Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BOREHOLE LOGGING SYSTEM AND METHOD
Field of the Invention
[0001] The present invention relates to borehole logging and in
particular to a
borehole logging system and method.
Background of the Invention
[0002] Changes in the gravitational field of the Earth from place to
place
reflect the distribution of its diverse geologic materials, as a result of
their differences
in density. These gravitational changes can be observed through precise
measurements by means of gravimeters, on the Earth's surface and, more
recently,
from aircraft and in boreholes. In mining applications, gravitational
measurements
have played an important role in the exploration, evaluation and development
of
mineral resources, including both metallics and non-metallics. Gravitational
measurements are also of use in the exploration and production phases in the
hydrocarbon field.
[0003] Surface gravity measurements naturally provide more information
about the distribution of densities of geologic material closer to the Earth's
surface
than about geologic material at greater depths. It is now common practice to
explore
and mine mineral deposits at depths of up to 1-2 km below the Earth's surface,
and
hydrocarbon bearing horizons are being exploited to even much greater depths.
Gravity measurements made at the Earth's surface are of limited help in such
deep
programs. For this reason, borehole gravity sondes have been developed which
are
capable of providing high quality gravity measurements at such great depths,
even
under the extreme conditions of pressure and temperature prevailing at those
depths.
[0004] Borehole gravity measurements are affected by the distribution
of rock
densities, both in the vicinity of the borehole and remote from the borehole.
For some
applications, borehole gravity measurements are employed to provide
quantitative,
bulk-density data about the geologic formations being intersected by the
borehole, and
about fluids in reservoirs. Accurate bulk-density measurements can be of
material
value in several stages of mining activity. In the hydrocarbon field, they can
yield
information of great value in the evaluation of the hydrocarbon potential of
specific
horizons, or to monitor the progress of secondary recovery methods.
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[0005] For other applications, borehole gravity measurements can be
interpreted to indicate the presence of density changes that are either remote
from the
borehole or that lie below the bottom of the borehole. Remote detection of non-
intersected ore bodies is a common objective of borehole gravity measurements
in
mining applications. In the hydrocarbon field, information may be sought about
the
presence of salt domes and structural features, such as faults and folds,
which have
not been intersected by the borehole. Whereas the effects of remote geologic
features
are inherent in gravity measurements taken at intervals along the borehole,
for lack of
any unique interpretation of gravity data, it is theoretically impossible to
resolve the
contribution of remote density variations from the much larger contributions
due to
proximal sources, that is, the geologic formations actually intersected by the
borehole.
Only in the rare case that the geologic founations intersected by the borehole
all have
a uniform density, will the gravity effects of more remote geologic formations
become resolvable.
[0006] As will be appreciated, being able to determine the effect remote
geologic formations have on gravity measurements taken along a borehole is
desired
as it provides insight as to the nature of the remote geologic formations. It
is
therefore an object of the present invention to provide a novel borehole
logging
system and method.
Summary of the Invention
[0007] According to the following, the gravitational effects of
geologic
formations intersected by a borehole may be determined and removed from the
gravity measurements taken along the borehole allowing the gravitational
effects of
geologic formations remote from the borehole to be resolved and revealed.
[0008] Accordingly, in one aspect there is provided a borehole
logging
method comprising: running a density probe down a borehole, and at intervals
during
the running, using the density probe to take density measurements; processing
the
density measurements taken at intervals by the density probe using a processor
to
determine gravitational effects of geologic formations intersected by the
borehole;
running a gravity probe down the borehole, and at intervals during the
running, using
the gravity probe to take gravity measurements; processing the gravity
measurements
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taken at intervals by the gravity probe using the processor to remove the
determined
gravitational effects of geologic formations intersected by the borehole from
the
gravity measurements thereby to determine gravitational effects of geologic
formations remote from the borehole; and utilizing the determined
gravitational
effects of geologic formations remote from the borehole to determine the
nature of the
remote geologic formations based on remote density variations.
