Abstract
Abstract
This paper investigates experimentally the effect of heterogeneity on buoyancy-driven flow in CO2 storage. The application of this work is for the storage of CO2 in aquifers, where the degree of heterogeneity affects its trapping capacity. The experimental set up idealizes the post-injection period when the migration of CO2 plume in an aquifer is mainly due to the buoyancy drive that results from the density contrast between the injected CO2 and formation brine.
A column of air was trapped beneath a brine-saturated sand pack at room temperature and the whole arrangement was placed in an Amott cell. The Amott cell was filled with brine to account for background regional groundwater support. The amount of air beneath the sand pack and the cumulative air production were measured throughout the experiments to determine the sand pack air saturation.
As the air plume migrates up the sand pack, the buoyancy drive decreases. This causes some regions in the sand pack that once permitted the upward migration of the air plume to serve as capillary barriers to the trailing air bubbles. In all the studies, the uniformity of the air front depends on the degree of heterogeneity in the sand pack. However, air migration through a homogeneous sand pack provided a more uniform front than the heterogeneous sand pack. The presence of a high-permeability streak in the sand pack reduced the trapping capacity of the entire pack. The presence of a low-permeability layer in the sand pack increased its trapping capacity. Irrespective of the degree of heterogeneity, a substantial amount of air was trapped in and below the sand packs (about 40% of the initial air volume). The results also showed that better consolidated sands trapped more air than poorly consolidated sands. Pulsing flows (fluctuation of air saturation with time) occurred in some experiments after air breakthrough.
Introduction
The concentration of CO2 in the atmosphere is steadily increasing. The high consumption of fossil fuels (oil, gas, and coal), which will continue into this century is the major contributor to the increased concentration of CO2 in the atmosphere (Hoffert et al., 1998). One of the environmental impacts of increased concentration of greenhouse gases in the atmosphere is global warming which results from the greenhouse gases trapping more of the earth's outgoing heat radiation (Sengul, 2006). A reduction in the rate of emission of CO2 into the atmosphere is considered an essential first step in the control of global warming. CO2 sequestration refers to the capture and long-term storage of anthropogenic CO2 in order to limit its emission to the atmosphere (Lanckner, 2003). There are four types of possible CO2 disposal: biological, mineral immobilization, deep ocean disposal and injection into geological formations (Bachu et al., 1994). The best available option to reduce the concentration of CO2 in the atmosphere will be to capture effluent gases after the combustion of fossil fuels and inject them into subsurface geological formations where they will be trapped for geological periods of time (Holloway, 1997; Bachu, 2000; Espie, 2005). Although the oceans provide possibly the largest potential for CO2 storage, ocean sequestration involves issues of poorly understood physical and chemical processes, sequestration efficiency, cost, technical feasibility and environmental impact (Bachu and Adams, 2003; Bennion and Bachu, 2005).
Storage of CO2 in geological formations can take four forms:Structural trapping: the CO2 remains as a mobile fluid beneath an impermeable cap rock that prevents its upward movement (Bachu et al., 1994; Sengul, 2006).Residual trapping: the CO2 phase becomes disconnected into an immobile fraction (Flett et al., 2004; Kumar et al., 2004; Mo and Akervoll, 2005; Spiteri et al., 2005; Hesse et al., 2006; Juanes et al., 2006; Ide et al., 2007; Qi et al., 2007; Juanes and MacMinn, 2008; Pentland et al., 2008; Saadatpoor et al. 2008).Dissolution Trapping: the injected CO2 dissolves in the brine as it migrates upward through the aquifer (Pruess and Garcia, 2002; Bachu and Adams, 2003; Van Der Meer and Van Wee, 2006; Burton and Bryant, 2007).Mineral trapping: the precipitation of dissolved gases as minerals (Gunter et al., 1997; Gallo et al., 2002; Pruess et al., 2003; Xu et al., 2003; Ozah et al., 2005).