Basic Research Needs for Geosciences - Energetics Meetings and ...

APPENDIX 1: TECHNICAL PERSPECTIVES RESOURCE DOCUMENTSTORAGE BASICSCO 2 behavior at depthStorage of large CO 2 volumes in geological formations requires that the CO 2 be relatively dense,so that storage capacity is efficiently used. Given typical geothermal gradients and hydrostaticloads, CO 2 is likely to be in a supercritical state (Figure 1) at most target sites (e.g., Bachu 2000).This requirement means that CO 2 will be injected typically at depths greater than 800 m.At the likely range of injection pressures and temperatures, CO 2 density would range from 400 to800 kg/m 3 (Table 3). Gravitational (buoyant) forces will drive CO 2 upward from the injectionpoint to the formation cap rock. Consequently, trapping mechanisms (listed in next section) areneeded to store CO 2 effectively. CO 2 viscosity will be less than most oils (~ a factor of 10); thisraises the concern of viscous fingering by the CO 2 through any reservoir into which it is injected,thereby potentially bypassing much of the potential pore storage space. CO 2 dissolves slowlyinto brines and then forms carbonic acid, which lowers pH. Both the carbonate alkalinity and thechange in pH can influence chemical reactions among the CO 2 (gas), the brine and the mineralsin the rock matrix. At the same time, water dissolves into the CO 2 , effectively drying out the porevolume within the body of the plume and reducing reaction rates therein. This process may alsosupersaturate pore brines and induce salt precipitation. These physical and chemical propertiesare summarized in Annex 1 of the IPCC Special Report (Freund et al. 2005) and referencestherein.Figure 1. CO 2 phase diagram (Bielinski 2006)Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems Appendix 1 • 7