GB1424023A - Methods and apparatus for measuring the density of geological formations - Google Patents

Abstract

1424023 Borehole logging SCHLUMBERGER INLAND SERVICES Inc 24 Jan 1973 [24 Jan 1972 11 Dec 1972] 3603/73 Heading G1A In borehole logging using a gamma-ray tool to measure the density of formations traversed by the borehole, and in which only pulses above a normal count threshold S are counted, the effects of barite contained in the mud cake of the borehole in deforming the amplitude spectrum of the detector pulses, are compensated for by modifying the threshold or substituting it by a higher value Amo. The threshold S is that above which the variation of the amplitude spectrum with density is regular, Fig. 1 (not shown). Barite is introduced into the borehole mud to adjust its density during drilling, but this produces logging measurement errors by deformation of the amplitude spectrum (dashed line, Fig. 6). Three basic circuits are described to compensate for this deformation. In the arrangement of Fig. 3, a scintillation counter 10, 11 senses gamma rays diffused by the formation 13 and mud cake 17. After amplification at 14, the detector pulses are applied to circuit 15 which corrects for spectrum deformation before the pulses are used to calculate density at 16. Pulses above threshold S are passed by comparator 21 and trigger monostable 22 which opens gate 19, so as to pass the original pulses, delayed at 18, to integrating amplifier 26. In opposition to pulses at amplifier 26 are applied reference amplitude Amo pulses from circuit 25. The output of 26 represents the deviation between the mean amplitude of pulses of amplitude above S and the mean reference amplitude Amo itself. This is used to provide a feedback signal to adjust 23 and thus S to maintain equality between Am and Amo. As shown in Fig. 6, Amo is the mean amplitude of pulses above S for an undeformed spectrum. In the arrangement of Fig. 5, (not shown) the output from amplifier 14 is supplied to two comparators (31, 32) having reference levels S and Amo. These are coupled via a logic circuit to the integrating amplifier as in Fig. 3. A flip-flop in the logic is reset by pulses between S and Amo and set by those above Amo, such that integrating amplifier produces a voltage representing the deviation between count rates N 1 and N 2 , Fig.6. Feedback to control S is used to maintain N 1 and N 2 equal. In the arrangement of Fig. 7, two comparators and logic 68 provide signals representing the count rates N 1 , N 2 of Fig. 6 at outputs 74, 76 respectively. While the ratio of N 1 :N 2 is near to unity, as measured by divider 84, (indicative of no deformation of the spectrum) the switch 82 is kept in position A and the signal delivered to 16 is the sum of the pulse trains N 1 , N 2 obtained by summer 78, divided by 2, which is equivalent to N 2 . When the ratio goes below 0À95, switch 82 changes to position B and a direct count of only N2 is made i.e. the unaffected part of the amplitude spectrum of Fig. 6. It is stated that automatic switching by calculation of the ratio need not be utilized, when an operator manually actuates 82 in accordance with the amount of barite introduced into the borehole. All circuits must be modified, when a reference source is utilized to enable stabilizing the detector gain. Since this is of a high amplitude compared with the spectrum shown in Fig. 6, the addition of voltage comparators having high energy thresholds A max, is a common feature of such modifications. In Fig. 3, the modification is shown dashed. For the Fig. 7 apparatus modified logic is provided, Fig. 8 (not shown) as it is for the Fig. 5 apparatus.