method and device for measuring the porositaet of borehole surrounding erdformationen using gepulster neutrons

摘要

1384357 Neutron well-logging TEXACO DEVELOPMENT CORP 15 March 1973 [17 May 1972] 12527/73 Heading G1A The porosity of earth formations surrounding a well borehole is measured by irradiating the formation with high energy neutron pulses, detecting during at least two intervals between successive pulses the gamma radiation resulting from the capture of thermalised neutrons, deriving from the measurements the thermal neutron decay time and combining this time with the neutron pulse duration and the number of 7-ray counts occuring during one of the detection intervals to derive a porosity related parameter. As shown, Fig.1, the apparatus consists of a log head containing a pulsed fast neutron source 11 and a detector comprising a scintillator 12 and a photo-multiplier 10. The fast neutrons are thermalised and then captured by the earth formations 3, the excited atoms then returning to the ground state with the emission of a γ-ray. A discriminator 18 rejects low energy pulses produced by background radiation and the remaining pulses are passed to the surface processing circuitry which uses a known sliding gate method involving the opening of three gates 21, 22, 23 in sequence between neutron pulses to produce counts representing the neutron decay characteristics. The processing circuitry 25 determines the thermal neutron decay time and produces a control signal to the neutron source pulsing circuit 14 so that a pulse of neutrons having a length equal to this decay time is emitted by source 11. The gates 21 to 23 are also controlled to open at times and for periods related to the decay time. Signals representing the thermal neutron decay time together with the number of counts received while the first gate 21 is open and the time for which the source is pulsed (if this differs from the decay time) are applied to a porosity computer 26 which determines the neutron moderation time by means of given equations. The moderation time depends on the hydrogen content of the earth formations which is related to the porosity. A recorder 27 plots the neutron decay time and the neutron moderation time (which may be calibrated in terms of porosity) as a function of the depth of the log as determined from the rotation of the cable sheave 9.