Method for exploiting a fluid within an underground formation comprising at least one layer passed through by a network of fractures and by at least one well, in which a meshed representation of said at least one layer is constructed from measurements of properties relating to said at least one layer, and at least for said layer, a zone of interest is defined combining a set of meshes of said meshed representation relating to said layer and comprising at least said well, characterized in that at least the following steps are carried out for each of said layers: A. from said measurements, said network of fractures is characterized by statistical parameters, and, from said parameters, a discrete fracture network model is constructed; B. for each of said wells contained in said zone of interest, at least three simplification zones (ZS1, ZS2, ZS3) are defined, a first simplification zone containing said well, a second simplification zone having as its internal boundary the external boundary of said first zone, and a third simplification zone having as its internal boundary the external boundary of said second zone, the set of said simplification zones of the set of said wells covering the set of the meshes of said zone of interest; C. from at least said statistical parameters, for at least each of said simplification zones (ZS1, ZS2, ZS3) of said zone of interest, there are determined an equivalent permeability tensor and a value of a parameter P characterizing the orientation and the continuity to the other layers of the fractures of said network; D. a simplification of said model is determined in each of said simplification zones, as a function of said zone, of said equivalent permeability tensor and of said value of said parameter P, said simplification consisting in the computation of at least one spacing value between said fractures, an opening of said fractures, and a permeability value; and in that, from said meshed representation, of said simplified fracture network model, and from a flow simulator, a scheme is defined for optimally exploiting said fluid of said formation and said fluid of said formation is exploited according to said optimal exploitation scheme. A. from said measurements, said network of fractures is characterized by statistical parameters, and, from said parameters, a discrete fracture network model is constructed; B. for each of said wells contained in said zone of interest, at least three simplification zones (ZS1, ZS2, ZS3) are defined, a first simplification zone containing said well, a second simplification zone having as its internal boundary the external boundary of said first zone, and a third simplification zone having as its internal boundary the external boundary of said second zone, the set of said simplification zones of the set of said wells covering the set of the meshes of said zone of interest; C. from at least said statistical parameters, for at least each of said simplification zones (ZS1, ZS2, ZS3) of said zone of interest, there are determined an equivalent permeability tensor and a value of a parameter P characterizing the orientation and the continuity to the other layers of the fractures of said network; D. a simplification of said model is determined in each of said simplification zones, as a function of said zone, of said equivalent permeability tensor and of said value of said parameter P, said simplification consisting in the computation of at least one spacing value between said fractures, an opening of said fractures, and a permeability value;