In the present thesis work, it was studied the accuracy of five thermodynamic models, i. e., the phi-phi model for multicomponent vapor-liquid equilibrium, and the van der Waals, Redlich-Kwong, Soave-Redlich-Kwong, and Peng-Robinson cubic equations of state, for predicting the composition of fluids coming from hydrocarbon reservoirs and fed to a flash separator in the well production stage of the petroleum and natural gas extraction industry. To achieve this, a computer program was developed, FlaSeHF, which was written in the C and C++ programming languages, and these cubic equations of state were implemented to describe the behavior of the hydrocarbon reservoir fluids liquid and gas phases being in equilibrium. Due to the fact that such fluids are hydrocarbon mixtures, it was necessary to wok with pseudocomponents for modeling the heavy fractions. In order to describe these pseudocompon...