U.S. patent application number 11/759030 was filed with the patent office on 2008-02-14 for sliding weight borehole gravimeter.
Invention is credited to Maximiliaan Peeters, Roelof K. Snieder.
Application Number | 20080034855 11/759030 |
Document ID | / |
Family ID | 39049242 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080034855 |
Kind Code |
A1 |
Peeters; Maximiliaan ; et
al. |
February 14, 2008 |
SLIDING WEIGHT BOREHOLE GRAVIMETER
Abstract
A borehole tool including an interferometer, a light source, a
chamber containing a sliding weight having a first optical prism, a
second optical prism located within the chamber, a tilt measuring
device, and a timing device operatively associated with the
interferometer. The light source, the interferometer, and the first
and second optical prisms are configured to cause light emitted by
the light source to form a first beam and a second beam that
interfere with each other. The interferometer measures distances
traveled by the sliding weight in the upward and downward direction
by counting the fringes caused by the interference between the
first beam and the second beam. The tilt measuring device measures
the angle of the chamber relative to vertical. The influence of
friction on the sliding weight's motion is eliminated by comparing
the distances traveled by it in its upward and downward path over
an equal time interval.
Inventors: |
Peeters; Maximiliaan;
(Golden, CO) ; Snieder; Roelof K.; (Golden,
CO) |
Correspondence
Address: |
DORSEY & WHITNEY, LLP;INTELLECTUAL PROPERTY DEPARTMENT
370 SEVENTEENTH STREET, SUITE 4700
DENVER
CO
80202-5647
US
|
Family ID: |
39049242 |
Appl. No.: |
11/759030 |
Filed: |
June 6, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60822188 |
Aug 11, 2006 |
|
|
|
Current U.S.
Class: |
73/152.05 ;
175/50; 73/152.59 |
Current CPC
Class: |
G01V 7/14 20130101 |
Class at
Publication: |
73/152.05 ;
175/50; 73/152.59 |
International
Class: |
E21B 47/022 20060101
E21B047/022; E21B 47/026 20060101 E21B047/026 |
Claims
1. A borehole tool comprising: an interferometer; a light source; a
chamber containing a sliding weight including a first optical
prism; a second optical prism located within the chamber; a tilt
measuring device; and a timing device operatively associated with
the interferometer; wherein: the sliding weight is movable within
the chamber; the light source, the interferometer, the first
optical prism, and the second optical prism are configured to cause
at least a portion of light emitted by the light source to form a
first beam and a second beam that interfere with each other; the
interferometer measures the interference between the first beam and
the second beam; and the tilt measuring device measures an angle of
the chamber relative to vertical.
2. The borehole tool of claim 1, further comprising a propulsion
device configured to propel the sliding weight upward in the
chamber.
3. The borehole tool of claim 2, wherein the sliding weight is
magnetic and the propulsion device comprises an induction coil.
4. The borehole tool of claim 1, wherein the timing device
comprises an atomic clock.
5. The borehole tool of claim 1, wherein the light source comprises
a laser.
6. The borehole tool of claim 1, wherein the tilt measuring device
comprises a borehole tilt-meter.
7. The borehole tool of claim 1, wherein a distance traveled by the
sliding weight during a select period of time is determined using
the interferometer and the timing device.
8. The borehole tool of claim 2, wherein a gravitational
acceleration is determined by propelling the sliding weight upward
in the chamber, determining an upward and a downward distance
traveled by the sliding weight during a select time period using
the interferometer and the timing device, measuring an angle of
tilt of the chamber during the select time period using the tilt
measuring device, and using the determined upward and downward
distances, a length of time of the select time period, and the
angle of tilt to calculate the gravitational acceleration.
9. The borehole tool of claim 1, further comprising: a second
interferometer; a second sliding weight contained within the
chamber and including a third optical prism; and a fourth optical
prism located within the chamber; wherein: the second sliding
weight is movable within the chamber; the light source, the second
interferometer, the third optical prism, and the fourth optical
prism are configured to cause at least a portion of light emitted
by the light source to form a third beam and a fourth beam that
interfere with each other; and the second interferometer measures
the interference between the third beam and the fourth beam.
10. The borehole tool of claim 1, further comprising a second
interferometer; a second chamber containing a second sliding weight
including a third optical prism; a fourth optical prism located
within the second chamber; a second tilt measuring device; and the
timing device operatively associated with the second
interferometer; wherein: the second sliding weight is movable
within the second chamber; the light source, the second
interferometer, the third optical prism, and the fourth optical
prism are configured to cause at least a portion of light emitted
by the light source to form a third beam and a fourth beam that
interfere with each other; the second interferometer measures the
interference between the third beam and the fourth beam; and the
second tilt measuring device measures an angle of the second
chamber relative to vertical.
