Modeling CO2 Storage in Aquifers : Assessing the key contributors to uncertainty

We study the influence of different physical properties o n the effectiveness of CO 2 storage in aquifers. We present the results of a numerical sensitivity analysis using experimental design to quantify and compare the contribution of the most impo rtant parameters to the trapping of CO 2. The work focuses on the impact of dissolution and residual tr apping. Simulations using a reservoir model with properties and geometries representative of the Stuggart Formation in K etzin (Germany) demonstrate how different trapping mechan isms are influenced by gravity segregation, fingering, and c hanneling. These studies show that horizontal permeability is the mos t influential parameter on the total amount of CO 2 dissolved, as it facilitates the lateral migration of CO 2, enhancing dissolution into the brine. Residual gas saturati on Sgr is found to be the greatest contributor to the amount of residual CO 2. As expected, higher S gr will reduce CO2 mobility, producing a higher residual trail left by the CO 2 plume as it migrates. In addition, permeability heterogeneity is a major contributor to both trapping mechanisms. After a suitable geological site for CO 2 storage has been selected, our studies suggest that wit appropriate well placement and injection strategy, at least 80% of the stored CO 2 can be trapped within a few decades of the end of injection . Introduction The global average temperature has increased over the last century by 0.6°C, snow precipitation and the ice extent have reduced considerably and the sea level and ocean temperature h ve risen ( Intergovernmental Panel on Climate Change , 2001). This problem is almost certainly exacerbated by the emis sion of anthropogenic greenhouse gases, and principally CO 2, into the atmosphere. More than 13 Gig tons (1Gt = 10 9 kg) of carbon dioxide is emitted from stationary sources eve ry y ar, of which more than 70% comes from burning fossil fuels ( Intergovernmental Panel on Climate Change , 2005). Several mitigation options are currently being used in order to slow down the upward trend of greenhouse gas concentrations in the Earth’s atmosphere. Carbon capture a nd storage (CCS) that consists of the separation of CO 2 from industrial and energy-related sources, transport and storage could play a major role in reducing greenhouse gas emi sions to the atmosphere ( Intergovernmental Panel on Climate Change , 2005). Geological CO2 storage in either oil or gas reservoirs, aquifers, and c oal beds, is currently being tested through several commercial or pilot projects. CO 2 has been used in the oil industry as a method to enhance the recovery of hydrocarbons (Blunt et al. , 1993) and this knowledge can be used to design safe storage i n other subsurface settings ( Intergovernmental Panel on Climate Change , 2005). This paper will focus on the storage of CO 2 in aquifers ( Bachu and Adams , 2003), as this is emerging as the storage option with the greatest potential, having the highest estimat ed capacity to store CO 2 in geological formations (upper estimates = 10 4 Gt CO2, lower estimates = 400 Gt CO 2; Intergovernmental Panel on Climate Change , 2005). The first investigations related to CO2 storage in aquifers date from the early nineties, when V a der Meer and colleagues published studies regarding CO 2 storage in aquifers in the Netherlands ( Van der Meer et al, 1992; Van der Meer, 1995; Van der Meer, 1996). Several simulations and studies on CO 2 storage in aquifers have been performed during the last dec ade, focusing on different aspects and using different approaches (see, e.g., Kumar et al ., 2005; Juanes et al ., 2006; Pruess et al., 2003; Spiteri et al. , 2005; Van der Meer and Van Wees , 2003; Bachu and Adams , 2003; Ennis-King and Paterson ., 2002; Mo et al., 2005; Pruess and Garcia , 2002; Qi et al., 2009; Doughty and Pruess , 2004; Ennis-King and Paterson , 2005). Several CO2 storage projects are currently under way or planned (pilot , research, commercial projects) and many of the commercial ones are related to major gas production facil ities (Intergovernmental Panel on Climate Change , 2005), which