Directional epithermal neutron detector

A directional epithermal neutron detector for use in well logging employs a plurality of neutron counters and a neutron moderating material. A first thermal neutron shield provides a cylindrical housing for the counters and moderating material. The counters are clustered to one side of the housing and the moderating material fills the remaining portion of the housing. A second thermal neutron shield separates the counters from the moderating material.

BACKGROUND OF THE INVENTION 
This invention relates to radioactive well logging processes and systems 
for irradiating subterranean formations under investigation with bursts of 
fast neutrons and, more particularly, to an improved epithermal neutron 
detector for use in characterizing the formation on the basis of the decay 
of the subsequently produced epithermal neutron population. 
Various techniques may be employed in order to characterize subterranean 
formations with regard to their fluid or mineral content, lithologic 
characteristics, porosity, or to provide for stratigraphic correlation. 
The neutron source may be a steady-state source or a pulsed source. For 
example, neutron porosity logging may be carried out using a steady-state 
neutron source in order to bombard the formation with fast neutrons. The 
porosity of the formation then may be determined by measuring thermal 
neutrons employing two detectors at different spacings from the source or 
by measuring epithermal neutrons with a single detector. 
In pulsed neutron logging procedures, the formations are irradiated with 
repetitive bursts of fast neutrons, normally neutrons exhibiting an energy 
greater than 1 Mev. When the fast neutrons enter the formation, they are 
moderated, or slowed down, by nuclei within the formation to form lower 
energy neutron populations. The fast neutrons are moderated to lower 
energy levels by the nuclear collision processes of elastic and inelastic 
scattering. In elastic scattering the neutron loses a portion of its 
energy in a collision that is perfectly elastic, i.e., the energy lost by 
the neutron is acquired as kinetic energy by the nucleus with which it 
collides. In inelastic scattering only some of the energy lost by the 
neutrons is acquired as kinetic energy by the nucleus with which it 
collides. The remaining energy loss generally takes the form of a gamma 
ray emitted from the collision nucleus. 
In the course of moderation, the neutrons reach the epithermal range and 
thence are further moderated until they reach the thermal neutron range. 
Thermal neutrons are neutrons which are in thermal equilibrium with their 
environment. The distribution in speed of thermal neutrons follows the 
so-called Maxwellian distribution law. The energy corresponding to the 
most probable speed for a temperature of 20.degree. C. is 0.025 electron 
volt. Epithermal neutrons are those neutrons which exhibit energies within 
the range from immediately above the thermal neutron region to about 100 
electron volts. While the boundary between thermal and epithermal neutrons 
is, of necessity, somewhat arbitrary, it is normally placed in the range 
of 0.1-1.0 electron volt. 
The populations of neutrons at the various energy levels decay with time 
following primary irradiation and thus offer means of characterizing the 
formation. For example, in the case of elastic scattering, which 
predominates for energies between a few electron volts and about 1 Mev, 
the number of collisions required for a neutron to moderate from one 
energy level to a second lower energy level varies more or less directly 
with the atomic weight of the nuclei available for collision. In 
subterranean formations, hydrogen nuclei present in hydrogenous materials 
such as oil, water, and gas tend to predominate in the slowing down 
process. Thus, the rate of decay of the epithermal neutron population 
gives a qualitative indication of the amount of hydrogenous material 
present which in turn may be indicative of the porosity of the formation. 
For example, U.S. Pat. No. 4,097,737 to Mills discloses a method and system 
for epithermal neutron die-away logging utilizing a 14-Mev pulsed neutron 
source and a neutron detector that is sensitive to epithermal neutrons and 
highly discriminatory against thermal neutrons. The detector is relatively 
insensitive to the high energy neutrons and has a filter that renders it 
sharply insensitive to thermal neutrons. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a borehole 
logging tool for directional epithermal neutron die-away logging of 
subterranean formations surrounding a borehole. 
More particularly, a source of neutrons irradiates the formations 
surrounding a borehole with repetitive bursts of fast neutrons and at 
least one neutron counter detects epithermal neutrons returning to the 
borehole from the irradiated formations. An outer thermal neutron shield 
provides a housing for the neutron counter as well as for a neutron 
moderating material. An inner thermal neutron shield divides the housing 
so as to provide a first compartment bounded by the inner thermal neutron 
shield and a first portion of the outer thermal neutron shield and a 
second compartment bounded by the inner thermal neutron shield and a 
second portion of the outer thermal neutron shield. The neutron counter is 
positioned in the first compartment and the neutron moderating material is 
positioned in the second compartment. The borehole tool is positioned 
against one side of the borehole wall and azimuthly oriented so that the 
first compartment housing the neutron counter is next to the borehole 
wall. Formation epithermal neutrons penetrate into the first chamber 
through the first portion of the outer thermal neutron shield and are 
detected by the neutron counter for die-away measurement while borehole 
fluid epithermal neutrons penetrate into the second chamber through the 
second portion of the outer thermal neutron shield and are slowed down and 
lowered in energy by the moderating material and absorbed by the inner 
thermal neutron shield. In this manner, the directional sensitivity of the 
neutron counter to formation epithermal neutrons is maximized, while the 
directional sensitivity of the neutron counter to borehole fluid 
epithermal neutrons is minimized.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is directed to a new and improved epithermal neutron 
detector for use in radioactivity well logging. 
