Geologic Carbon Sequestration Program

ZERT Task 2: Performance prediction for long-term fate and transport of injected CO2

Task 2 Leadership ZERT logo

Task Lead (primary contact)

Curt Oldenburg

Key Personnel/Participants

Tianfu Xu, Christine Doughty, Nic Spycher, Matt Reagan, Karsten Pruess

ZERT Task 2 Goal

  • To develop reliable techniques to predict and model CO2 migration and trapping mechanisms.

ZERT Task 2 Overview

The TOUGH codes will be further developed to accurately simulate the fate and transport of CO2 in the subsurface.  TOUGH2 and TOUGHREACT are available to the public through the Department of Energy’s National Energy Software Technology Center.  The TOUGH2 simulator ("transport of unsaturated groundwater and heat") is a multi-dimensional numerical model for simulating the coupled transport of water, vapor, gas components, and heat in porous and fractured media.  TOUGH2 offers added capabilities and user features, including the flexibility to handle different fluid mixtures through a wide variety of EOS (equation of state) modules.  TOUGH2 uses an integral finite difference method for space discretization, and first-order fully implicit time stepping.  Various preconditioned conjugate gradient and direct sparse matrix solvers are available for linear equation solution.  Thermophysical properties of water are represented, within experimental accuracy, by steam table equations provided by the International Formulation Committee.  The program provides options for specifying injection or withdrawal of heat and fluids.  Double-porosity, dual-permeability, and multiple interacting continua (MINC) methods are available for modeling flow in fractured porous media.  TOUGH2 takes account of fluid flow in both liquid and gaseous phases occurring under pressure, viscous, and gravity forces according to Darcy's law.  Interference between the phases is represented by means of relative permeability functions.  The code includes Klinkenberg effects and binary diffusion in the gas phase, and capillary and phase adsorption effects for the liquid phase.  Heat transport occurs by means of conduction (with thermal conductivity dependent on water saturation), convection, and binary diffusion, which includes both sensible and latent heat.

     TOUGHREACT uses the same basic architecture as TOUGH2, but includes geochemical reactions between CO2, water, solutes and rock.  Both equilibrium and kinetic calculations of mineral dissolution and precipitation can be carried out, thus allowing evaluation of mineral trapping.

ZERT Task 2 Subtasks

Subtask 2.1. Long-term CO2 migration and leakage:

Migration of CO2 away from the storage reservoir remains a primary concern for geologic CO2 storage.  As free-phase CO2 migrates upwards from the storage reservoir towards the land surface it decompresses and changes from a supercritical to gaseous phase.  The pressure decreases can lead to cooling of the CO2 and surrounding rock.  This has been shown to create complex periodic migration behavior which has the tendency to self-limit rates of upward migration.  CO2 injection may cause formation dry-out, which may be accompanied by precipitation of solids, and reduction in porosity, permeability, and injectivity.  In this subtask, we develop and quantify scenarios for CO2 migration to develop greater understanding of both self-limiting and self-enhancing processes controlling CO2 migration.  Additional capabilities will be added to TOUGH2 where needed.  TOUGH2 is available to the public through the Department of Energy’s National Energy Software Technology Center.

Subtask 2.2. TOUGHREACT V2:

The need to model reactive geochemical transport has emerged as one of the most critical performance prediction approaches with applications to both storage and impacts of CO2 and brine migration.  In this subtask, new capabilities will be added to TOUGHREACT, improvements will be made in existing treatments of processes in the code, new sample problems will be developed, and testing will be carried out.  Applications of TOUGHREACT to predict mineral dissolution and reaction and related changes to hydrologic properties are undertaken to advance the understanding of CO2 trapping and demonstrate the capabilities of TOUGHREACT to the broader community.  TOUGHREACT is available to the public through the Department of Energy’s National Energy Software Technology Center, whereas TOUGHREACT V2 will be distributed by LBNL.

Subtask 2.3. Hysteresis and Heterogeneity:

In this subtask, we investigate the effects of heterogeneity and length scale (from core to reservoir scale) on the long-term residual gas trapping of CO2.  Relative permeability (the fundamental property that allows for residual phase trapping) in multiphase systems, such as supercritical CO2 and brine, is dependent on the history of fluid-phase occupancy.  For example, porous media that is draining (water exiting, CO2 entering) has a different permeability to water and CO2 than porous media that is imbibing (water entering, CO2 exiting) as occurs when CO2 migrates over long time periods up dip.  TOUGH2 models for this hysteretic effect are implemented, tested, and applied in this subtask.

Subtask 2.4. Near-surface modeling:

CO2 in the shallow subsurface will encounter soil gas that is predominantly air.  For modeling near-surface CO2 leakage and seepage, we have developed a module called EOS7CA where the CA stands for CO2 and Air.  Although we have applied this module extensively to the ZERT shallow-release experiment, we have not developed a user guide or set of test problems to make it accessible to others.  In this subtask, we are enhancing the code, developing test problems, and writing a user guide for TOUGH2/EOS7CA.

Subtask 2 (supplemental). WebGasEOS:

WebGasEOS, a free online application, provides a fast and robust means of computing real gas properties using common cubic equations of state. In addition to basic PVTx relationships, WebGasEOS offers derivative quantities, including fugacity coefficients, viscosities, binary diffusivities, and thermal conductivity. Using the interactive form, you may select an equation of state, specify temperature, pressure, and composition, choose properties of interest, and set convenient units. WebGasEOS will calculate real gas properties for one or a number of T-P points. Results can be generated interactively, or output in a columnar format for export to other applications.

Disclaimer:  WebGasEOS Copyright (c) 2005 by The Regents of the University of California, through Lawrence Berkeley National Laboratory (subject to receipt of any required approvals from the U.S. Department of Energy).  (Berkeley Lab Reference CR-2237)