A method comprising: (a) generating fracture data for a subsurface formation from geomechanical simulation of mechanical properties associated with the subsurface formation; (b) defining a fracture network within a plurality of three-dimensional cells of a three-dimensional model of the subsurface formation using the fracture data, the defining including: generating a plurality of geometric primitives arranged within the plurality of three-dimensional cells of the three-dimensional model;and determining a combined area of the plurality of geometric primitives within at least a subset of the plurality of three-dimensional cells by summing areas of individual geometric primitives within each of the subset of three-dimensional cells, wherein determining the combined area of the plurality of geometric primitives includes determining an area of a first geometric primitive among the plurality of geometric primitives within a first cell in the subset of three-dimensional cells by projecting the first geometric primitive onto each of first, second and third orthogonal planes respectively aligned with faces of the first cell to define respective first, second and third two-dimensional projections and calculating areas of each of the first, second and third two-dimensional projections, and clipping individual geometric primitives that are partially within each of the subset of three-dimensional cells; (c) determining a fracture abundance parameter for the fracture network from the defined fracture network by determining the fracture abundance parameter from the determined combined area of the plurality of geometric primitives; (d) running a fluid flow simulation using the determined fracture abundance parameter to estimate fluid flow through the fracture network; and (e) outputting a result of the fluid flow simulation; and (f) using the result of the fluid flow simulation in performing an oilfield operation. (a) generating fracture data for a subsurface formation from geomechanical simulation of mechanical properties associated with the subsurface formation; (b) defining a fracture network within a plurality of three-dimensional cells of a three-dimensional model of the subsurface formation using the fracture data, the defining including: generating a plurality of geometric primitives arranged within the plurality of three-dimensional cells of the three-dimensional model;and determining a combined area of the plurality of geometric primitives within at least a subset of the plurality of three-dimensional cells by summing areas of individual geometric primitives within each of the subset of three-dimensional cells, wherein determining the combined area of the plurality of geometric primitives includes determining an area of a first geometric primitive among the plurality of geometric primitives within a first cell in the subset of three-dimensional cells by projecting the first geometric primitive onto each of first, second and third orthogonal planes respectively aligned with faces of the first cell to define respective first, second and third two-dimensional projections and calculating areas of each of the first, second and third two-dimensional projections, and clipping individual geometric primitives that are partially within each of the subset of three-dimensional cells; generating a plurality of geometric primitives arranged within the plurality of three-dimensional cells of the three-dimensional model;and determining a combined area of the plurality of geometric primitives within at least a subset of the plurality of three-dimensional cells by summing areas of individual geometric primitives within each of the subset of three-dimensional cells, wherein determining the combined area of the plurality of geometric primitives includes determining an area of a first geometric primitive among the plurality of geometric primitives within a first cell in the subset of three-dimensional cells by projecting the first geometric primitive onto each of first, second and third orthogonal planes respectively aligned with faces of the first cell to define respective first, second and third two-dimensional projections and calculating areas of each of the first, second and third two-dimensional projections, and clipping individual geometric primitives that are partially within each of the subset of three-dimensional cells; (c) determining a fracture abundance parameter for the fracture network from the defined fracture network by determining the fracture abundance parameter from the determined combined area of the plurality of geometric primitives; (d) running a fluid flow simulation using the determined fracture abundance parameter to estimate fluid flow through the fracture network; and (e) outputting a result of the fluid flow simulation; and (f) using the result of the fluid flow simulation in performing an oilfield operation.