[0009] In one embodiment, the density and gravity measurements are
taken
independently, at either different times or simultaneously. The method may
further
comprise, prior to the processing, taking density measurements at first
vertical
intervals along the borehole and taking gravity measurements at second longer
vertical intervals along the borehole. The method may further comprise pre-
processing the gravity measurements prior to the processing to compensate for
the
effects of at least one of instrumental drift, effects of the Earth's tides
and the normal
variation with depth of the Earth's gravitational attraction.
[00010] According to another aspect, there is provided a borehole logging
system comprising: a density probe configured to take density measurements at
intervals when run-in along a borehole; a gravity probe configured to take
gravity
measurements at intervals when run-in along the borehole; and a processor
communicating with the density probe and the gravity probe and configured to:
process the density measurements taken at intervals by the density probe to
determine
gravitational effects of geologic formations intersected by the borehole;
process the
gravity measurements taken at intervals by the gravity probe to remove the
determined gravitational effects of geologic formations intersected by the
borehole
from the gravity measurements thereby to determine gravitational effects of
geologic
formations remote from the borehole; and utilize the determined gravitational
effects
of geologic formations remote from the borehole to determine the nature of the
remote geologic formations based on remote density variations.
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Detailed Description of the Drawings
[00011] Embodiments will now be described more fully with reference to
the
accompanying drawings in which:
[00012] Figure 1 is a side elevational view of a gamma-gamma density
probe
that has been lowered into a borehole;
[00013] Figure 2 is a side elevational view of a gravity sensor that
has been
lowered into the borehole; and
[00014] Figure 3 is a side elevational view of a sonde, comprising a
gamma-
gamma density probe and a gravity sensor that has been lowered into the
borehole.
Detailed Description of the Embodiments
[00015] According to the following, a borehole logging system and
method are
provided that allow the gravitational effects of geologic formations remote
from a
borehole to be resolved and revealed. During the method, density measurements
are
taken along the borehole at intervals to yield a density log and are processed
to
determine the gravitational effects of the geologic formations intersected by
the
borehole. Gravity measurements are also taken at intervals along the borehole
to
yield a gravity log. The determined gravitational effects of the geologic
formations
intersected by the borehole are removed from the gravity measurements allowing
the
gravitational effects of geologic formations remote from the borehole to be
determined. These determined gravitational effects signify density variations
remote
from the borehole and provide insight as to the nature of remote geologic
formations.
[00016] A number of techniques may be used to acquire the density
measurements at intervals along the borehole in order to generate the density
log. For
example, gamma-gamma (GG) density measurements, neutron gamma (ND) density
measurements or other non-acceleration based density measurements that provide
information about the density of geologic formations within one meter or less
of the
borehole maybe used. GG density measurements however are the one most
commonly preferred in practice for wallrock density determination and will be
referred to hereinafter by example only.
[00017] The model 2GDA-1000DX device, manufactured by Mt. Sopris
Instruments, Denver, Colorado, USA, is a suitable device to take GG density
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measurements at intervals along a borehole in order to generate the density
log. This
device comprises a source of gamma radiation (e.g. 100milliCuries of Cesium
137),
and one or two radiation detectors (typically thalium- activated NaI
crystals),
separated from the radioactive source by a distance of between about 20-35cm.
The
detectors are shielded from direct gamma radiation emitted by the radioactive
source.
The emitted gamma radiation creates both back-scattered Compton radiation and
photoelectric absorption in the geologic material surrounding the borehole.
This
secondary radiation impinges on the detectors. For a range of common densities
of
geologic materials (1-4 gm/cc), the count rate sensed by the detectors
provides a
reasonable linear estimate of the density of the geologic material surrounding
the
borehole, out to about 7-12cm into the surrounding geological material. The
typical
accuracy of such gamma-gamma density measurements is of the order of 0.1
gin/cc,
with a resolution of about 0.05 gm/cc.
