ECO2N – A fluid property module for the TOUGH2 code for studies of CO2 storage in saline aquifers
Introduction
Numerical modeling is an important tool for studying the injection of CO2 into saline aquifers, which is being considered as a means for reducing atmospheric emissions of this greenhouse gas. Simulating CO2 storage in saline aquifers requires an accurate representation of the thermodynamics and thermophysical properties of brine–CO2 mixtures. Our development of simulation capabilities for CO2/aquifer systems started from the EWASG fluid property module [1]. EWASG was designed for applications to geothermal reservoir engineering, which generally involve elevated temperatures and modest CO2 partial pressures, typically of order 10 bar1 or less. In contrast, the thermodynamic regime of interest for CO2 storage in saline formations typically involves moderate (near ambient) temperatures, and high CO2 pressures of the order of a few hundred bars. Lower CO2 pressures are of interest in connection with studies of CO2 leakage from the primary storage reservoir.
Compared to the PVT formulation used in EWASG, the main enhancements required for CO2 storage studies include (1) accurate correlations for density, viscosity, and specific enthalpy of CO2 for temperatures from ambient to about 100 °C or more, and pressures from ambient to several hundred bar; (2) accurate representation of thermophysical properties of brines for the same temperature and pressure range, and (3) accurate partitioning of H2O and CO2 between aqueous and CO2-rich phases.
CO2 storage in saline aquifers would be made at super-critical conditions. However, if CO2 leaks from the primary storage reservoir its temperature and pressure could drop below the critical point (Tcrit = 31.04 °C, Pcrit = 73.82 bar), and two CO2-rich phases (gaseous and liquid) could form. ECO2N is capable of accurately representing thermophysical properties of CO2 in a gaseous as well as in a liquid CO2-rich phase, but it has no provisions for modeling transitions between those phases. In ECO2N, the CO2-rich phase is modeled as a single non-wetting phase. For convenience, the CO2-rich phase will in the remainder of this paper often be referred to as “gas,” while the aqueous phase will be referred to as “liquid.”
Mixtures of H2O–NaCl–CO2 with phase change between liquid and gaseous CO2 at sub-critical conditions can be modeled with another fluid property module “EOSM,” which includes all seven possible phase combinations in the system aqueous–liquid CO2–gaseous CO2 [2]. The development of EOSM is ongoing, and we hope to be able to release this code to the public at some future time.
Our earlier efforts at simulating CO2–brine mixtures retained much of the original EWASG formulation [3]. In particular, CO2 dissolution in the aqueous phase was modeled by means of an extended Henry’s law that included fugacity effects for CO2 and salting-out corrections for the aqueous phase. ECO2N represents a more profound departure from this approach, as we employ newly developed correlations for the partitioning of H2O and CO2 between aqueous and gas phases that do not use the concept of CO2 partial pressure, and include effects of aqueous concentrations of NaCl [4].
The process modeling capabilities of TOUGH2/ECO2N include single and multiphase flow with relative permeability and capillary pressure effects, multiphase diffusion of all mass components (H2O, NaCl, CO2), transport of sensible and latent heat, heat conduction, heat of dissolution effects for CO2, mutual dissolution of H2O and CO2 in H2O and CO2-rich phases, and precipitation and dissolution of solid salt with associated porosity and permeability changes. User options allow disabling certain processes, such as running in isothermal mode, ignoring diffusion, and neglecting permeability change from salt precipitation.
Section snippets
Thermophysical properties
Thermodynamic conditions are specified in ECO2N with the same primary variables as in EWASG, namely (P, Xsm, X3, T) for single-phase conditions, and (P, Xsm, Sg + 10, T) for two-phase. When discussing phase conditions, we refer to the fluid phases only; in all cases solid salt may precipitate or dissolve, adding another active phase to the system. Here P is pressure, T temperature, X3 mass fraction of CO2, and Sg gas saturation. Xsm pertains to the salt and denotes salt (NaCl) mass fraction Xs in
Applications of TOUGH2/ECO2N
An executable for the TOUGH2/ECO2N code is generated by compiling and linking the source code file eco2n.f with standard TOUGH2 V 2.0 modules, exactly like any of the other fluid property modules. The sample problems provided with ECO2N and described in the user’s guide [9] include the three saline aquifer flow problems that had been part of a recent code intercomparison study [17], [18]. Results for these problems are generally similar to the earlier LBNL submissions to that study, but show
Concluding remarks
ECO2N is a new fluid property module for the multiphase, multicomponent simulator TOUGH2, Version 2.0. It provides capabilities for modeling advective and diffusive flow and transport in multidimensional heterogeneous systems containing H2O–NaCl–CO2 mixtures. Process capabilities include coupling between fluid and heat flow, partitioning of H2O and CO2 among different phases, and precipitation/dissolution of solid salt. The code represents mutual dissolution of water and CO2 generally within
Acknowledgements
Thanks are due to Tianfu Xu and two anonymous reviewers for a careful review of the manuscript and the suggestion of improvements. I thank Richard Fuller of Princeton University for stimulating discussions. This work was supported by the Zero Emission Research and Technology project (ZERT) under Contract No. DE-AC02-05CH11231 with the US Department of Energy.
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