Investigation of gridding effects for numerical simulations of CO2 geologic sequestration

Potential errors caused by grid shape and resolution are investigated for numerical simulations of CO2 geologic sequestration. The spatial orientation of finite difference grids can strongly influence the calculated shapes of CO2 fronts due to so-called “grid orientation effect”. A coarse vertical discretization of a reservoir can impede gravity override (i.e., less-dense CO2 flows over denser groundwater) of CO2 plumes, resulting in underestimation of the maximum plume size. It is known that injection of CO2 into a saline aquifer may cause formation dry-out and precipitation of solid salt near the injection well, which may reduce porosity and permeability of the aquifer. Numerical simulation of salt precipitation may require very fine grid size near the injection well, because dry-out would be greatly underestimated in a large grid block containing a large amount of water. In this study, these gridding effects are demonstrated using one-dimensional and two-dimensional idealized models as well as a three-dimensional field-scale simulation model of a large-volume CO2 injection in a saline formation in California's Central Valley. For the field-scale modeling, we generated a high-resolution grid model utilizing Voronoi tessellation. To solve the high-resolution model efficiently TOUGH-MP, a parallelized version of general purpose multi-phase flow simulator TOUGH2, was used. Our results indicate that (1) the use of higher-order Voronoi tessellation significantly reduces the “grid-orientation effects”; (2) coarse grids considerably underestimate gravity override, and thus the maximum lateral extent of a CO2 plume is also underestimated to a few tens of percent; (3) a fine gridding in the vicinity of the injection well may be needed to simulate near-well phenomena accurately, especially when the capillary-driven backflow to the well is significant.