Method for enhanced hydrocarbon recovery in an underground reservoir rock, comprising the injection of an aqueous conditioning solution comprising an alkaline agent, in which the following steps are carried out: - the in situ effects of said injection of said aqueous conditioning solution comprising said alkaline agent into an underground reservoir rock are determined by means of a simulation of flows on a model of the reservoir discretized into mesh cells, said in situ effects comprising the pH value, the interfacial tension, the viscosity and displacement parameters, • said simulation taking into account the mesh-cell to mesh-cell transport of the alkaline agent as principal agent, without taking into account the mesh-cell to mesh-cell transport of the "intermediate" species which result from the injection of the principal agent in aqueous solution, and • the concentrations of said intermediate species are determined analytically in each of the mesh cells, from the dissociation, adsorption on rock and, where appropriate, precipitation and/or reaction equilibria, - the injection conditions and the physicochemical characteristics of said aqueous conditioning solution are deduced from said simulation and from said concentrations determined analytically, said injection conditions comprising the volumes and concentrations of the products to be injected and the injection modes, such as the flow rate and the injection scheme, - said aqueous conditioning solution comprising said alkaline agent is injected into said underground reservoir rock according to said injection conditions and said physicochemical characteristics deduced, in order to perform the enhanced hydrocarbon recovery. - the in situ effects of said injection of said aqueous conditioning solution comprising said alkaline agent into an underground reservoir rock are determined by means of a simulation of flows on a model of the reservoir discretized into mesh cells, said in situ effects comprising the pH value, the interfacial tension, the viscosity and displacement parameters, • said simulation taking into account the mesh-cell to mesh-cell transport of the alkaline agent as principal agent, without taking into account the mesh-cell to mesh-cell transport of the "intermediate" species which result from the injection of the principal agent in aqueous solution, and • the concentrations of said intermediate species are determined analytically in each of the mesh cells, from the dissociation, adsorption on rock and, where appropriate, precipitation and/or reaction equilibria, • said simulation taking into account the mesh-cell to mesh-cell transport of the alkaline agent as principal agent, without taking into account the mesh-cell to mesh-cell transport of the "intermediate" species which result from the injection of the principal agent in aqueous solution, and • the concentrations of said intermediate species are determined analytically in each of the mesh cells, from the dissociation, adsorption on rock and, where appropriate, precipitation and/or reaction equilibria, - the injection conditions and the physicochemical characteristics of said aqueous conditioning solution are deduced from said simulation and from said concentrations determined analytically, said injection conditions comprising the volumes and concentrations of the products to be injected and the injection modes, such as the flow rate and the injection scheme, - said aqueous conditioning solution comprising said alkaline agent is injected into said underground reservoir rock according to said injection conditions and said physicochemical characteristics deduced, in order to perform the enhanced hydrocarbon recovery.