Modeling CO2 Storage in Aquifers: Assessing Key Contributors to Uncertainty

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 123582, ’Modeling CO2 Storage in Aquifers: Assessing the Key Contributors to Uncertainty,’ by W. Sifuentes, SPE, Schlumberger; M.J. Blunt, SPE, Imperial College; and M.A. Giddins, SPE, Schlumberger, prepared for the 2009 SPE Offshore Europe Oil and Gas Conference & Exhibition, Aberdeen, 8-11 September. The paper has not been peer reviewed. The influence of different physical properties on the effectiveness of CO2 storage in aquifers was studied. A numerical sensitivity analysis based on experimental design was used to quantify and compare the contribution of the most important parameters to the trapping of CO2. The work focused on the effects of dissolution and residual trapping. Reservoir simulations with properties and geometries representative of the Stuttgart formation in Ketzin, Germany, demonstrated how different trapping mechanisms are influenced by gravity segregation, fingering, and channeling. Introduction This focus was on CO2 storage in aquifers because this storage option is emerging with great potential, having a high estimated capacity to store CO2 in geological formations (estimates range from 400 to 1000 Gt of CO2). Several CO2-storage projects are under way or planned (pilot, research, commercial projects), and many of the commercial projects are related to major gas-production facilities, at which gas-production streams contain high levels of CO2 (e.g., Sleipner in Norway, In Salah in Algeria, and Gorgon in Australia). The first commercial project related to CO2 storage in aquifers was the Sleipner project in Norway, where approximately 1 Mt/a of CO2 captured from the gas-production stream has been injected since 1996 in the Utsira saline formation. The efficiency of long-term storage in aquifers is related directly to the efficiency of each of the trapping mechanisms involved. In the context of CO2 storage, particularly in aquifers, there are four major trapping mechanisms. Hydrodynamic (structural or stratigraphic) trapping—caprock prevents mobile CO2 from flowing back to the surface. Residual or capillary trapping—capillary forces and relative permeability effects contribute to converting the CO2 injected into an immobile phase. Solubility trapping—CO2 dissolves in the aqueous phase. Mineral trapping—chemical reactions between CO2 and rock minerals form a solid carbonate. During the injection phase, structural or stratigraphic trapping is the main contributor preventing CO2 from escaping to the surface. Mineral trapping is the safest long-term way to trap CO2 because it transforms CO2 into an immobile solid. However, this process can be very slow. The focus here was on capillary and dissolution trapping, which are likely to be effective on an intermediate time scale.