Abstract: The centralizer comprises a main assembly suitable for centering a well tool in casing, and a removable adapter assembly (75) for use in the open hole portion of a borehole. The main assembly comprises a body (30), longitudinally spaced-apart supports (36, 41) slidably mounted on the body, and pairs of hinged arms (45.sub.1, 47.sub.1) disposed between the slidable supports. The central hinges (50.sub.1) of the pairs of arms carry wheels (51.sub.1) and are spaced apart laterally from the body under the effect of blade springs (52.sub.1, 53.sub.1). The adapter assembly (75) comprises curved spring blades (80.sub.1) hinged on link collars (77, 82) removably fixed on the sliding supports. The wheels are laterally spaced apart from the spring blades. The centralizer is easily modified to operate either in the case hole portion or in the open hole portion of a borehole.