SPE Reservoir Evaluation & Engineering
Volume 10, Number 4, August 2007, pp. 406-414

Summary

The geochemical changes caused by carbon dioxide (CO₂) injection into aquifers include acidification and carbonation of the native brine. There are also potential mineral-dissolution and mineral-precipitation reactions caused by the aqueous- composition changes. The latter are important for evaluating the potential CO₂-storage capacity in the form of minerals. Reactions also may influence the performance of the injection well.

The theories of geochemical flows and of fractional flow provide useful insight into several aspects of CO₂ sequestration. This paper gives the mathematical formalism of combined geochemical reactions and multiphase flow. If the local-equilibrium assumption applies, the theory leads to a graphical solution, from which it is easy to see when and under what conditions mineralization will occur. The theory also illustrates the modes of CO₂ trapping (hydrodynamic, solubility, mineral, residual saturation). Trapping mechanisms are identified analytically. We also show that the natural groundwater flow alters the modes of trapping significantly.

Introduction

CO₂ sequestration was first discussed in the late 1970s (Baes et al. 1980). However, serious research and development into CO₂ sequestration began only in the early 1990s. The technical literature (Lohuis 1993; Gunter et al. 1993, 1997, 2000; Bachu et al. 1994; Pruess et al. 2003; Wellman et al. 2003) about CO₂ disposal in aquifers includes feasibility studies in The Netherlands and in the Alberta basin, Canada. A field test is being performed in the North Sea in the Sleipner Vest project, which is the first CO₂-sequestration project in a brine-bearing formation (Korbol and Kaddour 1995).

CO₂ can be sequestered in geologic formations by four principal mechanisms (Hichon et al. 1996; Kumar et al. 2005):

• CO₂ can be trapped as a gaseous phase or supercritical fluid under a low-permeability caprock, similar to what occurs in natural-gas reservoirs (hydrodynamic trapping).
• Dissolution into an aqueous phase (solubility trapping) can occur.
• CO₂ can react with the minerals and the organic matter in geologic formations to become a part of the solid (mineral trapping). Formation of carbonate minerals such as calcite or siderite and the adsorption onto coal are examples of the mineral trapping. Mineral trapping will create stable repositories of CO₂ that decrease mobile hazards such as leakage to the surface.
• CO₂ trapping as a residual gas saturation is also considered. Here, CO₂ remains as a gaseous phase, such as for hydrodynamic trapping, but it is immobile because the gaseous phase is trapped by capillary forces. In this study, the immobile gas trapping is called the residual-saturation trapping.

Siliciclastic aquifers should have greater potential for the mineral trapping of CO₂ than carbonate aquifers (Gunter et al. 1997). Depending on whether the basic aluminosilicate minerals, such as feldspars, zeolites, illites, chlorites, and smectites, contain an alkali- or alkaline-earth cation, two types of mineral trapping can be considered. Na/K-bearing minerals result in the development of bicarbonate brines. Fe/Ca/Mg-bearing minerals result in the precipitation of siderite, calcite, or dolomite. Both types show a substantial amount of trapping and immobilization of CO₂. Gunter et al. (1997) performed an experimental and numerical study on CO₂-trapping reactions in a glauconitic-sandstone aquifer, which is a typical sandstone aquifer in the Alberta basin. Their study indicated that geochemical trapping of CO₂ is slow, but still fast enough to form an effective CO₂ trap compared to the fluid flow in aquifers.

During CO₂ injection into geologic formations, geochemical processes are affected by multiphase fluid flow and solute transport. The dissolution of primary minerals and the precipitation of secondary minerals could change formation porosity and permeability and subsequently affect fluid-flow patterns. These reactions also determine the mass of CO₂ that can be stored by mineral trapping. The theory of propagation of geochemical fronts (mineral precipitation/ dissolution) provides insight into the time scales, spatial extents, and composition changes associated with these reactions (Lake et al. 2002). The theory is well established for single-phase aqueous flows, but it requires extension when a second fluid, in this case CO₂, is also flowing because the velocity and saturation of the aqueous phase varies with position. When mass transfer between the flowing phases is possible, the fractional flow and geochemical changes are tightly coupled. In this study, we show an analytical approach to characterizing the semimiscible displacement of water by CO₂. The specific velocity of a concentration discontinuity is derived from the mass-balance equation (see the Appendix). For verification, analytical solutions are compared with simulation results.

This paper is structured as follows. We first present the mathematical model. Next, the saturation distribution and mineral reaction in CO₂ sequestration are discussed. With fractional-flow theory, each of the trapping mechanisms in CO₂ sequestration is identified analytically, and its effects are compared with numerical simulation.

© 2007. Society of Petroleum Engineers

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History

• Original manuscript received: 19 January 2004
• Meeting paper published: 17 April 2004
• Revised manuscript received: 16 October 2006
• Manuscript approved: 20 March 2007
• Version of record: 20 August 2007