Rock in the Context of Geologic Carbon Sequestration

Abstract

Geological carbon sequestration is a promising means of reducing atmospheric carbon dioxide emissions by capturing CO2 gas and storing it long term in subsurface porous rock. The goal of geologic carbon sequestration is to sequester CO2 in an aqueous, solid, or gaseous form so it cannot re-enter the atmosphere. In this study, a gypsum-rich rock sample from Cassville, Georgia was analyzed as a potential injection site for carbon sequestration. Before injection, it is of the utmost importance to have an accurate prediction of the geochemical processes that will occur with the addition of CO2 to the formation. The specific reactions studied include the initial acidification of the brine present in this sample and the mineral dissolution and precipitation reactions between CO2 and gypsum, dolomite, quartz, and illite (the minerals present in this sample). These dissolution reactions are highly dynamic, and the goal of the models is to provide an accurate prediction of the rate, extent, and impact of the geochemical reactions on the formation. The reactive transport modeling software Crunchflow was used to model these dynamic processes. Images of the sample were taken using a scanning electron microscope (SEM) with a backscatter electron (BSE) detector. These images were used to calculate the porosity of the sample. This data was used to inform the reactive transport model. The results of this model provide predictions of changes in this formation including porosity, mineral dissolution, and potential mineral precipitation. The results from this model suggest that gypsum will remain stable in these conditions. The simulation results also show that dolomite will dissolve completely triggering an increase in porosity from 26% to 33%. In addition to modeling, this study also includes batch reactor experiments to validate the stability of gypsum in CO2 storage conditions. Two sets of experiments are carried out exposing samples to conditions pertinent for storage formations: one for pure gypsum, and one for Sample 9 from the Cassville, GA site. The concentration of calcium ions in the effluent solution was used as an indicator of the amount of gypsum dissolved. SEM imaging was utilized to note the morphological differences in the sample pre and post experiment.