11. A method for determining a gravitational acceleration
comprising: providing a borehole tool comprising a chamber with a
sliding weight; propelling the sliding weight upward in the
chamber; determining an upward distance and a downward distance
traveled by the propelled sliding weight during a select time
period; determining an angle of the chamber relative to vertical
during the select period of time; and determining the gravitational
acceleration using the determined upward and downward distances
traveled by the sliding weight, a time length of the select period
of time, and the determined angle.
12. The method of claim 11, wherein the borehole tool further
comprises a tilt measuring device to measure the angle of the
chamber relative to vertical.
13. The method of claim 11, wherein: the borehole tool further
comprises an interferometer, a timing device, a first optical
prism, and a light source; the sliding weight includes a second
optical prism; and the interferometer, the timing device, the first
and second optical prisms, and the light source are operatively
associated for determining the upward and downward distance
traveled by the sliding weight during the select period of
time.
14. The method of claim 11, wherein the select period of time
includes a time when the sliding weight reaches a maximum upward
position.
15. The method of claim 11, wherein the select period of time
includes an upward time period and a downward time period.
16. The method of claim 15, wherein the upward and the downward
time period are substantially equal.
17. The method of claim 16, wherein the gravitational acceleration
is determined using the following equation:
g.sub.0=(X.sub.i+X.sub.j)/(cos (.theta.)t.sup.2) where g.sub.0 is
the gravitational acceleration; X.sub.i is the downward distance
traveled by the sliding weight during the upward time period;
X.sub.j is the upward distance traveled by the sliding weight
during downward time period; .theta. is the angle of the first
chamber relative to vertical; and t is either the upward time
period or the downward time period.
18. A method for determining a density of an underground rock layer
comprising: providing a borehole tool comprising an interferometer,
a light source operatively associated with the interferometer, a
chamber containing a sliding weight including a first optical prism
operatively associated with the interferometer, a propulsion device
operatively associated with the sliding weight, a second optical
prism located within the chamber and operatively associated with
the first optical prism and the interferometer, a tilt measuring
device operatively associated with the chamber, and a timing device
operatively associated with the interferometer; positioning the
borehole tool at a first select depth underground near the
underground rock layer; launching the sliding weight upward;
determining an upward distance and a downward distance traveled by
the launched sliding weight during a select period of time;
determining an angle from vertical of the chamber during the select
period of time; and determining a gravitational acceleration using
the determined upward and determined downward distances traveled by
the sliding weight, a time length of the select time period, and
the determined angle from vertical of the chamber.
19. The method of claim 18, further comprising: positioning the
borehole tool at a second select depth underground near the
underground rock layer; and repeating the steps of launching the
sliding weight, determining an upward distance and a downward
distance, determining an angle, and determining a gravitational
acceleration.
20. The method of claim 19, further comprising determining a
density of the underground rock layer using the first and second
determined gravitational accelerations, the first select depth, and
the second select depth.
21. The method of claim 20, wherein the density of the underground
rock layer is determined using the following equation: .rho. b /
.DELTA. g .DELTA. z ##EQU00002## where .rho..sub.b is the density
of the underground rock layer; .DELTA.g is the difference between
the first and second determined gravitational accelerations; and
.DELTA.z is the difference between the first and second select
depths.
22. A method of claim 18, further comprising: the borehole tool
further comprising a second interferometer, the light source
operatively associated with the second interferometer, a second
chamber containing a second sliding weight including a third
optical prism operatively associated with the second
interferometer, a second propulsion device operatively associated
with the second sliding weight, a fourth optical prism located
within the second chamber and operatively associated with the third
optical prism and the second interferometer, a second tilt
measuring device operatively associated with the second chamber,
and the timing device operatively associated with the second
interferometer; launching the second sliding weight upward;
determining an upward distance and a downward distance traveled by
the launched second sliding weight during a second select period of
time; determining an angle from vertical of the second chamber
during the second select period of time; and determining a second
gravitational acceleration using the determined upward and
determined downward distances traveled by the second sliding
weight, a time length of the second select time period, and the
determined second angle from vertical of the second chamber.
23. The method of claim 22, further comprising determining a
density of the underground rock layer using the first and second
determined gravitational accelerations.
24. The method of claim 23, wherein the first interferometer and
the second interferometer are a select distance apart.
25. The method of claim 24, wherein the density of the underground
rock layer is determined using the following equation
.eta..sub.b/.DELTA.g/.DELTA.z where .rho..sub.b is the average
density of the underground rock layer proximate the first select
depth; .DELTA.g is the difference between the first and second
determined gravitational accelerations; and .DELTA.z is the select
distance between the first and second interferometers.
