Patent GB-765100-A (GB)
Title: Neutron logging apparatus
Inventor: TITTMAN JAY
IPC: G01V5/10
Filed: 1954-02-11
Abstract: 765,100. Neutron logging apparatus. SCHLUMBERGER WELL SURVEYING CORPORATION. Feb. 11, 1954, No. 4087/54. Class 40 (3). An apparatus for neutron logging of formations traversed by a borehole containing hydrogenous fluid comprises means for displacing the fluid from a zone contiguous with a wall portion of the borehole, means for radiating high energy neutrons through the portion into a formation means for detecting only low energy neutrons passing through the zone from the formation and means for transmitting signals out of the borehole indicative of the flux of the low energy neutrons. Slow neutron detector 22 is housed within a cylindrical shield 23 of slow-neutron absorbing material, e.g. cadmium or boron carbide formed with an opening 24 facing the borehole wall and subtending an angle of about 60 degrees at the centre of the detector. In the opening is disposed a block 26 of material, e.g. A1 transparent to slow neutrons which bears against the borehole wall thereby displacing borehole fluid from in front of the opening. A neutron source, e.g. a Ra-Be pellet is placed in a cavity 25 in the block. Flanges 28, 29 extend along the sides of the block to the borehole wall and plates with extensions registering with the flanges complete the casing. Resilient flaps 33, 34 of material opaque to slow neutrons extend along the borehole walls. They are made, e.g. of spring steel backed with Cd or of B4 C incorporated in polytetrafluorethylene polymer. - The assembly is forced against the borehole wall by springs 14, 15 mounted on a tubular casing 13 movable vertically in the bore by cable 21 through which the detector output is communicated to the surface. The casing 13 is centralized in the borehole by a pad 19, mounted on similar springs 15, 17 opposite the probe, and which may carry electrodes for simultaneous electrical logging. Fast neutrons from the source are slowed by collisions with nucleii of atoms of low mass, particularly hydrogen, and return through the A1 block to the detector. The number slowed down depends on the nature of the formations forming the borehole wall the number being much greater when the formation is porous and therefore containing oil or water. In earlier arrangements a large percentage of slowed neutrons reaching the detector were slowed wholly or in part by hydrogenous material in the borehole rather than by adjacent formations so masking the effect attributable to differences in the formations. The block 26, shield 23 and flexible extensions 33, 34 thereof reduce the number of neutrons slowed down partly or wholly in the bore which reach the detector relative to the number slowed down by the formations so that the count rate from the detector more sharply indicates changes in the porosity of formations forming the borehole wall. When a BF3 filled detector is used it is operated in the ionization chamber or proportional region to avoid interference from gammaradiation. In these regions gamma-rays give smaller pulses than the neutrons, which may be clipped. When a detector operating in the Geiger region is used parts of the detector exposed to spurous gamma-radiation must be screened, e.g. by lining with lead. An arrangement for measuring the dip of strata traversed by a borehole comprises three assemblies of the type described arranged as shown in Fig. 9. The dip is indicated by the relative displacement of corresponding changes in the logs recorded from each of assemblies 18, 18<1>, 18<11> as the apparatus is raised through the borehole. The assembly shown in Fig. 8 (not shown), differs from that described above in that the aperture does not extend the full length of the shield. This arrangement gives a greater difference in countrate between porous and non-porous formations.