Geochemical Modelling of CO2 in Saline Aquifers

Abstract

Carbon dioxide, CO2, disposal into saline aquifers could reduce emissions of anthropogenic greenhouse gases into the atmosphere. To ensure that the CO2 is trapped securely and will not escape to the surface, storage in such formations must be designed carefully. The geochemical reactions involved depend on the composition of the injected fluid introduced in the aquifer and the composition of the initial minerals assemblage and the aquifer brine. This thesis studies mineral dissolution/precipitation during CO2 storage, both in the cap rock and the storage aquifer itself. The overall objective is to propose an ideal storage design for long term, inexpensive and safe CO2 disposal in saline aquifers. The rate-limiting effects of CH4 impurities in gas streams on the CO2 reactivity in the cap rock and aquifer (carbonate and sandstone) at conditions representative of storage locations are studied. Representative geochemical data of formation water and mineralogy assemblages of cap rock (Nordland shale) overlying the Sleipner field, Dogger carbonate aquifer from Paris basin, France and Frio sandstone aquifer from Texas, US, are used. Kinetic batch and one-dimensional and two-dimensional reactive transport models are run to predict mineral alteration induced in the cap rock and in the aquifer. Cap rock models are run using PHREEQC for 10,000 years. The model considers both pure CO2 and mixtures of CO2 with CH4 (1-4 (w/w)%) in the injected gas stream. The simulations demonstrate that mixtures of CO2 with CH4 suppress the porosity increase in the cap rock, leading to more secure storage. The aquifers models incorporate 1 (w/w)% organic matter and are run using TOUGHREACT. CO2 was initially injected at constant rate of 30kg/s for 25 years and the models were subsequently run for 10,000 years to study long-term storage. The simulations demonstrate that injection of CO2 in microbial-mediated aquifers enhances the precipitation of secondary carbonates minerals and decreases the porosity of both sandstone and carbonate aquifers. Overall, this study proposes two main design criteria for safe and cost-effective CO2 storage: CO2 injection with CH4 (1-4 (w/w)%) and CO2 injection into microbial-mediated aquifers to enhance mineral precipitation, rendering storage secure.