Important modelling techniques for CO2 storage in saline aquifers are discussed, in particular solubility trapping, residual gas trapping and geochemistry for mineral trapping. These modelling techniques are applied to the simulation of several important aspects of CO2 storage, which include optimizing total trapping through water injection, assessing the security of residual trapping with regards to leakage through the cap rocks and evaluating the conversion of CO2 into minerals.
Saline aquifers represent the most important venue for CO2 storage since they have the largest capacity among all other venues (coal seams, depleted gas and oil fields). Table 1 (DOE and NETL, Carbon and Sequestration Atlas for the United States and Canada, 2008) shows the dominance of saline aquifers for large scale deployment of CO2 storage.
There are several CO2 trapping mechanisms in saline aquifers:
residual gas trapping;
solubility trapping and
Structural trapping involves the storage of CO2 in a geological structure as a free gas or super-critical fluid. CO2 can flow and escape through the cap rock or sealing faults if the integrity of the latter is compromised. Residual gas trapping consists of storing CO2 as an immobile gas in the porous media. This process has been identified as one of the most important processes for safe CO2 storage as the immobile gas can be kept away from the cap rock. CO2 is highly soluble in brine and solubility trapping is essentially the impetus for CO2 storage in saline aquifers. As CO2 dissolves in brine, it decomposes into H+ and HCO−3. These ions in turn react with the minerals in place. Depending on the mineralogy of the formation, these reactions could induce precipitation of carbonate minerals such as Calcite, Dolomite and Siderite, which corresponds essentially to the conversion of CO2 into minerals. This paper discusses the modelling of the physics associated with CO2 storage in saline aquifers and illustrates through examples the important storage processes.
Table 1: Storage Capacity for CO2 Storage in North America (Available in full paper)
CO2 has a critical pressure of 7,376 kPa and a critical temperature of 304.2 K (31 °C). Thus, CO2 is normally a supercritical fluid at aquifer conditions. For convenience, CO2 is referred to as a "gas" in this paper. To be general, the gas phase is treated as a multicomponent mixture.
The simulation and modelling techniques described in this paper are based on the geochemical equation-of-state compositional simulator, GEM ™ with the greenhouse gas option (Nghiem et al., 2004). This simulator has been used extensively for modelling CO2 storage in saline aquifers in the past five years. Pruess et al. (2003) and Zhang et al. (2007) describe a simulator for CO2 storage in aquifers with similar features.
The accurate modeling of gas solubility in the aqueous phase (brine) is important as brine can dissolve a large amount of CO2.