The Enhanced Geothermal System (EGS) offers a great potential for developing the use of geothermal energy, allowing the creation of hydrothermal reservoirs in deep and hot geological formations, where energy production was impossible due to a lack of fluid or permeability. In order to guaranteee the economic flow rates of the system, this technology allows to improve the permeability of the rock pumping high-pressure fluid from an injection well. With the injection of fluid, the stress equilibrium of the rock formation could change and lead to enhanced seismic activity. Thus making necessary to investigate the physical and chemical role of pore fluid (i.e., water pressure, flow rate water chemistry, and water density and viscosity) on the fault stability and lubrification, in order to optimise the hydraulic stimulation in a EGS. With this purpose,a first serie of experiment has been set up in dry and wet conditions using fluids with different viscosity for direct shear tests and rotary shear tests on precut samples of Westerly granite. Normal stress varied from 0.3 MPa to 10 MPa. Slip velocity was set at ~ 0.008 mm/s. Water and glycerol with different concentrations (0% and 80%) have been used for the wet tests thereby the kinematic viscosity ranges between about 1 mm2/s to 50 mm2/s. Glycerol has been chosen because it is fully miscible with water and is not considered as a lubricant. First results showdifferences in stick-slip motions that occur with the different configurations. The amplitude of stress drops are larger for wet conditions than for dry one, in particular it grows as the viscosity increases. At the same time, the viscosity of the fluid layer negligiblely affects the macroscopic frictional resistence, reducing the mean static friction coefficient of only 10% above the dry condition.