The limited durability of the cement in acidic fluids is one of the main issues controlling the lifetime of a geothermal well. The most critical region is the outermost layer of the cement adjacent to the formation. This paper provides insight into the way in which well cementing formulations interact with a common rock formation in geothermal environments, at 150 degrees C in brine, with and without CO2 exposure. The results show that chemical reactions occur in the ITZ, which are driven by Ca2+ and OH- migration from the cement into the rock, which are dependent on the type and amount of silica added to the cement. The presence of CO2 influences the extent of cement/rock interaction which is dependent on the cement formulation. The current work indicates that the carbonation in these systems is a very complex set of reactions, where poorly crystalline and amorphous calcium carbonates may form.