  The physical mechanism of CO2 trapping in porous media by capillary trapping
(pore scale) incorporates a number of related processes, i.e. residual
trapping, trapping due to hysteresis of the relative permeability, and trapping
due to hysteresis of the capillary pressure. Additionally CO2 may be trapped in
heterogeneous media due to difference in capillary pressure entry points for
different materials (facies scale). The amount of CO2 trapped by these
processes depends upon a complex system of non-linear and hysteretic
relationships including how relative permeability and capillary pressure vary
with brine and CO2 saturation, and upon the spatial variation in these
relationships as caused by geologic heterogeneity. Geological heterogeneities
affect the dynamics of CO2 plumes in subsurface environments. Recent studies
have led to new conceptual and quantitative models for sedimentary architecture
in fluvial deposits over a range of scales that are relevant to the performance
of some deep saline reservoirs. We investigated how the dynamics of a CO2
plume, during and after injection, is influenced by the hierarchical and
multi-scale stratal architecture in such reservoirs. The results strongly
suggest that representing small scales features (decimeter to meter), including
their organization within a hierarchy of larger-scale features, is critical to
understanding trapping processes.
