Method implemented by computer for simulating flows of a fluid in an underground formation comprising a network of cracks in order to exploit said fluid of said underground formation, wherein, based on measurements of properties relating to said formation, a meshed representation of said formation is constructed and at least statistical parameters relating to said network of cracks are determined, characterized in that said method comprises at least the following steps: A) based on at least said statistical parameters relating to said network of cracks, a model of porosity of said underground formation comprising said network of cracks is constructed, said model of porosity comprising a first medium representative of a non-cracked matrix of said formation, a second medium representative of said cracks of said formation oriented in a first direction, and a third medium representative of said cracks of said formation oriented in a second direction, said first and second directions being orthogonal to one another; B) based on at least said measurements of properties relating to said formation, on statistical parameters relating to said network of cracks, and on said model of porosity, flow parameters in each of the meshes of said meshed representation are determined, said flow parameters comprising at least: - between two neighbouring meshes in said second direction, a transmissibility of convection and a transmissibility of conduction of said fluid in said first and second directions in said second medium, said transmissibilities of convection and of conduction of said fluid being zero in said second direction in said second medium, - between two neighbouring meshes in said first direction, a transmissibility of convection and a transmissibility of conduction of said fluid in said first and second directions in said third medium, said transmissibilities of convection and of conduction of said fluid being zero in said first direction in said third medium, - within a mesh of said meshed representation, exchanges of weight by convection, exchanges of energy by convection and exchanges of energy by conduction between said second and third media, between said first and second media, and between said first medium and said third medium, C) based on said meshed representation and on said flow parameters in each of the meshes of said meshed representation, said flows of said fluid in said underground formation comprising said network of cracks are simulated by means of a flow simulator implementing said model of porosity. A) based on at least said statistical parameters relating to said network of cracks, a model of porosity of said underground formation comprising said network of cracks is constructed, said model of porosity comprising a first medium representative of a non-cracked matrix of said formation, a second medium representative of said cracks of said formation oriented in a first direction, and a third medium representative of said cracks of said formation oriented in a second direction, said first and second directions being orthogonal to one another; B) based on at least said measurements of properties relating to said formation, on statistical parameters relating to said network of cracks, and on said model of porosity, flow parameters in each of the meshes of said meshed representation are determined, said flow parameters comprising at least: - between two neighbouring meshes in said second direction, a transmissibility of convection and a transmissibility of conduction of said fluid in said first and second directions in said second medium, said transmissibilities of convection and of conduction of said fluid being zero in said second direction in said second medium, - between two neighbouring meshes in said first direction, a transmissibility of convection and a transmissibility of conduction of said fluid in said first and second directions in said third medium, said transmissibilities of convection and of conduction of said fluid being zero in said first direction in said third medium, - within a mesh of said meshed representation, exchanges of weight by convection, exchanges of energy by convection and exchanges of energy by conduction between said second and third media, between said first and second media, and between said first medium and said third medium, - between two neighbouring meshes in said second direction, a transmissibility of convection and a transmissibility of conduction of said fluid in said first and second directions in said second medium, said transmissibilities of convection and of conduction of said fluid being zero in said second direction in said second medium, - between two neighbouring meshes in said first direction, a transmissibility of convection and a transmissibility of conduction of said fluid in said first and second directions in said third medium, said transmissibilities of convection and of conduction of said fluid being zero in said first direction in said third medium, - within a mesh of said meshed representation, exchanges of weight by convection, exchanges of energy by convection and exchanges of energy by conduction between said second and third media, between said first and second media, and between said first medium and said third medium, C) based on said meshed representation and on said flow parameters in each of the meshes of said meshed representation, said flows of said fluid in said underground formation comprising said network of cracks are simulated by means of a flow simulator implementing said model of porosity.