Summary The possible influence of the well casing in reservoir-deformation measurements by the radioactive-marker technique (RMT) is investigated. The issue is quite important because RMT data may be used for a most-representative estimate of the in-situ vertical rock compressibility cM (i.e., a basic parameter to predict the land settlement caused by gas-/oilfield development or the land uplift caused by underground fluid injection). A geomechanical finite-element (FE) model is implemented to evaluate the disturbance caused by the stiffness of the steel casing and the surrounding cement on the amount of deformation around the borehole as detected by RMT. The FE model is integrated by a class of elastoplastic interface FEs (IFEs) specifically designed to account for the potential sliding of the different materials (i.e., along the contact surfaces between the steel casing and the cement, and the cement and the exploited formation). The numerical simulations make use of real casing data and geomechanical information from the Northern Adriatic basin, Italy. The results show that sliding is not likely to occur along the contact surfaces and that RMT appears to be a reliable tool for assessing the actual geomechanical properties of the depleted formation at a depth larger than 1000 m, where the in-situ deformation is negligibly affected by the casing stiffness. In shallow softer units, the compaction as measured by RMT is influenced progressively by casing, with a corresponding likely underestimate of cM.