pads and correct orientation of the barrel in relation to the axis of the well before the barrel performs its longitudinal motion in order to displace the pads orientation of the pads in relation to the vertical is achieved by means of a sufficient ballast 15 arranged for example along a generatrix of the outer shell and running on from one of pads 3 when circulation of the drilling fluid is stopped all the pads are folded by means of their return systems the assembly consisting of barrel 8 and annular jack 9 is in a resting position by means of spring return system 13 in the case where the well is inclined thanks to the gravity force the ballast goes into a lowered position in relation to the borehole the friction of the seals and of the roller bearings must be sufficiently reduced to allow rotation of body 2 about shaft 1 under the action of the ballast two pads are thus oriented in a vertical plane and the two others in an inclined plane perpendicular to this vertical plane the pads being thus positioned in relation to the vertical the position of the barrel in relation to body 2.