Drill string enabling information to be transmitted

#A drilling installation constituted by drill strings enabling information to be transmitted between the bottom of a borehole being drilled and the surface of the ground is already described in French patent 2 777 594 in the name of the Applicant. As explained in that document, while a borehole is being drilled, it is very important to be able to transmit to the surface information that is collected by sensors mounted in the vicinity of the drilling tool, which tool is fixed to the bottom end of the drill string.

#One of the solutions proposed in the above-mentioned French patent is described below with reference to accompanying FIG. 1.

#FIG. 1 shows a drill string made up of a rod 10a constituting the bottom rod, a rod 10b constituting the top rod, and intermediate rods. A drilling tool 12 is fixed to the bottom end of the rod 10a and has measurement sensors 14. The inside face 16 of the rod 10 is coated in a layer of insulating material 18 over the entire length of the drill string. Furthermore, the drill string is disposed inside a borehole 20 that is being drilled, which borehole is filled with drilling mud 22 that conducts electricity. To enable information to be transmitted, there is a first induction coil 24 disposed close to the bottom end of the rod 10a and connected to the measurement sensor 14. There is also an inductive coupler coil 26 mounted inside the top rod 10b and connected to conductors such as 28 for transmitting electrical signals picked up by the coil 26 to a processor device.

#Other characteristics and advantages of the invention will appear better on reading the following description of various embodiments of the invention given as non-limiting examples. The description refers to the accompanying drawings, in which:

#FIG. 1, described above, is a simplified vertical section view through a drill string fitted with prior art information transmission means;

#FIG. 2 is a simplified longitudinal section view of a drill string fitted with information transmission means constituting a first embodiment of the invention;

#With reference initially to FIG. 2, there follows a description of a first embodiment of the drill string that enables information to be transmitted.

#FIG. 2 is a simplified diagram of a borehole 40 while it is being bored. This borehole contains a drill string and, more particularly, the bottom rod Ti having the drilling tool mounted thereon (not shown in the figure) and the top rod Ts. Intermediate rods are naturally interposed between these two end rods. Each rod is constituted by a conductive metal wall 42 having its inside face covered in an insulating layer 44.

#As already explained, the borehole 40 is filled with a mud that conducts electricity more or less well and that fills not only the inside 46 of the drill string, but also the annular space 48 between the drill string and the wall of the borehole 40. This establishes a closed conductive loop constituted firstly by the mud on the inside 46 and secondly by the conductive wall 42 together with the mud in the outer annular space 48. These two branches of the electrically conductive circuit are separated by the insulating layer 44 and they are interconnected at the bottom by the tool itself and at the top by appropriate means. In this figure, reference 40F designates the borehole and 40S the top end of the borehole close to the surface of the ground S.