An apparatus for studying the migration of a gas in a cement slag comprises a cell 21 filled at least partially with a slag 22 to be used during the cementation of a well pipe. A first strainer 23 made of a sintered material which simulates a formation is disposed below a second strainer 24 made of a sintered material and connected to a gas-detection apparatus, the levels at which the two strainers are positioned being such that they are embedded in the slag being studied. A pressurized gas source 27 is connected to the first strainer, and to a tank 28 partially filled with water 29, the water-gas interface being located at a level lower than that of the first strainer 23. A line 30 is connected between the bottom of the tank and the upper part of the cell, above the upper level of the slag 22, and actual formation conditions can be simulated by suitably adjusting the vertical height H1 between the water-slag interface in the cell and the water-gas interface in the tank.