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In the ECLIPSE output format grids with dual porosity are (to some extent ...) modeled as two independent grids stacked on top of eachother, where the fracture cells have global index in the range [nx*ny*nz, 2*nx*ny*nz). The physical connection between the matrix and the fractures in cell nr c is modelled as an nnc: cell[c] -> cell[c + nx*ny*nz]. In the ert ecl library we only have cells in the range [0,nx*ny*nz), and fracture is a property of a cell, we therefor do not included the nnc connection between matrix and fracture in the same cell in the nnc setup.