In FIG. 1, a well logging system is illustrated with which the directional 
epithermal neutron detector of the present invention may be employed. A 
borehole tool 10, supported by cable 11, comprises a high energy pulsed 
neutron source 12 and an epithermal neutron detector 13. A high voltage 
power supply 14 is provided for the source 12 and a module 15 is provided 
with circuits for utilization and modification of signals from detector 
13. Also included are circuits for the control of the high voltage power 
supply 14. Cable 11 extends to a surface unit 16 where the signals from 
the epithermal neutron detector 13 are recorded along with an indication 
of borehole depth. The use of such a well logging system for epithermal 
neutron die-away logging is disclosed in the aforementioned patent to 
Mills. 
A particularly suitable epithermal neutron detector, as disclosed in the 
Mills patent, is a proportional counter filled with helium-3 gas and 
surrounded by an outer thermal neutron filter, preferably a thin layer of 
cadmium or gadolinium. This filter prevents thermal neutrons which are in 
thermal equilibrium with their environment from entering the neutron 
detector. The energy corresponding to the most probable speed for thermal 
neutrons in equilibrium at a temperature of 20.degree. C. is 0.025 
electron volt. Epithermal neutrons are those neutrons which exhibit 
energies within the range from immediately above the thermal equilibrium 
region to about an energy of 100 electron volts. While the boundary 
between thermal and epithermal neutrons is, of necessity, somewhat 
arbitrary, it is normally placed in the range of 0.1 to 1.0 electron volt. 
Thus, the thermal neutron filter allows only epithermal neutrons above 
about 0.1 to 1.0 electron volt to enter the space between the neutron 
counter and the filter. The neutron counter measures the rate of decay of 
the epithermal neutron population passing through the filter as a 
qualitative indication of the amount of hydrogenous material present in 
the formation surrounding a borehole, which in turn may be indicative of 
the porosity of the formation. 
In borehole logging operations involving the production of neutrons, it is 
usually possible to increase the efficiency of this epithermal neutron 
measurement by inserting an appropriate amount of neutron moderating 
material between the neutron counter and its thermal neutron filter, as 
shown in U.S. Pat. No. 4,241,253 to Allen, et al. A neutron moderating 
material, such as polyethylene, slows down epithermal neutrons which 
penetrate the thermal neutron shield to enable these neutrons to be more 
readily absorbed by the neutron counter. This occurs due to the 
moderator's effect in spreading the neutron counting rate in time. Energy 
loss due to scattering in the moderator lowers the speed of the neutrons 
and this delays their arrival at the counter. This delay in turn reduces 
the instantaneous maximum counting rate. 
Such epithermal neutron detectors, as shown in the aforementioned patents 
to Mills and to Allen, et al., have omni azimuthal sensitivity. In a well 
logging operation, the borehole logging tool is not normally centered 
within the borehole, but rather rests along one side of the borehole wall. 
The borehole is usually filled with a drilling fluid of high hydrogen 
content. Neutrons within this borehole fluid influence the neutron 
detector of the logging tool as do those neutrons in the formation on the 
side of the borehole in contact with the logging tool. These neutrons 
within the borehole fluid include neutrons that have never been in the 
formation surrounding the borehole as well as those neutrons returning to 
the borehole following irradiation of the formation from the neutron 
source of the borehole logging tool. This gives rise to a neutron detector 
response that is made up of both a formation epithermal die-away component 
and a borehole fluid epithermal die-away component. 
It has been found that the borehole fluid epithermal die-away component is 
quite dominant to the point that sensitivity to the formation epithermal 
die-away component is minimal. It is, therefore, a specific feature of the 
present invention to provide for a well logging operation in which the 
formation epithermal die-away sensitivity is maximized while the borehole 
fluid epithermal die-away sensitivity is minimal. In accordance with such 
aspect, the present invention provides for a directional epithermal 
neutron detector such as illustrated in FIG. 2. 