26. A borehole tool system comprising: a weight in a tube, wherein
the weight is displaceable along the tube an upward distance and a
downward distance during a displacement cycle, wherein the upward
and downward distances are substantially similar; an incline
measuring device configured to measure a tube incline during the
displacement cycle; a timer configured to measure a duration of the
upward and downward distances; and a processor for calculating a
gravitational acceleration from the upward and downward distances,
the tube incline and the duration of the upward and downward
distances.
27. The system of claim 26, further comprising a second weight in a
second tube.
28. The system of claim 26, wherein the tube is a vacuum tube and
the system does not include an ion vacuum pump.
29. The system of claim 26, further comprising a light source, a
stationary prism, and an interferometer that receives a light beam,
that is generated by the light source and reflected from the
stationary prism.
30. The system of claim 29, wherein the weight includes a prism
that reflects the light beam.
31. A system of claim 26, further comprising a second weight in the
tube.
32. A method for determining a density of an underground rock layer
comprising: providing a borehole tool including a first borehole
gravimeter and a second borehole gravimeter positioned a select
distance from the first borehole gravimeter; positioning the
borehole tool at a first select depth underground near the
underground rock layer; determining a first gravitational
acceleration using the first borehole gravimeter and a second
gravitational acceleration using the second borehole gravimeter;
and determining a density of the underground rock layer proximate
the first select depth using the first gravitational acceleration
and the second gravitational acceleration.
33. The method of claim 32, wherein the density of the underground
rock layer at the first select depth is determined using the
following equation: .rho. b / .DELTA. g .DELTA. z ##EQU00003##
where .rho..sub.b is the average density of the underground rock
layer proximate the first select depth; .DELTA.g is the difference
between the first and second determined gravitational
accelerations; and .DELTA.z is the select distance between the
first and second gravimeters.
34. The method of claim 32, wherein the distance between the first
and second gravimeters is calibrated on the surface of the earth
using an absolute surface gravimeter.
35. The method of claim 32, wherein the determined density
represents an average density of the underground rock layer
adjacent to the borehole tool between the first borehole gravimeter
and the second borehole gravimeter.
36. A borehole tool comprising: a first interferometer; a light
source; a chamber containing a first sliding weight including a
first optical prism and a second sliding weight including a second
optical prism; a third optical prism located within the chamber; a
tilt measuring device; and a timing device operatively associated
with the first interferometer; wherein: the first sliding weight is
movable within the chamber; the second sliding weight is movable
within the chamber; the light source, the first interferometer, the
first optical prism, and the third optical prism are configured to
cause at least a portion of light emitted by the light source to
form a first beam and a second beam that interfere with each other;
the first interferometer measures the interference between the
first beam and the second beam; and the tilt measuring device
measures an angle of the chamber relative to vertical.
37. The borehole tool of claim 36, further comprising: a second
interferometer; a fourth optical prism located within the chamber;
the light source, the second interferometer, the second optical
prism, and the fourth optical prism are configured to cause at
least a portion of light emitted by the light source to form a
third beam and a fourth beam that interfere with each other; and
the second interferometer measures the interference between the
third beam and the fourth beam.
38. The borehole tool of claim 37, wherein a product of a
gravitational acceleration by a coefficient of friction is
determined by dropping the first and second sliding weights in the
chamber, determining a downward distance traveled by the first
sliding weight during a select time period using the first
interferometer and the timing device, determining a downward
distance traveled by the second sliding weight during the select
time period using the second interferometer and the timing device,
measuring an angle of tilt of the chamber during the select time
period using the tilt measuring device, and using the determined
downward distances of the first and second sliding weights, a
length of time of the select time period, and the angle of tilt to
calculate the product of the gravitational acceleration by the
coefficient of friction.
39. A borehole tool comprising: a means for containing a weight,
the weight displaceable within the weight containing means for an
upward and a downward distance during a displacement cycle and the
upward and downward distances are substantially similar; a means
for measuring an incline of the weight containing means during the
displacement cycle; a means for measuring a duration of the upward
and downward distances; and a means for calculating a gravitational
acceleration from the upward and downward distances, the incline of
the weight containing means, and the durations of the upward and
downward distances.
40. The borehole tool of claim 39, further comprising a second
weight in a second means for containing a weight.
41. The borehole tool of claim 39, further comprising: a means for
producing a light beam; a means for at least partially redirecting
the light beam; and a means for determining interferences between a
first and a second portion of the light beam.
42. The borehole tool of claim 41, wherein the weight includes a
second means for at least partially redirecting the light beam.
