1. A method for exploiting a geological reservoir according to an exploitation scheme defined by a reservoir model, the reservoir model comprising a grid associated with at least one parameter of the reservoir, in which static data are measured within the reservoir and dynamic data are acquired during exploitation, comprising: a) constructing the reservoir model by: i) providing a first initial reservoir model, including a first grid comprising a number N 1 of mesh cells and at least one second initial reservoir model including a second grid comprising a number N 2 of mesh cells, with N 2 being greater than N 1 , generated stochastically according to the static data, the stochastic generation of the second model being conditional on the generation of the first model; ii) scaling the second model to a simulation reservoir model which is usable by a flow simulator; iii) determining an objective function measuring a deviation between the acquired dynamic data and dynamic data simulated by the simulation reservoir model and a flow simulator; iv) reiterating the steps ii) and iii) to minimize the objective function after modification of at least one of the first and second reservoir models and by applying the modification to an unmodified model of the first and second reservoir models; b) determining an optimum exploitation scheme for the reservoir by simulating the exploitation of the reservoir by the second reservoir model and the flow simulator; and c) exploiting the reservoir by implementing the optimum exploitation scheme.