Invasion of fluids from the well to the reservoir is a problem encountered in all oil well drilling processes, as it causes a considerable increase in the cost of the operation as well as in the time for its completeness. In an oil well it is common for perforated rock to have porosities, and through them the fluid, which is used both to maintain the structural integrity of the channel and to withdraw the sediments from the perforation, penetrates, and with this there is a considerable loss of the fluid during the process. In this work, numerical simulations are performed to determine the relationship between flow and pressure drop through a slit in order to study the phenomenon of drilling fluid loss from the well to the reservoir. The simulations are performed using the finite volume method and the OpenFOAM software. The results obtained are compared to the values obtained through experiments conducted at PUC-Rio. The simulations were performed for fluids with different mechanical behavior, such as glycerin, which presents a Newtonian behavior and mixtures of drilling fluids with Laponite, CMC and PAM in different concentrations. Since the mixtures in question do not present Newtonian behavior, the Power-Law model will be used to determine the viscosity of the fluid.