TWO DETECTOR PULSED NEUTRON POROSITY AND WATER SALINITY APPARATUS AND METHODS

摘要

1471788 Borehole logging SCHLUMBERGER Ltd 1 May 1974 [1 May 1973] 19103/74 Heading G1A Borehole logging apparatus, operating by irradiating the formation with neutrons and monitoring (thermal) neutron populations in the formation at two locations by means of two detectors spaced at different distances along the logging tool from the neutron source, is used to measure formation porosity # and formation fluid salinity, WS by deriving a neutron population measurements ratio R and, combining this with a neutron absorption characteristic such as thermal neutron decay time # or microscopic capture cross-section #, in accordance with predetermined relationships to calculate the required quantities. In the embodiment described, Fig. 1, the near and far detectors 20, 22 are sodium-iodide scintillation crystal detectors coupled to photomultipliers monitoring 7-radiation from thermal neutron capture. Alternatively, thermal neutron sensitive detectors e.g. He-3 filled proportional counters may be used. The neutron source 18 is arranged to produce discrete pulses of 14MeV high energy neutrons of one decay time (#) duration, at intervals of 10#. The neutron population is sampled in the interval between the irradiation pulses: at 2# after the irradiation pulse has ended for a duration of #; at 3# after irradiation has ended for a 2# duration; and at 6# after irradiation has ended for 3# duration. The first and second sample period counts N 1 , F 1 and N 2 , F 2 represent neutron population counts and the third N3, F3 represent gamma backgrounds to be subtracted from N 1 and/or N 2 and F 1 and/or F 2 during calculations. # is calculated from the ratio of the net N 1 , N 2 counts by adjusting the sample intervals until the ratio equals 2À0 when the first interval is of length # and may be used to control the timing and duration of neutron source energization in order to optimize the measurement system. The neutron population function calculated is R, equal to NSP1/SP-SP1/SP/ 3 N 3 /F 1 +F 2 -F 3 a more accurate function than the simple ratio of the net N 1 , F 1 counts, and is calculated using the circuitry shown in Fig. 1. A suitable computation circuit for combining # (obtained from #) and R to calculate # and WS is detailed in Fig. 4 (not shown), or an appropriately programmed computer may be used. The quantities shown in Fig. 1 at 40 may be recorded on magnetic tape or visual recording apparatus. Parts of the circuit may be on the borehole tool. The invention need not be carried out using a pulsed source and measuring neutron populations in the intervals between pulses. A steady state system using a continuously operating source may be used and R obtained from counts for the near and far detectors, in which cell #, or #, is derived by other appropriate apparatus.