Shale-drilling fluid interaction problems are generally associated with the exchange of ions between shale pore fluid and the drilling mud, being this change caused by the chemical gradient that holds between both. These problems are treated in general by using chemical products, so that the hydration and the expansion of the shales can be inhibited (Bol, 1986). Among these products, we found saline solutions (Steiger, 1976),which attempt to reduce the ionic migration by means of chemical balance between rocks and fluids.The objective of this work is to develop a methodology that allows measuring the variation of salt concentration, by using optic fiber sensors, in order to allow concentrations to be assessed without the withdrawal of the pressures applied into the sample, so that the variation of the concentration during the assays of diffusion and advection can be measured in real time, during the entire assay. The variation of the concentration with time is of fundamental importance for the attainment of the coefficients of ionic transport that occur during the assays with the shales and the sandstone. Assays in a sample of shale from Campos basin, as well as one sample of sandstone from Botucatu, have been carried out. Through the development of a methodology of assays specific for analyzing the interaction rock-fluid, it was possible to obtain resuls, which will allow to foresee instabilities generated in the wells during the digging process.