3D VORONOI PRE- AND POST- PROCESSING TOOLS FOR THE MODELING OF DEEP SEDIMENTARY FORMATIONS WITH THE TOUGH2 FAMILY OF CODES

Full three-dimensional (3D) unstructured grids offer a great degree of flexibility to perform accurate reservoir numerical simulations. However, when the space discretization is done using the Integral Finite Difference Method (IFDM), the requested orthogonality between the segment connecting the nodes and the blocks interface area complicates the construction of grids with irregular shape blocks. Nevertheless, the full 3D Voronoi approach guarantees the IFDM constraints and allows reproducing geological formations geometry, to follow the shapes of objects such as faults and directional wells, and allows increasing grid resolution in volumes of interest (local grid refinement). Here we present applications of the 3D Voronoi preand post-processing software tools dedicated to the TOUGH family of codes, (developed by the Geothermal research group of the DICAM Department, University of Bologna), to study the migration of non- condensable gases (NCG) in deep sedimentary formations at basin scale. Several algorithms, mainly developed by the scientific community, are already available to calculate the Voronoi tessellation from a given set of seed points. In particular, the voro++ library is a well-known and powerful open source code to carry out 3D computations of the Voronoi tessellation. Based on voro++, VORO2MESH is a new software coded in C++ able to rapidly compute the 3D Voronoi tessellation for a given domain and to create a ready to use TOUGH2 MESH file, up to million blocks. The program can also directly generate the set of Voronoi seed points, using a set of geological surfaces as input. The resulting grid is a mixed grid with regular blocks (orthogonal prisms) and irregular blocks (polyhedron Voronoi blocks) at the contact between different geological formations. The use of regular blocks in regions sufficiently far from the contact surface allows limiting the number of connections. In order to easily inspect the 3D Voronoi discretization and to better visualize the subsequent numerical simulation results, the functionality of the TOUGH2Viewer post-processor has been extended. The software handles the visualization of 3D grids (structured and unstructured), the 3D vector representation of heat and mass fluxes, the iso-surfaces of the simulated variables and 2D contour maps. Different 3D grids of a deep sedimentary formation have been created with VORO2MESH, and the simulation results analysed with TOUGH2Viewer. In particular, the migration of NCG in a large sedimentary formation occupying an area of about 25,000 km2 and with an average thickness of 800 m, extending from -2000 to -7000 m asl, has been simulated using TOUGH2-TMGAS. The use of unstructured grids as compared to the use of structured regular grids has substantially improved both the reproduction of the geological model and the simulation results of the NCG migration.