1. A method for optimizing developments of an underground medium containing a fluid according to a development scheme utilizing petrophysical measurements acquired relative to the medium and/or the fluid comprising the steps: a) constructing a geological model with a programmed computer having a first grid having a set of cells discretizing the medium with a value of at least one petrophysical property being assigned to each cell from a geostatistical simulation and from the measurements; b) constructing by upscaling the geological model a reservoir model having a second grid including a set of cells discretizing the medium with each cell of the second grid containing a set of cells of the first grid; c) identifying for each cell of the second grid, cells of the first grid that are contained in the cell of the second grid by use of the coordinates of the cells of the first and second grids, defining a geometrical link between the identified cells of the first grid and the cell of the second grid and storing the links to reduce subsequent computer computation time; d) performing flow simulations with a programmed computer using a reservoir simulator, the reservoir model, and the links; e) repeating steps a), b) and d) without repeating c), by modifying parameters of at least the development scheme, the geostatistical simulation, the upscaling, and the flow simulation, to optimize the development scheme; and f) drilling wells according to the optimized development scheme for producing the fluid from the underground medium.