Simulation of CO2 Storage in Saline Aquifers

Abstract

Summary This paper describes the essential features for the simulation of CO2 storage in saline aquifers. Important physical processes that occur during CO2 storage are addressed: solubility trapping, residual gas trapping, mineral trapping and caprock integrity. The paper shows how these modeling techniques can be used to design a CO2 storage process in saline aquifers. The following results are obtained: Solubility trapping and residual gas trapping are competing mechanisms. Residual gas trapping is important in low-permeability aquifers and water injection can be used to accelerate and enhance residual gas trapping. Mineral trapping is a very slow process and depends on the mineralogy of the aquifers. Geomechanics calculations coupled with the flow simulation can be used to predict potential failure of the caprock and subsequent leakage of CO2 to upper formations. Introduction 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:structural trapping;residual gas trapping;solubility trapping andmineral trapping. Structural trapping involves the storage of CO2 in a geological structure as a free gas or supercritical fluid. CO2 can flow and escape through the caprock 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. The latter has been identified as one of the important processes for safe CO2 storage as the immobile gas can be kept away from the caprock. 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 HCO3 ions. 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. One of the major risks associated with sequestration in aquifers is the potential leakage of CO2 through caprock and overburden leading to escape of CO2 into shallow potable aquifers or to the atmosphere. Factors responsible for rock failure need to be investigated with geomechanics analysis before starting a CO2 injection program to avoid potential leakage problems. This paper discusses the modelling of the physics associated with CO2 storage in saline aquifers and illustrates through examples the important storage processes. It also shows how geomechanics can be used to predict failure, if any, of the caprock and subsequent leakage of CO2.