100018] The Gravilog sonde offered by Scintrex Limited, Concord,
Ontario,
Canada is a suitable device to take gravity measurements at intervals along
the
borehole in order to generate the gravity log. The gravity measurements are
typically
pre-processed to undergo certain corrections in order to more accurately
provide
useful geologic infoiniation. These corrections include instrumental drift
with time,
the effects of the Earth's tides and the normal variation, with depth, of the
Earth's
gravitational attraction (the so-called Free-Air correction). After such
corrections
have been made to the gravity measurements, changes (Ag) in the resultant
gravity
data with depth are predominantly dependent on the density of the geologic
formations being traversed by the borehole, and are given by the formula shown
in
Equation (1) below:
Ag = -0.08382 pAz (1)
where:
Ag is in units of milliGals;
p is the local geologic formational density, in units of
gm/cc; and
Az is the change in depth, in meters
[00019] The change of resultant gravity with depth reflects the upward
attraction (i.e. the reduction of gravity) of the geologic formations which
overlie the
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gravity station (i.e. the position along the borehole at which the gravity
measurement
is made), and may be inverted to derive the mean bulk density of the geologic
formations, for those applications where this is the factor of greatest
interest.
Inverting Equation (1) provides Equation (2) as follows:
p = 11.93Ag / Az (2)
where:
p is the mean geologic formational density, in units of gm/cc.
[00020] According to Equation (2), p is the mean value of the geologic
formational density, the so-called "bulk density" between two gravity stations
Az
meters apart in depth. The effective radius from the borehole of these bulk-
density
values is several times the separation between the two gravity stations, i.e.
normally
10-100 meters.
[00021] However, for applications requiring the detection of remote density
variations, a basic impediment is the dominant effect of the near-borehole
geologic
formational densities on the gravity measurements. Attempts may be made to
estimate the mean value of p to be used for the near-borehole geologic
formation
densities, in Equation (1) and therefore to estimate their contribution to the
observed
gravity measurements, but this is an exercise which is subject to considerable
subjective error.
[00022] It has been found that the gravity contribution of the near-
borehole
geologic formations can be removed through the use of the GG density log. Due
to
the shallow depth penetration of GG density measurements into the geologic
material
.. surrounding the borehole, the densities in the density log pertain only to
the geologic
formations actually intersected by the borehole. The densities derived from
the GG
density log can be designated as py and can be used in Equation (1) to
determine the
gravity effects that arise from these geologic formations, on the assumption
of their
being of a uniform GG density, tabular, horizontally lying and of very large
horizontal
extent.
[00023] These calculated gravity effects can then be subtracted from
the
corrected gravity log measurements, on the basis of the measured GG densities.
The
residual gravity after this subtraction is attributed to density changes that
are remote
from the borehole. This GG-derived residual gravity is referred to as GRG and
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embodies the effect of any departures, at a distance from the borehole, of the
actual
density distribution from the idealized, one-dimensional density model based
on py.
[00024] Typically, in a gravity log of a borehole, the gravity station
highest in
the borehole is designated as a base station for drift determination, and also
is the
arbitrary base level for all subsequent relative gravity measurements. If the
corrected
gravity values are defined as g, and the GG calculated gravity values are
defined as gy,
then based on Equation (1), Equation (3) can be derived as follows:
gy = -0.08382 E py8 (3)
where:
the 1, summation is over the vertical depth from the base
station to the moving station; and
8 is the vertical spacing between successive values of py.
[00025] Based on the above, GG-derived residual gravity GRG may be
resolved as shown in Equation (4) below:
GRG = g - = g + 0.08382 Epy8 (4)
[00026] The graph of GRG may be interpreted to derive basic information
about density changes occurring remote from the borehole, not just laterally
but also
at depths below the bottom of the borehole.