43. A borehole tool comprising: a means for determining
interferences between at two light beams; a means for producing a
light beam; a means for containing a sliding weight including a
first means for at least partially reflecting the light beam; a
second means for at least partially reflecting the light beam, the
second means located within the sliding weight containing means; a
means for measuring an angle; and a means for measuring time
operatively associated with the interference determining means;
wherein: the sliding weight is movable within the sliding weight
containing means; the light producing means, the interference
determining means, the first reflective means and the second
reflective means are configured to cause at least a portion of
light emitted by the light producing means to form a first beam and
a second beam that interfere with each other; the interference
determining means measures the interference between the first beam
and the second beam; and the angle measuring means measures an
angle of the sliding weight containing means relative to
vertical.
44. The borehole tool of claim 43, further comprising a means for
propelling the sliding weight upward in the sliding weight
containing means.
45. The borehole tool of claim 43, further comprising: a second
means for determining interferences between at least two light
beams; a second sliding weight contained within the sliding weight
containing means, the second weight including a third means for at
least partially reflecting the light beam; a fourth means for at
least partially reflecting the light beam located within the
sliding weight containing means; wherein: the second sliding weight
is movable within the sliding weight containing means; the light
producing means, the second interference determining means, the
third and fourth reflective means are configured to cause at least
a portion of light emitted by the light producing means to form a
third beam and a fourth beam that interfere with each other; and
the second interference determining means measures the interference
between the third beam and the fourth beam.
46. The borehole tool of claim 43, further comprising a second
means for determining interferences between at least two light
beams; a second means for containing a second sliding weight
including a third means for at least partially reflecting the light
beam; a fourth means for at least partially reflecting the light
beam, the fourth reflective means located within the second sliding
weight containing means; a second means for measuring an angle; and
the timing measuring means operatively associated with the second
interference determining means; wherein: the second sliding weight
is movable within the second sliding weight containing means; the
light producing means, the second interference determining means,
the third and fourth reflective means are configured to cause at
least a portion of light emitted by the light producing means to
form a third beam and a fourth beam that interfere with each other;
the second interference determining means measures the interference
between the third beam and the fourth beam; and the second angle
measuring means measures an angle of the second sliding weight
containing means relative to vertical.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims under 35 U.S.C. .sctn.119(e) the
benefit of U.S. Provisional Application No. 60/822,188, entitled
"Sliding Weight Borehole Gravimeter" and filed on Aug. 11, 2006,
which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates generally to instruments for
measuring a rock layer's density, and more particularly to a
borehole gravimeter.
BACKGROUND OF THE INVENTION
[0003] Rock density is an important parameter for assessing
underground oil and gas reservoirs because it can provide an
indication of the storage capacity (porosity) and the fluid type
(gas/oil/water) in a rock's pores. Density measuring instruments
such as induced gamma-gamma probes and gravimeters may be run at
the end of a wireline in a well to measure the density of rock
layers around the hole. Conventional density measuring instruments
can typically measure rock density up to about 30 cm from the hole.
In contrast, gravimeters can potentially measure rock density up to
100 m from the hole and can provide information on the behavior of
a much larger volume of a hydrocarbon reservoir (Gournay, Luke S.
and Maute, Robert E., Detection of Bypassed Gas Using Borehole
Gravimeter and Pulsed Neutron Logs, The Log Analyst, v. 23, No. 3,
pp. 27-32, 1982; Van Popta, J. et al., Use of Borehole Gravimetry
for Reservoir Characterization and Fluid Saturation Monitoring, SPE
20896, 1990).
[0004] Gravimeters are routinely used on the earth's surface.
Surface gravimeters typically use either a Lacoste-Romberg spring
sensor, quartz or a falling weight sensor. For measuring rock
densities underground, gravimeters using Lacoste-Romberg sensors
are used in boreholes. Such gravimeters generally have diameters
greater than 4 inches (10.2 cm). These relatively large diameters
severely limit their use in oil well boreholes since oil well
production tubing typically has an inside diameter of 2 to 4 inches
(5.1 to 10.2 cm). These gravimeters also require near vertical
holes, which further limits their use since many oil well boreholes
deviate significantly from the vertical, up to 90.degree..
[0005] Accordingly, what is needed in the art is an improved device
and method for measuring rock densities around boreholes.
BRIEF SUMMARY OF THE INVENTION
[0006] A borehole tool is disclosed herein. In one embodiment, the
borehole tool includes an interferometer, a light source, a chamber
containing a sliding weight including a first optical prism, a
second optical prism located within the chamber, a tilt measuring
device, and a timing device operatively associated with the
interferometer. The sliding weight is movable within the chamber.
The light source, the interferometer, the first optical prism, and
the second optical prism are configured to cause at least a portion
of light emitted by the light source to form a first beam and a
second beam that interfere with each other. The interferometer
measures the interference between the first beam and the second
beam. The tilt measuring device measures the angle of the chamber
relative to vertical.
