1. A computer-implemented method for modeling three-dimensional (3D) geological fractures, the method comprising: receiving a set of 3D fractures with geometry that has been discretized by 2D surfaces by a collection of polygonal facets; generating a set of 3D stadia at specified radii around each facet in the collection of polygonal facets; generating closed 3D stadia surfaces around intersecting facets; discretizing all 3D stadia surfaces using various shape cells; and generating 3D cells within each of the closed 3D stadia surfaces.