Referring now to FIG. 2, there is shown a cross-sectional view of the 
directional epithermal neutron detector of the present invention. The 
detector 20, as part of the borehole logging tool, is held against the 
formation 24 on one side of the borehole by conventional means (not 
shown), such as by the use of bow springs. A plurality of helium-3 thermal 
neutron counters 22 are clustered to one side of the neutron detector 20 
and are enclosed by an outer thermal neutron shield 25 and by an 
additional inner thermal neutron shield 26, thereby forming a first 
compartment 27 which houses the counters 22 and a second compartment 28 
which is filled with a thermal neutron moderating material 29. Suitable 
thermal neutron shielding material is cadmium or gadolinium, while 
suitable thermal neutron moderating material is polyethylene. 
To utilize the neutron detector of FIG. 2 in a borehole logging operation 
to maximize directional sensitivity of the counters 22 to the formation 
epithermal neutron component and to minimize directional sensitivity to 
borehole fluid epithermal neutron component, the borehole logging tool is 
held against the formation 24 on one side of the borehole by means of bow 
springs (not shown) and is oriented such that the chamber 27 housing the 
neutron counter 22 is in juxtaposition with the borehole wall as pictured 
in FIG. 2. In this manner, neutron detector 20 becomes directional for 
epithermal neutron die-away logging of the formation adjacent the 
borehole. 
More particularly, epithermal neutrons shown at 30 returning to the 
borehole from that portion of the formation in the vicinity of contact 
with the neutron detector 20 pass through the outer thermal neutron filter 
25 into chamber 27 housing the counters 22 without thermal moderation. 
However, epithermal neutrons shown at 31 entering the neutron detector 20 
by way of the borehole fluid must first primarily pass through the outer 
thermal neutron filter 25 into chamber 28 and then through the inner 
neutron filter 26 into chamber 27. Within chamber 28 these borehole fluid 
epithermal neutrons are scattered and lowered in speed by the moderating 
material 29, thus decreasing the likelihood that these neutrons will reach 
the chamber 27. In addition, the inner thermal neutron shield 26 serves to 
further reduce the number of borehole fluid moderated neutrons from 
reaching the neutron counters 22. In this way, the neutron detector 20 
becomes directionally sensitive through a maximizing of the formation 
epithermal neutron die-away component and a minimizing of the borehole 
fluid epithermal neutron die-away component. Ideally, the surface area 
contact of that portion of the outer thermal neutron filter forming 
chamber 27 with the borehole wall should be maximized so as to minimize 
the amount of borehole fluid between chamber 27 and the formation. For a 
circular configuration of the outer thermal neutron shield, that portion 
of the outer thermal neutron shield forming chamber 27 should subtend an 
angle of no greater than about 90.degree.. 
In a further aspect of the invention illustrated in FIG. 3, that portion of 
the outer epithermal neutron shield 25 which forms the chamber 27 along 
with the inner epithermal neutron shield 26, designated as 25b, is 
comprised of a different shielding material from the remaining portion of 
such outer shield, designated as 25a. For example, shield 25b may 
preferably comprise a thin layer of gadolinium of no greater than about 10 
mils thickness, while the shields 25a and 26 may preferably comprise a 
much thicker layer of cadmium of at least about 100 mils thickness. The 
gadolinium filter 25b defines the area of the neutron detector 20 through 
which formation epithermal die-away neutrons more readily pass to the 
neutron counters 22. The cadmium filter 25a, along with the moderating 
material 29, defines the absorbing area of the neutron detector 20 which 
minimizes the passage of borehole fluid epithermal die-away neutrons to 
the neutron counters 22. Cadmium absorbs neutrons of all energy levels to 
some degree. Above about 0.4 electron volt the absorption cross-section of 
cadmium is proportional to 1 /v, wherein v is the velocity of neutrons, or 
to 1/.sqroot.E.sub.n, where E.sub.n is the energy of neutrons, thus, the 
thicker the cadmium shielding, the greater the number of higher energy 
neutrons that are absorbed. The moderating material serves to moderate 
those higher energy neutrons that are not absorbed. Those moderated 
neutrons are then subject to absorption by the inner cadmium shield 26 to 
further reduce borehole fluid neutrons from reaching the neutron counters 
22. 
The foregoing described embodiments of the invention permit directional 
epithermal neutron die-away logging which was not possible with prior art 
epithermal neutron detectors. Other neutron filtering and moderating 
materials than those described above may be successfully utilized as well 
as an infinite variety of combinations of materials. It is to be 
understood that the present invention relates to a directional epithermal 
neutron die-away detector for maximizing sensitivity to formation 
epithermal die-away neutrons while minimizing sensitivity to borehole 
fluid epithermal die-away neutrons and that modifications or alterations 
may be made without departing from the spirit and scope of the invention 
as set forth in the appended claims.