[0007] A method for determining a gravitational acceleration is
disclosed herein. In one embodiment, the method includes providing
a borehole tool including a chamber with a sliding weight,
propelling the sliding weight upward in the chamber, determining an
upward distance and a downward distance traveled by the propelled
sliding weight during a select time period, determining an angle of
the chamber relative to vertical during the select period of time,
and determining the gravitational acceleration using the determined
upward and downward distances traveled by the sliding weight, a
time length of the select period of time, and the determined
angle.
[0008] A method for determining a density of an underground rock
layer is disclosed herein. In one embodiment, the method includes
providing a borehole tool, positioning the borehole tool at a first
select depth underground near the underground rock layer, launching
a sliding weight upward, determining an upward distance and a
downward distance traveled by the launched sliding weight during a
select period of time, determining an angle from vertical of the
chamber during the select period of time, and determining a
gravitational acceleration using the determined upward distance and
the determined downward distances traveled by the sliding weight, a
time length of the select time period, and the determined angle
from vertical of the chamber. In one embodiment, the borehole tool
includes an interferometer, a light source operatively associated
with the interferometer, the chamber containing the sliding weight,
a propulsion device operatively associated with the sliding weight,
a second optical prism located within the chamber and operatively
associated with the interferometer, a tilt measuring device
operatively associated with the chamber, and a timing device
operatively associated with the interferometer. The sliding weight
includes a first optical prism operatively associated with the
interferometer and the second optical prism.
[0009] A borehole tool system is disclosed herein. In one
embodiment, the system includes a weight in a tube, an incline
measuring device, a timer and a processor. The weight is
displaceable along the tube an upward distance and a downward
distance during a displacement cycle, wherein the upward and
downward distances are substantially similar. The incline measuring
device is configured to measure a tube incline during the
displacement cycle. The timer is configured to measure a
displacement cycle time period. The processor calculates a
gravitational acceleration from the upward and downward distances,
the tube incline and time period.
[0010] Yet another method for determining a density of an
underground rock layer is disclosed herein. In one embodiment, the
method includes providing a borehole tool including a first
borehole gravimeter and a second borehole gravimeter positioned a
select distance from the first borehole gravimeter, positioning the
borehole tool at a first select depth underground near the
underground rock layer, determining a first gravitational
acceleration using the first borehole gravimeter, determining a
second gravitational acceleration using the second borehole
gravimeter, and determining a density of the underground rock layer
at the first select depth using the first gravitational
acceleration and the second gravitational acceleration.
[0011] Yet another borehole tool is disclosed herein. In one
embodiment, the borehole tool includes a first interferometer, a
light source, a chamber containing a first sliding weight including
a first optical prism and a second sliding weight including a
second optical prism, a third optical prism located within the
chamber, a tilt measuring device, and a timing device operatively
associated with the interferometer. The first and second sliding
weights are movable within the chamber. The light source, the first
interferometer, the first optical prism, and the third optical
prism are configured to cause at least a portion of light emitted
by the first light source to form a first beam and a second beam
that interfere with each other. The first interferometer measures
the interference between the first beam and the second beam. The
tilt measuring device measures an angle of the chamber relative to
vertical.
[0012] Yet another borehole tool is disclosed herein. The borehole
tool includes a means for containing a weight, a means for
measuring an incline of the weight containing means during a
displacement cycle of the weight, a means for measuring a duration
of upward and downward distances of the weight, and a means for
calculating a gravitational acceleration from the upward and
downward distances, the incline of the weight containing means, and
the durations of the upward and downward distances. The weight is
displaceable within the weight containing means for the upward and
downward distance during the displacement cycle. The upward and
downward distances are substantially similar.
[0013] Still yet a further borehole tool is disclosed herein. The
borehole tool includes a means for determining interferences
between at least two light beams, a means for producing a light
beam, a means for containing a sliding weight including a first
means for at least partially reflecting the light beam, a second
means for at least partially reflecting the light beam, a means for
measuring an angle, and a means for measuring time. The second
light reflecting means is located within the sliding weight
containing means. The means for measuring time is operatively
associated with interference determining means. The sliding weight
is movable within the sliding weight containing means. The light
producing means, the interference determining means, the first
reflective means and the second reflective means are configured to
cause at least a portion of light emitted by the light producing
means to form a first beam and a second beam that interfere with
each other. The interference determining means measures the
interference between the first beam and the second beam. The angle
measuring means measures an angle of the sliding weight containing
means relative to vertical.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 depicts a schematic view of a first embodiment of a
borehole tool with a borehole gravimeter.
[0015] FIG. 2 depicts a schematic view of a second embodiment of a
borehole tool with two borehole gravimeters.
