Durability of materials used for the completion of oilwell is of utmost importance for oil and gas industry. We carried out ageing tests on a hardened cement paste in two types of fluid by varying the experimental procedure. We show that the observed alterations are highly dependent on the way of conducting the tests. To correctly assess the long-term durability of cement-based materials, it is necessary to renew periodically the ageing fluid. By doing so, a severe impairment of the macroscopic properties of an hardened cement paste aged in a monthly-replaced brine can be observed.