Electromagnetic logging apparatus.

The present invention relates to electromagnetic logging devices for measuring the electrical conductivity and / or the dielectric constant of geological formations traversed by a borehole.

This category includes measures electrical conductivity of formations traversed by a hole-drilling electromagnetic induction. Embodiments of methods and induction logging tools are, for example, described in United States Patent No. 2,582,314 filed by HC Doll .. A transmitting coil mounted on a probe is excited by an oscillator a frequency of the order of 20 kHz, for example, for inducing currents in the surrounding geological formation. The importance of these currents depends on the conductivity of formations- in which they originate. They circulate in substantially circular lines centered on the axis of the borehole and cause themselves the appearance of an electromotive force in one or more receiving coils mounted on the logging sonde at determined distances from the transmitter coil. The analysis of parameters of the output signal of the receiving coils relative to the transmitted signal provides information on the conductivity of formations traversed by these currents.

More recently, it has been proposed (US Patent 4,185,238) tools to measure certain characteristics of circles surrounding a drill involving the propagation of electromagnetic energy in these environments at frequencies significantly higher than the frequency used to perform induction logs. In these techniques, radio frequencies are used in an area that can range from a low frequency of approximately 1 megahertz to one gigahertz about. The measurements allow. determine both the conductivity and the dielectric constant of geological formations.

It has long been recognized that in electromagnetic logging tools, the sealed envelope in contact with the drilling fluid must be non-magnetic material and not electrically conductive (US Patent 2,857,451; see also application EPO 0 035 421), in order not to disturb the measurements. At the same time as the enclosure has to withstand the fluid pressure of the drilling and the forces s'exercent during movement of the tool in the borehole, its realization has been difficult.

Furthermore, in the case of high frequency devices, the insulating jacket forms with the inner metal tube (for antenna and serving for the passage of the conductors) with the conductive drilling fluid, a structure similar to a coaxial cable, which allows the transverse spurious mode TEM to spread almost without losses of the issuer (s) receptor (s). As the resulting interference signals have the same frequency and the same phase as the useful signals, extraction of these is an extremely difficult problem.

The problem is to design an electromagnetic logging device which does not have the disadvantages mentioned above.

This problem is solved with the device according to claim 1.

Other aspects and advantages of the invention will appear on reading the following description given by way of example, with reference to the accompanying drawings, in which:<ul><li>1 shows a logging sonde equipped with antennas on mandrel according to one embodiment of the invention;</li><li>Figure 2 is an enlarged view of a detail of Figure 2 cut by a longitudinal diametral plane.</li></ul>

According to one embodiment, a probe 200 (Figure 1) comprises a cylindrical core surrounded by a metallic outer envelope. Around this casing are arranged in a plurality of longitudinally spaced locations 204, the biplates antennas 206 of the coil type described in the parent application No. 0,051,018, Figure 6. As shown the upper part of Figure 1 shows that the probe partially intersected by a longitudinal diametral plane, the metal outer shell 202 has, at each location 204, a portion 208 of narrowed external diameter which constitutes by itself a cylindrical mass member for each antenna 206. each of these narrowed portions 208 is coated with a dielectric sleeve 209 around which is wound a helical metal blade 210 whose one end is short-circuited with the narrowed metal portion 208 of the conductive frame 202. the combined thickness of the dielectric 209 and the winding 210 is such that the diameter of the assembly is less than the diameter of the casing 202 in portions 212 which separate the slots 204. These portions 212 are interconnected by a series of bars 214-longitudinal direction through over each winding 210 which thus form a cage parallel bars around the antennas 206 for the purpose of mechanical protection. The longitudinal bars 214 are integral with the housing 202. In this embodiment, the cylindrical mass element antenna is in direct electrical contact with the mud spread 202. No TEM mode can not take place in the absence of a coaxial type of structure with dielectric between an inner conductor and the column of mud surrounding the tool.

According to another embodiment (Figure 2a), the envelope of a probe 220 is constituted by a cylindrical metallic cylindrical tube 222 extending over the entire height of the tool. Around the casing 222 are mounted in longitudinally spaced positions, a transmitting antenna 224 to the lower part of the tool and a set of receiving antennas 226₁, 226₂, 226₃ and 226₄. Each of the antennas 224; 226-226₄ comprises a dielectric coating 229 attached directly around the surface of external. the envelope 222, which forms a cylindrical element of common ground with all these antennas. Around each dielectric sleeve 229 (Figure 2b) is a helically wound metal strip radiant 230 electrically connected to the casing 222 at one of its ends 231. The coil 230 is embedded in an insulating coating 232 made of fiberglass which gives it protection against both mechanical shock and abrasion due to movement of the tool within the drilling and chemical against corrosion.

A coaxial cable 234 to the antenna feed 224 passes through the casing 222. The sheath is electrically connected to this envelope. The core 235 is connected as described above to achieve impedance matching.

If the protection offered by the coating 232 is effective mechanical and chemical point of view, it is not necessary from an electrical point of view. No propagation TEM mode is to be feared.

Internal electronics necessary for operation of tools is housed (cut portions - of Figures 1 and 2a) in the inner space delimited by the conductive tubes 202 and 222 at their upper part. Supports 240 mounted electronic card own treatment to be connected to pairs of receivers 226₁, 226₂ and 226₃, 226₄ via coaxial cables 241, 243, 245 and 246.

The creation of electronic circuits and connections suitable for powering the transmitter 224 and to allow processing of signals from receivers 226₁ 226₄Is well known and, for example, described in US Patent 4,185,238 cited 22 January 1980 issued to Messrs Huchital and Tabanou.

Thus, it was realized a logging sonde by propagation of electromagnetic waves comprising a metallic envelope which has, besides the suppression of the propagation of waves in the TEM mode, a number of advantages for carrying tools, both plane of the robustness of the ease of assembly and therefore the manufacturing cost. They can in particular minimize or take into account the distance changes of the coils along the mandrel under the effect of thermal expansion.