[00027] As well, the bulk-density values of p determined by Equation
(2) using
the gravity measurements, may be compared with the mean GG density values py
over
the same vertical intervals. Any differences between these two density values
indicates an increase or decrease of density away from the borehole (usually
laterally)
from the geologic formations intersected by the borehole. These differences
may be
designated as GG-derived residual densities, or GRD, equal to p- py. However,
GRD
data are of lesser value than the GRG values, because they are not as readily
subject
to quantitative modeling.
[00028] The practice of generating a GG density log of a borehole is a
long
established, standard procedure in the case of boreholes drilled for
hydrocarbon
purposes, so that in such boreholes at-hole density measurements are usually
available
for use. In the case of boreholes drilled for mining and other applications,
GG density
logs are not necessarily commonplace, and a density log may not be available.
For
such case, a GG density log will have to be carried out. For example, a GG
sensor
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may be incorporated in the same sonde as a gravity sensor, or alternatively a
GG
density log may be carried out independently, either prior to or subsequent to
the
gravity log. Where it is intended that a borehole will be metal cased prior to
conducting a gravity log, the GG density log will have to be done prior to the
insertion of the casing. This is because GG density measurements arc not
possible in
metal-cased boreholes, due to the effect of the casing. When carried out, GG
density
measurements are commonly recorded at short intervals down the borehole,
typically
of the order of 15 cm. Gravity measurements are typically made at much larger
intervals, commonly 10 meters or more. The value of the GG py to be used will
be the
mean of the GG py values over the gravity station intervals.
[00029] As will be appreciated, the subject method employs density
logs to
calculate the theoretical gravitational effect of geologic formations in the
immediate
vicinity of the borehole and then removes these gravitational effects from
gravity
measurements to reveal the presence of density changes which are remote from
the
borehole. The mean GG density values, over the vertical interval between
successive
gravity measurements, are compared with the gravity-derived bulk-density
values,
thereby to directly indicate an increase or decrease of density away from the
borehole.
[00030] Figures 1 to 3 show examples of probes that can be used to
acquire
density and/or gravity measurements at intervals along a borehole in order to
enable
density and gravity logs to be generated. In particular, Figure 1 is a
schematic
drawing of a gamma-gamma density probe 2, having a gamma source 3 and a
radiation detector 4 that is suspended in borehole 5 by a cable 1. Cable 1
incorporates
a strength member, as well as electrical members which both control the
operation of
the probe 2 and transmit density measurements from the probe 2 to the top of
the
.. borehole. The region of geologic material 6 around the borehole 5which
influences
the gamma-gamma density measurements is shown.
[00031] In Figure 2, a gravity sensor 7 that is suspended in the
borehole 5 by
the cable 1 is shown. As is suggested by the configurations shown in Figures 1
and 2,
the density probe 2 and gravity sensor 7 allow independent density and gravity
measurements to be taken along the same borehole. However, as shown in Figure
3, a
sonde including both a gamma-gamma density probe 2 and a gravity probe 7 may
be
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used, whereby both gamma-gamma density and gravity measurements can be made in
one logging survey of the borehole.
[00032] The gamma-gamma density measurements and the gravity
measurements are transmitted via one or more of the electrical members in the
cable 1
to processing structure (not shown). The processing structure processes the
density
and gravity measurements as described above to resolve the gravity
contribution due
to formations intersected by the borehole from the gravity contribution due to
density
variations that are remote from the borehole.
[00033] The processing structure in this embodiment is a general
purpose
computing device comprising a processing unit, system memory (volatile and/or
non-
volatile memory), other non-removable or removable memory (e.g. a hard disk
drive,
RAM, ROM, EEPROM, CD-ROM, DVD, flash memory, etc.), a display and a system
bus coupling the various computer components to the processing unit. The
computing
device may also comprise networking capabilities using Ethernet, WiFi, and/or
other
suitable network format, to enable connection to shared or remote drives, one
or more
networked computers, or other networked devices.
[00034] Although embodiments have been described, those of skill in
the art
will appreciate that variations and modifications may be made without
departing from
the scope thereof as defined by the appended claims.
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