Implications of Coupling Fractional Flow and Geochemistry for CO2 Injection in Aquifers
- Myeong Hwan Noh (University of Texas at Austin) | Larry Wayne Lake (University of Texas at Austin) | Steven Lawrence Bryant (University of Texas at Austin) | Aura N. Araque-Martinez (University of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2007
- Document Type
- Journal Paper
- 406 - 414
- 2007. Society of Petroleum Engineers
- 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology, 5.4.2 Gas Injection Methods, 6.5.2 Water use, produced water discharge and disposal, 5.2.1 Phase Behavior and PVT Measurements, 4.6 Natural Gas, 5.5 Reservoir Simulation, 5.3.1 Flow in Porous Media, 5.8.8 Gas-condensate reservoirs, 5.4 Enhanced Recovery, 6.5.3 Waste Management, 5.4.3 Gas Cycling, 5.6.5 Tracers, 5.3.2 Multiphase Flow, 5.1 Reservoir Characterisation
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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.
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.
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