Method of downhole neutron monitoring

A method of downhole neutron monitoring using an oxygen containing scintillating material to generate scintillator light as a function of electrons and gamma rays emitted from the product of the .sup.16 O (n,p) reaction in a scintillator, converting the generated scintillator light to an electrical signal, and using the electrical signal to record the flux of measured neutrons.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the oil exploration industry and particularly to 
the area of neutron well logging. 
2. Prior Art 
Nuclear techniques have been extensively applied to the problems of the oil 
exploration industry, particularly in the area of well logging. Nuclear 
particle detectors are used to differentiate between strata containing 
oil, natural gas, water and the type of strata containing these fluids. 
These investigations estimate the extent of the oil-bearing strata and the 
amount of oil that these strata contain. 
The scientific basis for this survey technique is based on the difference 
in the neutron reactions of the various materials. The varying reactions 
are monitored by the detection of gamma rays or neutrons returning from 
the strata. A probe containing a neutron source is drawn through the bore 
hole and a detector measures the energy and intensity of the emitted 
incident particles. Oil, gas, water and various geological formations 
possess a distinctive gamma ray signature that permit identification of 
the chemical constituents of the strata. 
Two types of neutron sources are now used in the downhole probe. One type 
employs radioactive sources, such as americium/beryllium or californium, 
that continuously emit neutrons. The other employs a pulsed 14-MeV neutron 
generator. Each of these methods has serious drawbacks. The radioactive 
source requires special licenses in every country in which the source will 
be used, and must have massive shielding and complicated transportation 
arrangements. Most importantly, radioactive sources are occasionally lost 
downhole, which can potentially cause the loss of the hole. The 
radioactive sources, however, possess the advantage of a well known 
neutron emission rate. 
The pulsed generators are safer to use and allow a wider variety of 
measurements downhole. The disadvantage of pulsed sources is that they may 
not emit neutrons at a constant rate. 
A high efficiency detector has been proposed for the measurement of short 
pulses of d+T neutrons. It is based on the activation of sodium in a 
sodium iodide scintillation detector via the .sup.23 Na (n,.alpha.) 
.sup.20 F reaction and the immediately postpulse measurement of the 
induced beta activity which has an 11 second half life. The detector is 
insensitive to neutrons with energies below about 7 MeV, and may be 
calibrated directly against an absolute technique using a "steady current" 
d+T neutron generator. It is noted that the threshold required is low and 
encompasses the oxygen background, thereby greatly reducing the usefulness 
of the method disclosed. V. E. Lewis and T. B. Ryves Nuclear Instruments 
and Methods in Physics Research A 257 (1987) 462-466. 
OBJECTS OF THE INVENTION 
Principal objects of the present invention are to provide a method of 
detection of emissions from pulsed neutron generators that is safe to use, 
accurate and that can be advantageously used for downhole monitoring, 
i.e., bore hole surveys, activation analysis and source quality control. 
Other objects are to provide a method of measuring neutron emission rates 
that is fast and that will effectively measure varying neutron emission 
rates from a neutron source drawn through a bore hole with widely varying 
rock stratas. 
Still other objects are to provide a method utilizing an neutron detector 
that is minimally subject to background distractions and particularly to 
the large amounts arising from oxygen found in wells and the like. 
FEATURES OF THE INVENTION 
Principal features of the invention include the use of an oxygen containing 
scintillating material with a suitable photomultiplier tube to convert 
generated scintillator light to an electrical signal. The electrical 
signal is used to record the varying flux of measured neutrons. The 
measurements are made during beam-off periods to avoid detection of 
unwanted background events caused by lower energy gamma rays. 
Additional objects and features of the invention will become apparent to 
persons skilled in the art to which the invention pertains from the 
following detailed description.

DETAILED DESCRIPTION 
As previously noted, the movement of probes containing a neutron emitting 
source through a neutron capturing medium and determination of the nature 
of the medium by detection of low energy neutrons and of gamma rays 
released by the medium has been known. 
The method of the present invention constitutes an improvement over the 
known processes in that it utilizes an oxygen containing scintillating 
material such as bismuth germanate or a lithium glass scintillator as the 
neutron flux monitor. 
The scintillator provides a light signal that is coupled to a suitable 
conventional photomultiplier tube that will convert the scintillator light 
to an electrical signal. 
The .sup.16 O (n,p) .sup.16 N reaction is used to record the flux of 
neutrons with energies greater than 11 MeV (the effective threshold) 
through observation of the decay of .sup.16 N nuclei. This decay occurs 
with a seven second half life with the emission of high energy electrons 
and gamma rays. The above seven second half life permits the obtaining of 
the data during beam-off periods with the consequent reduction in unwanted 
background events and the emission of high energy electrons and gamma rays 
which permits the discrimination against lower energy background gamma 
rays. It is noted that approximately 13% of the .sup.16 N decays results 
in a combined energy release of 6 MeV or greater. It has been found that 
the use of an oxygen containing scintillating material permits 
discrimination against the oxygen background arising from the surrounding 
media and therefore makes the method very suitable for use in downhole 
monitoring or where present oxygen may otherwise adversely affect detected 
data. The same is true for other high energy backgrounds i.e., from 
.sup.11 B, arising from the surrounding media. 
Laboratory tests have been performed to validate the system proposed. The 
response of the detector system was measured using a Van de Graaff 
accelerator to produce 14-MeV neutrons. The detector system was placed 20 
cm away from the neutron source on a low mass aluminum stand. The detector 
was irradiated for 30 seconds to establish an equilibrium decay rate for 
the .sup.16 N produced (4 half lives). 
The neutron beam was then turned off and the N decays were counted for 15 
sec. This process was repeated several times with the bare detector. The 
detector was then surrounded by 450 pounds of concrete to simulate the 
conditions of a bore hole. (The space between the source and detector was 
left open for the test.) The above irradiation procedure was repeated with 
the concrete in place and the ratio taken. Table 1 lists this ratio as a 
function of bias energy for gamma rays interacting in the crystal. 
TABLE 1 
______________________________________ 
Bore-Hole Neutron Detector Response 
Bias Energy Concrete in 
Relative 
(MeV) Concrete out 
Efficiency 
______________________________________ 
3.7 1.16 1.0 
4.3 1.23 0.68 
4.9 1.26 0.54 
5.5 1.30 0.41 
6.1 1.22 0.27 
6.7 1.05 0.15 
7.3 0.99 0.084 
7.9 1.02 0.043 
8.5 1.18 0.022 
9.2 1.34 0.011 
______________________________________ 
It can be seen from the Table that the detector is sufficiently insensitive 
to changes in the bore hole environment for a bias level between 6.7 and 
7.9 MeV. 
Although a preferred embodiment of my invention has been disclosed, it is 
to be understood that the present disclosure is by way of example and that 
variations are possible without departing from the subject matter coming 
within the following claims, which subject matter I regard as my 
invention.