Michael Celia and colleagues continue to develop a suite of models to simulate CO2 injection, migration, and potential leakage, with applications to a number of active or potential injection sites. These models predict pressure buildup in the formation, movement of both CO2 and brine, capillary trapping of CO2, and dissolution of CO2 into the brine phase (that is, solubility trapping).


Multi-scale model development and evaluation

The group’s work in the past year has focused on applications of models to real field sites, on explicit identification and evaluation of differences among a wide range of models with different complexity, and on new numerical algorithms and analytical solutions to enhance model capabilities. Applications include large-scale analysis of the Basal Aquifer of Canada and the northern United States, covering more than a million square kilometers (Figure 2); the top layer of the Utsira formation and CO2 migration associated with the Sleipner injection; and ongoing simulations focused on the Gorgon Project in Australia.

The researchers have written a manuscript (Bandilla et al., 2014) that provides details on the range of models that have been, and can be, used for CO2 injection problems, from fully coupled multi-component multi-phase models to simplified dynamic models to very simple percolation-type models.

Figure 2. Pressure increases after 50 years of injection at nine sites across the Canadian portion of the Basal Aquifer using five different models. a) VESA (Vertical Equilibrium with Sub-scale Analytical method) two-phase numerical model with capillary transition zone; b) VESA with sharp interface; c) single-phase numerical simulator with brine injection; d) ELSA two-phase semi-analytical simulator, e) Theis single-phase analytical solution. From Huang et al., 2014.

This year Celia and colleagues also developed new numerical algorithms for their multi-scale models. One advance maintains much of the computational advantages of vertical equilibrium models while allowing for a relaxation of the equilibrium assumption (see Guo et al. (2014)). They have also developed a new analytical solution for leakage into a leaky fault; this analytical solution can be used to provide local-scale information within coarse grid blocks of larger-scale simulations. Much of their ongoing modeling work is in collaboration with partners at Lawrence Berkeley National Laboratory and the University of Bergen.


Pore-scale and geochemical models

The Celia group continues to study additional aspects of CO2 injection, migration, and leakage, including pore-scale models for geochemically reactive systems, and multi-phase models for trapping and hysteresis.

The geochemical modeling work includes explicit modeling of both precipitation and dissolution with the associated changes in porosity and permeability tracked as the reactions proceed. The researchers also identify conditions under which unique relationships between porosity and permeability should be expected.

Pore-scale models also provide new insights into how nonwetting fluids are trapped in porous media, including underlying mechanisms and the extent to which continuum-scale trapping functions are hysteretic (Figure 3). The hysteretic nature of all multi-phase constitutive functions can be carried from the small scale to the large scale; Doster et al. (2013) shows how this can be done for Vertical Equilibrium models and the conditions under which hysteresis may be important at the large scale.

Figure 3. Hysteretic constitutive relationships based on pore-scale network simulations. The results show the importance of continuous wedges and corners of wetting fluid. Part (a) shows the volume fraction of nonwetting fluid that is disconnected or trapped. Figure from Joekar-Niasar et al. (2013).