GB925630A - Nuclear magnetism well logging apparatus - Google Patents

Abstract

925,630. Gyromagnetic well-logging. CALIFORNIA RESEARCH CORPORATION. Aug. 5, 1959, No. 26758/59. Class 37. In an apparatus for detecting the presence of a natural hydrogenous fluid in association with another hydrogenous fluid in a borehole, a coil 15, Fig. 1, the length of which is great compared with its width is used to polarize the protons of the water and oil in an earth formation around a borehole 43 in a direction at right-angles to the earth's field, the relaxation time of the precession which occurs when the polarizing current is suddenly switched off, being determined from an oscillogram on cathode-ray tube 30. A camera 26, Fig. 5, is arranged to make a permanent record of the oscillogram, and the reading on sonde depth indicator 28. D.C. power from source 11 is applied to the coil through a fast electronic switch 13 which comprises a triode having a high resistance between anode and cathode, under the control of a square-wave pulse generator 17, the output of which may be varied in respect of the negative pulse duration, and the interval between pulses. The pulses serve also to gate preamplifier 19 which is gated off until the D.C. polarizing field returns to zero, the signal from processing protons being picked up for display by an additional coil 21 or by the coil 15 when switched for pick-up by a vacuum switch. In general, water with magnetic impurities has a much shorter relaxation time than oil and in certain circumstances paramagnetic impurities, such as molecular oxygen, may be introduced into the drilling mud as a gas or a compound such as hydrogen peroxide which is capable of liberating molecular oxygen, so that the relaxation times of the mud and oil are clearly distinguishable. Alternative impurities include magnetite. The molecular paramagnetic materials may be slightly ionized. The same apparatus may be used to measure the polarization time as an alternative to the relaxation time by a study of the signals picked up from a series of polarizing pulses of varying duration or the density of responsive protons from which the liquid content or porosity of the earth formation can be determined, in the following manner:- (i) Determining the instrument constant by immersion in a large quantity of magnetically uncontaminated water at S.T.P. (ii) Determining the density of protons in the drilling fluid by use of the instrument or an alternative laboratory method; and (iii) Analysing a borehole reading. Wave-form analysis is described, and a mathematical basis for the method is included.