Patent GB-1364572-A (GB)
Title: Well logging method and apparatus
IPC: G01V5/10
Filed: 1972-05-05
Abstract: 1364572 Borehole logging TEXACO DEVELOPMENT CORP 5 May 1972 [21 May 1971] 20976/72 Heading G1A In a borehole logger particularly suitable for determining the presence of oil-bearing strata, the earth formation is bombarded with high energy neutrons and the gamma rays produced are detected, the resultant pulses representative of gamma rays produced as a result of inelastic scattering of neutrons by carbon nuclei and by oxygen nuclei respectively and during particular time intervals, being selected and counted in four energy ranges to derive a backgroundcompensated ratio of carbon to oxygen in the formation. In the apparatus of Fig. 1, the logging tool includes a source of neutrons 6, which may be an encapsulated radio-active source e.g. of radium and beryllium, plutonium and beryllium &c. but is preferably a deuterium-tritium accelerator 7 which can be actuated at 8 to produce 14À4 MeV neutrons cyclically in short bursts e.g. of 5-10 micro-seconds during each 1000 micro-seconds (periods 55 in Fig.2). The effect of each neutron burst 55 is to produce a distribution 56 of fast neutrons. There is also a build up of the thermal neutron population 57, but by keeping interval 55 short, the major proportion of neutrons during the irradiation interval 55 will be fast. The fast neutrons interact with the neuclei in the earth formation to produce y-rays of different energies characteristic of the different nuclei. These are detected by a scintillation counter 10 (using a thallium activated NaI or CsI crystal), the output of which may be amplified before delivery to the circuitry at the top of the borehole. Either the detector is switched on and off or, preferably, 'its output is gated, in synchronism with the accelerator 7 actuation (see 58), Fig. 2. This is carried out by timer 23 pulse generator 24 and gate 25. (The detection period 58 may be slightly longer than the irradiation period 55, if required.) Thus the detector output delivered to pulse height analyzer 26 is representative of y-rays resulting from substantially only fast neutron interactions. The pulse height analyzer divides the pulses up into four pulse height ranges, Fig. 3 representative of the contributions due to carbon (C) carbon background (C B  , unwanted pulses due to thermal neutron capture, scattering by housing 5 &c.), oxygen (O) and oxygen background (O B ). From pulse rate counts at 31 and 32-35, various measurements are possible: (a) Direct ratio of carbon-oxygen, obtained by ratioing device 230. (b) Compensated ratio of carbon-oxygen, by ratioing device 30 operating on the counts rate for the carbon and oxygen channels, which have had the respective backgrounds subtracted therefrom, providing a ratio value less dependent on the porosity and composition of the formation and more on oil content. (c) Ratio of C B  to O B , by ratioing device 130, indicating the proportion of limestone as compared to that of sandstone in the formation. (d) The total count rate, obtained at 31, indicating formation porosity. In a modification, a further detection period (58B), Fig. 5 (not shown) is provided, by delaying the timer output at 37 and use of a further gate 39 of the detector output, whereby to utilize the y-spectrum resulting from thermal neutron capture. Two further measurements provided are: (e) Total count rate, by meter 36, providing a further porosity measurement. (f) Ratio of silicon to calcium, by meter 43 operating on count rates due to pulses in silicon and calcium pulse height analyzer channels.