[0016] FIG. 3 depicts a schematic view of a third embodiment of a
borehole tool with a borehole gravimeter.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Oilfield operators typically want to minimize pulling
production tubing from an oil well borehole because it interrupts
production and carries the risk that production of the entire well
will be lost. Production tubing typically has an inside diameter of
2 to 4 inches (5.1 cm to 10.2 cm). Thus, rock density measuring
devices should preferably fit within an instrument with a diameter
of 1 11/16 inches (4.28 cm), which is an oil industry standard for
production logging tools. Described herein are density measuring
devices that take the form of borehole gravimeters. The borehole
gravimeters may be fitted in an 1 11/16'' (4.28 cm) diameter
logging tool and may be run in wells with deviations from the
vertical of up to 80 degrees. Further, these borehole gravimeters
may withstand temperatures and pressures of at least 350.degree. F.
(177.degree. C.) and 20,000 psi.
[0018] FIG. 1 depicts a schematic view of a first embodiment of a
borehole tool 100 containing a borehole gravimeter 105. The logging
tool 100 may include a housing 110 for containing various
components of the borehole gravimeter 105. The borehole gravimeter
105 includes a tilt measuring device 115, a timing device 120, a
vacuum chamber or tube 125, an interferometer 130, and a light
source 135. The tilt measuring device 115 measures the angle of
tilt of the borehole gravimeter 105 relative to vertical. The tilt
measuring device 115 may be any commercially available tilt-meter
used with borehole technology (such as a Lippmann two-channel
borehole tilt-meter), an array of tilt-meters, or any other tilt
measuring device or system that can withstand high borehole
temperatures and pressures. The timing device 120 may be an atomic
clock or other suitable timing device for obtaining required
measurement accuracies in the range of 1 in 10.sup.9.
[0019] The vacuum chamber 125 contains a magnetic sliding weight
140, which may slide up and down within the vacuum chamber 125 and
may be generally semi-cylindrically shaped or any other suitable
shape. The sliding weight 140 may be propelled upward using an
induction coil or other suitable device contained within a catcher
145 for catching the sliding weight 140. Launching the sliding
weight upwards with a magnetic field generated by the coil, or by
other means using another suitable device, starts a movement cycle
in which the sliding weight 140 moves upward in the vacuum chamber
125 until it reaches the top of its trajectory and then moves
downward in the vacuum chamber 125 until stopped by the catcher
145.
[0020] The vacuum chamber 125 may have an internal surface with a
Teflon or other suitable coating to reduce friction between the
contact surfaces of the sliding weight 140 and the vacuum chamber
125. Further friction reductions between the contact surfaces may
be obtained by maintaining the temperature of the vacuum chamber
125 above approximately 80.degree. C., using a sledge design,
and/or equipping the sliding weight 140 with ball bearings or other
suitable friction reduction devices.
[0021] The sliding weight 140 has an optical prism 150 for
reflecting a portion of a light beam 155 to a stationary optical
prism 160 located near the bottom end of the vacuum chamber 125.
The stationary optical prism 160, in turn, reflects light received
from the sliding weight's optical prism 150 to a mirror 175 for
reflection to an interference area 165 in the interferometer 130,
which also receives another portion of the light beam 155 emitted
by the light source 135. The light source 135 may be a laser or
other suitable light emitting device, which can be located outside
the chamber 125 or at any other suitable location relative to the
chamber 125. The interferometer 130 may have two or more fully
reflective and/or semi-transparent mirrors that cause the portion
of the light beam 155 received from the light source 135 to be
directed to the sliding weight's prism 150 and the second portion
to the interference area 165. The mirrors may be located within the
chamber 125, outside the chamber 125, or some combination of inside
and outside the chamber 125. In one embodiment, as shown in FIG. 1,
the fully reflective mirror 170 is located outside the chamber 125
to reflect light 155 from the light source 135 into the chamber 125
and onto the semi-transparent mirror 175, and the semi-transparent
mirror 175 is located within the chamber 125.
[0022] With reference to FIG. 1, the light beam 155 exits the light
source 135 and travels along a first light path 180 to the fully
reflective mirror 170. The fully reflective mirror 170 redirects
the light beam 155 towards the semi-transparent mirror 175 along a
second light path 185. A first portion of the light beam 155 is
transmitted through the semi-transparent mirror 175 and travels
along a third light path 190 to the light interference area 165 in
the interferometer 130. A second portion of the light beam 155 is
redirected by the semi-transparent mirror 175 to travel towards the
sliding weight 140 along a fourth light path 195. The optical prism
150 of the sliding weight 140 redirects the second portion of the
light beam 155 towards the stationary optical prism 160 along a
fifth light path 200. The stationary optical prism 160 redirects
the second portion of the light beam 155 back towards the
semi-transparent mirror 175 along a sixth light path 205. The
semi-transparent mirror 175 then redirects the second portion of
the light beam 155 towards the light interference area 165 along a
seventh light path 210. The first portion of the light beam 155
interferes with the second portion in the light interference area
165.
