The present work focuses on the determination of absolute permeability in a hydrocarbon reservoir whose pressure is under the bubble point. The simulation of the homogenic reservoir and well with the simultaneous ow of oil and gas is performed using the Eclipse100 software. The main purpose of the simulation is to obtain pressure build-up tests for two simulated reservoir systems, since the well testing technology requires the shut-in period, which increases the costs of the real well production cycle. In the rst simulation it is presumed that the well is not damaged, having the skin e ect equal to zero, while in the second one, the skin e ect of the well in the same reservoir is 9.44. The aim is to compare the results of the pressure build-up test analysis for both cases. In each case, permeability is rstly calculated using four methods, given by di erent authors. The theoretical background and the process of obtaining the required results are given for each approach. Results are compared with the permeability as determined by the pressure transient analysis, using the Saphir software, for both the ideal and the damaged well. Di erences in results are presented and discussed. Conclusions could be applied to similar real reservoir systems with the multiphase uid ow in the porous media.