Trapping mechanisms

In general there are four basic mechanisms that will prevent the CO₂ from leaking back to the atmosphere:

• Structural/stratigraphical trapping - as the CO₂ plume migrates upward, it may encounter a local maximum (structural trap) in the top surface created by faulting or folding of the caprock. The CO₂ will start to accumulate inside this structural trap and gradually push the interface between CO₂ and brine deeper and deeper until this interface touches a local minimum of the trap, at which the CO₂ can 'spill over' and continue its upward migration. CO₂ accumulated inside structural traps will remain permanently trapped unless the height of the plume is so high that it creates a capillary presure that enables the CO₂ to enter the top seal. CO₂ can also be trapped by changes in the rock character, which is referred to as stratigraphical trapping.
• Residual trapping - when the concentration of CO₂ at the tail of the migrating plume falls below a certain level, CO₂ becomes trapped by capillary pressure from the water in void space between rock grains and stops migrating.
• Solubility trapping - the injected CO₂ will dissolve in water. This increases the density of the water, which starts to sink downwards, thereby enabling a mixing process that will enable the CO₂ to become more dispersed in the resident brine over time
• Mineral trapping - the dissolved CO₂ will react with the reservoir rock to form and precipitate carbonate minerals.

These processes will take place across a wide span of time scales. MRST-co2lab focusses primarily on the first three mechanisms, which are often assumed to take place on a somewhat shorter time scale than mineral trapping.

Upper bounds on the structural trapping can be determined at a low computational cost by geometrical and topological algorithms that only utilize a description of the surface that separates the aquifer from the overlying caprock. For more details, consult our discussion of structural trapping and spill-point analysis.

Modeling injection and migration

To also describe residual and solubility trapping, one generally has to solve a coupled system of partial differential equations that describe the conservation of the fluid phases and chemical components that are present in the aquifer. In their simplest form, these equations describe a system of two fluid phases, supercritical CO₂ and brine. How these fluid phases flow together and interact during the injection and migration stages is governed by delicate balances of a number of physical mechanisms (see the Figure 1). The balances may change with time and spatial location, and resolving the long-term CO₂ migration is a challenging multiscale problem that involves a large disparity in spatial and temporal scales.

Figure 1. Illustration of injection and trapping of CO2 under a sealing caprock. The plots show forces active during the injection (left) and migration phase (right).

Vertical equilibrium models

CO₂ is very mobile and can travel large distances, but the flow is usually confined to thin layers underneath a sealing caprock or other low-permeable vertical barriers. A typical saline aquifer considered for CO₂ storage can be viewed as a thin, slightly inclined sheet that spans thousands of square kilometers. This, in combination with differences in density between the supercritical CO₂ plume and the resident brine, means that the vertical fluid segregation will be almost instantaneous compared with the up-dip migration. The tendency of forming a relative flat fluid interface is an effect of the pressure distribution, which in turn depends strongly on the flow in the vertical direction, particularly near the interface. The vertical fluid distribution must also be accurately represented to avoid introducing large errors in the forecast of the updip migration.  The thin plume and sharp transition between CO₂ and brine means that high vertical resolution is required to compute the vertical phase distribution.

In MRST-co2lab, the primary assumption is that the flow system is in vertical equilibrium so that the vertical distribution of fluid phases can be determined from analytical expressions. One can now integrate the flow equations vertically to obtain a reduced model. This not only reduces the number of spatial dimensions, and hence the required number of grid cells, but will also lessens the coupling between pressure and fluid transport and improves the characteristic time constants of the problem. As a results, so-called vertical-equilibrium simulators will typically be orders of magnitude faster and consume signficantly less memory than conventional 3D simulators (which are also available in MRST). Using these vertical-equilbrium models, it is easy to simulate pressure buildup and plume migration, and provide reliable forecasts of the various trapping mechanisms, e.g., as illustrated in Figure 2.

You can read more about VE models here or in one of our papers.

Figure 2. Schematic illustration of how a vertical-equilibrium model is used to describe the migration and resulting trapping of CO2 under a sloping caprock.