[0023] The interference of light received directly from the light
source 135 (i.e., the first portion of the light beam 155) with
light received from the stationary prism 160 (i.e., the second
portion of the light beam 155) creates light interference fringes,
which provide an indication of the distance that the sliding weight
140 travels during a certain measuring period. The measuring period
is determined using the timing device 120 (e.g., an atomic clock).
The interferometer 130 may include an optical fringe detector 215
or other suitable device to measure the interference fringes to
enable the distance traveled by the sliding weight 140 during the
measuring period to be determined. Further, the timing device 120
may be used in conjunction with the optical fringe detector 215 to
determine the distance traveled by the sliding weight 140 during
any select time period. More particularly, the timing device 120 is
utilized in conjunction with the interferometer 130 to establish
the number of fringes measured during the select period of time,
which is then used to establish the distance traveled by the
sliding weight 140 during the select period of time.
[0024] For the borehole gravimeter 105 depicted in FIG. 1, the
downward distance X.sub.i traveled by the sliding weight 140 over a
time period t.sub.i during the downward motion may be calculated,
if second and higher order powers of time multiplied by the
gravitational gradient (.gamma.) are ignored, using the
equation:
X.sub.i=X.sub.oi+V.sub.oit.sub.i+1/2g.sub.0(cos .theta.-.eta. sin
.theta.)t.sub.i.sup.2 (1)
[0025] where X.sub.oi is the initial position of the sliding
weight; [0026] V.sub.oi is the initial velocity of the sliding
weight; [0027] g.sub.0 is the gravitational acceleration; [0028]
.theta. is the tilt angle of the borehole gravimeter relative to
vertical; and [0029] .eta. is the coefficient of friction between
the sliding weight and the vacuum chamber; and the upward distance
X.sub.j traveled by the sliding weight 140 over a time period
t.sub.j during the upward motion may be calculated using the
equation:
[0029] X.sub.j=X.sub.oj+V.sub.ojt.sub.j+1/2g.sub.0(cos
.theta.+.eta. sin .theta.)t.sub.j.sup.2 (2)
[0030] where X.sub.oj is the initial position of the sliding
weight; and [0031] V.sub.oj is the initial velocity of the sliding
weight. Further, when the sliding weight 140 is propelled upwards
by the induction coil, it will momentarily have zero velocity as it
stops at the top of its travel trajectory. This point in time can
be determined with great accuracy using the interferometer 130
because this is when the first derivative of the phase versus time
is zero. By taking this point as the starting point for the time
periods of upward and downward travel used in equations 1 and 2,
X.sub.oi, X.sub.oj, V.sub.oj, and V.sub.oi are equal to zero.
Accordingly, equations 1 and 2 may be rewritten as:
[0031] X.sub.i=1/2g.sub.0(cos .theta.-.eta. sin
.theta.)t.sub.i.sup.2 (3)
and
X.sub.j=1/2g.sub.0(cos .theta.+.eta. sin .theta.)t.sub.2 (4)
Setting X.sub.oi, X.sub.oj, V.sub.oj, and V.sub.oi equal to zero
additionally results in the elimination of the terms for the second
and third order powers of time multiplied by the gravitational
gradient, which were ignored in equations 1 and 2. This leaves only
time to the fourth power multiplied by the gravitational constant
(i.e., .gamma.g.sub.0t.sup.4/24) as the term ignored when using
equations 1 and 2. This fourth power of time term will generally be
very small compared to numbers resulting from the terms used in
equations 1 and 2, and thus will generally not significantly impact
the calculated distances using equations 1 and 2. If the length of
the time period of the sliding weight's downward travel is selected
equal to the length of the time period of the sliding weight's
upward travel (i.e., t.sub.i=t.sub.j=t), then equations 3 and 4 may
be added together to obtain the following equation:
X.sub.i+X.sub.j=g.sub.0(cos .theta.)t.sup.2 (5)
and equation 3 may be subtracted from equation 4 to obtain the
following equation:
X.sub.j-X.sub.i=.eta.g.sub.0(sin .theta.)t.sup.2 (6)
[0032] By combining the downward and upward distances traveled by
the sliding weight 140 during a trajectory cycle, the effect of the
coefficient of friction is eliminated. By subtracting the downward
distance traveled by the sliding weight 140 from its upward
distance, the coefficient of friction can be determined using, as
described in more detail below, the gravitational acceleration
determined from equation 5. Additionally, the fourth order power of
time term ignored in equations 1 and 2 will cancel out in equation
6, thus resulting in substantially no difference in coefficient of
friction values obtained between using equations that take into
account the fourth order power of time terms and using those that
ignore this term.
