Computer-implemented method for optimizing the exploitation of a reservoir of fluid traversed by a fracture network, wherein said reservoir is discretized into a set of mesh cells, an image representative of the fracture network delimiting porous blocks and fractures being associated with each of said mesh cells on the basis of direct and indirect observations of the reservoir, the method comprising the following steps: - a transient exchange flux occurring between the porous blocks and the fractures is determined, in each mesh cell, by means of processing of said image representative of the fracture network, based on estimation of a relationship ( A, V) representing, for a given mesh cell, an advance of an exchange front within said porous blocks, as a function of a distance of advance of said front, and by way of the following steps: • at a time t, the distance of advance X(t) of said front is computed from results of simulations of fluid flows at the previous time t - 1, and said relationship ( A, V) representing the advance of an exchange front is computed, • a transient geological form factor I A ( X ( t)), I V ( X ( t)) is computed depending on the distance of advance of said front and on said relationship representing the advance of the front, said transient geological form factor I A ( X ( t)), I V ( X ( t)) corresponding to the geometric coefficient C divided by the unit area or volume of porous rock, said geometric coefficient C being the ratio between the exchange area A or the exchange volume V and the distance between on the one hand the fractures and on the other hand the porous blocks at the scale of the mesh cell, the exchange area A or the exchange volume V being dependent on said distance of advance X(t) of said front; • said transient exchange flux is computed using said form factor, - fluid flows in said reservoir are simulated by means of a flow simulator and of said exchange flux, and - the exploitation of the reservoir is optimized using said simulation of the fluid flows. - a transient exchange flux occurring between the porous blocks and the fractures is determined, in each mesh cell, by means of processing of said image representative of the fracture network, based on estimation of a relationship ( A, V) representing, for a given mesh cell, an advance of an exchange front within said porous blocks, as a function of a distance of advance of said front, and by way of the following steps: • at a time t, the distance of advance X(t) of said front is computed from results of simulations of fluid flows at the previous time t - 1, and said relationship ( A, V) representing the advance of an exchange front is computed, • a transient geological form factor I A ( X ( t)), I V ( X ( t)) is computed depending on the distance of advance of said front and on said relationship representing the advance of the front, said transient geological form factor I A ( X ( t)), I V ( X ( t)) corresponding to the geometric coefficient C divided by the unit area or volume of porous rock, said geometric coefficient C being the ratio between the exchange area A or the exchange volume V and the distance between on the one hand the fractures and on the other hand the porous blocks at the scale of the mesh cell, the exchange area A or the exchange volume V being dependent on said distance of advance X(t) of said front; • said transient exchange flux is computed using said form factor, • at a time t, the distance of advance X(t) of said front is computed from results of simulations of fluid flows at the previous time t - 1, and said relationship ( A, V) representing the advance of an exchange front is computed, • a transient geological form factor I A ( X ( t)), I V ( X ( t)) is computed depending on the distance of advance of said front and on said relationship representing the advance of the front, said transient geological form factor I A ( X ( t)), I V ( X ( t)) corresponding to the geometric coefficient C divided by the unit area or volume of porous rock, said geometric coefficient C being the ratio between the exchange area A or the exchange volume V and the distance between on the one hand the fractures and on the other hand the porous blocks at the scale of the mesh cell, the exchange area A or the exchange volume V being dependent on said distance of advance X(t) of said front; • said transient exchange flux is computed using said form factor, - fluid flows in said reservoir are simulated by means of a flow simulator and of said exchange flux, and - the exploitation of the reservoir is optimized using said simulation of the fluid flows.