The downhole pulsed neutron generator is used to simulate the linear transmission of detector signals. The oscillator generates a square wave of 1 ~ 10 kHz, the anode pulse delay delays the trigger signal by 15μs, the anode pulse shaping shapes the trigger signal into a pulse with a width of 10μs, and the power of the driver is amplified to drive a high-power and high-back pressure transistor. An anode pulse with a width of 12μs and an amplitude of 1800 ~ 2000 V is generated in the secondary coil of transformer T, and applied to the anode of the neutron tube ion source to ionize the neutron tube. The ionization signal is converted into a negative DC signal through a п-type filter consisting of a resistor R and a capacitor C, and input into an operational amplifier to control two current regulating tubes for deuterium storage, so that deuterium is ionized. After the neutron tube is ionized stably, a deuterium-tritium nuclear reaction occurs when a high voltage of 120kV is applied to the titanium-tritium target of the neutron tube, resulting in a fast neutron of 14. 1MeV. The square wave of another oscillator is input into the synchronous signal to shape a pulse with a square wave shaping width of 5μs and an amplitude close to+15V, which is transmitted to the ground instrument through a cable and used as a simulated carbon-oxygen specific energy spectrum logging tool to generate a dual-gate spectrum measurement synchronous signal.
(2) Pulse neutron generator in digital coded signal transmission.
The numerical control C/O ratio downhole pulsed neutron generator is shown in Figure 2-4-7. Compared with the underground pulse neutron generator when the signal is transmitted linearly, it has the following points: (1) the synchronous measurement signal is not transmitted to the underground surface instrument; Two synchronous measurement signals are generated by the single chip microcomputer numerical control system and supplied to the underground pulse neutron generator.
(3) Compared with the downhole pulsed neutron generator, carbon and oxygen should be as coarse as possible. Its advantage is that it can discharge the liquid in the well, reduce its influence, and increase the sensitivity of C/O ratio logging to distinguish oil-water layers. In addition, ceramic neutron tubes with diameters of φ50mm, φ55mm or φ60mm should be used as targets. Its advantages are high neutron yield and long service life. Do not use commercial target neutron tubes or small diameter neutron tubes, which have short working life and low neutron yield, especially small diameter neutron tubes.
(4) Neutron life underground pulsed neutron generator, because the neutron impact frequency of 10 ~ 1000Hz is low and the neutron consumption is low, the effect of using commercial target neutron tube or small diameter neutron tube is also very good.
(5) The downhole pulsed neutron generator of bidirectional flow oxygen activation instrument is shown in Figure 2-4-8. The neutron hit period is 30 ~ 60s, which is suitable for small diameter neutron tubes, but the neutron yield should not be too low, which is estimated to be 105 ~ 106. Because the neutron impact period is very long and the neutron impact width is 2 ~ 12s, it is especially suitable for applying DC voltage between the anode and cathode of the neutron tube ion source of 1800 ~ 2000 V, and the target voltage of 120kV is pulsed.
(6) The successful development of120kV high-voltage power supply with small diameter and short length has opened the way for people to develop bidirectional nuclear logging tools, and successively developed bidirectional liquid pulse neutron oxygen activation logging tools, integrated logging tools for simultaneous measurement of carbon-oxygen ratio and neutron lifetime PNN (γ, N full spectrum) and four detectors neutron-neutron (N or γ) integrated logging tools.
(7) The expansion coefficient of 201methyl silicone oil is 15% ~ 20%, so silicone oil needs to fill the expansion space, and a bicycle valve core can be reversed. When SF6 gas is filled, insulation can be ensured as long as there is a high vacuum in the pipe, generally 0.65 ~ 1.25 atmospheric pressure. If there is no good vacuum pump, it can be charged to 3 ~ 8 atmospheres.