[0033] As further described below, in some embodiments, multiple
measurements of upward and downward distances traveled by the
sliding weight 140 during a period of time at a specific location
are taken to enable multiple calculations for the gravitational
acceleration and the coefficient of friction to be done. These
multiple calculations help to average out the error in the
gravitational acceleration calculations, and provide a continuous
check for the constancy of the coefficient of friction for the
vacuum chamber.
[0034] Elimination of the coefficient of friction from the
determination of the gravitational acceleration by adding together
the upward and downward distances traveled by the sliding weight
140 means that the sliding weight 140 may contact the walls of the
vacuum chamber 125 so long as the upward and downward paths of the
sliding weight 140 are substantially similar and the angle of the
gravimeter 105 relative to vertical remains substantially constant
during the upward and downward travel cycle of the sliding weight
140. Further, the effect of any remaining air resistance in the
vacuum chamber 125 also cancels out in a manner similar to the
canceling out of the coefficient of friction when both upward and
downward movement of the sliding weight 140 are combined to
determine the gravitational acceleration. Thus, the vacuum
requirements in the vacuum chamber 125 may be relaxed. With less
strict vacuum requirements, an ion vacuum pump, which is typically
used in surface falling weight gravimeters, may be eliminated.
Elimination of the ion vacuum pump reduces the required size of the
borehole gravimeter 105 since this component tends to be rather
large and generally does not fit in a 2'' (5.08 cm) diameter or
less wireline borehole logging tool.
[0035] Returning to equation 5, the distance traveled upward by the
sliding weight 140 for the selected time period length prior to the
sliding weight 140 reaching its top trajectory may be determined.
Additionally, the distance traveled downward by the sliding weight
140 from its top trajectory over the same time period length may
also be determined. Further, since the angle of the borehole
gravimeter 105 related to the vertical at any time is known, the
angle of the borehole gravimeter relative to vertical during the
selected time periods may be determined. Thus, every variable of
equation 5 is known except for the gravitational acceleration,
which may be determined by inputting the data for the known
variables into the equation.
[0036] Once the gravitational acceleration is determined, the rock
density around the borehole may be determined. Specifically, the
density .rho..sub.b of a rock layer with thickness .DELTA.z is
proportional to the difference in gravitational acceleration
.DELTA.g measured over the layer:
.rho. b / .DELTA. g .DELTA. z ( 7 ) ##EQU00001##
Therefore, once the gravitational accelerations are known along the
depth of a rock layer, the rock layer's density may be determined
using equation 7.
[0037] In operation, the tool 100 containing the borehole
gravimeter 105 is inserted into a borehole. At a desired depth for
measuring the gravitational acceleration, the borehole gravimeter
105 is maintained in a substantially stationary position by
clamping the tool 100 to the borehole wall or by using any other
suitable mechanism or method to maintain the borehole gravimeter
105 in a substantially stationary position. The sliding weight 140
is propelled upwards using the induction coil or any other suitable
means. Measurements using the tilt meter 115, the interferometer
130, and the timing device 120 are taken during the sliding
weight's upward and downward path of travel. These measurements are
provided to a processor 220, which utilizes the measurements to
calculate the gravitational acceleration at the location of the
measurements. In one embodiment, the processor 220 is part of the
gravimeter 105. In another embodiment; the processor 220 is
separate from the gravimeter 105, but electrically coupled to the
gravimeter 105 via a hardwire or wireless connection.
[0038] If desired, the sliding weight 140 may be launched upwards
multiple times to obtain multiple measurements at a desired depth
in the borehole. In some embodiments, at least one hundred upward
and downward cycles are measured at the desired depth. This allows
for the estimation of the uncertainty in the gravitational
acceleration measurement and compensates for small movements that
could effect one measurement. Similarly, in some embodiments during
any one upward and downward cycle, multiple measurements are taken
using the tiltmeter 115 and averaged to account for small
variations in the angle of the gravimeter 105 relative to vertical
that occur due to micro-seismic movements in the rock.
[0039] After completion of at least one upward and downward
movement cycle of the sliding weight 140, the borehole gravimeter
105 is moved to another depth in the borehole. At the new depth,
the steps for measuring the required parameters for determining the
gravitational acceleration at the new location are repeated, and
the new measurements are also provided to the processor 220 to
calculate a second gravitational acceleration. The processor 220
utilizes the two gravitational accelerations to determine a
gravitational gradient, which is utilized to determine the mass
density of the rock layer.
[0040] To take direct underground rock density measurements, two
borehole gravimeters 300, 305 may be arranged within a housing 110
of a wireline logging tool 310 to work in tandem as shown in FIG.
2. In such an embodiment, the density of a rock layer is the ratio
of the difference between the gravitational accelerations measured
by each borehole gravimeter 300, 305 and the distance between the
two borehole gravimeters 300, 305. The two borehole gravimeters
300, 305 may be similar to the borehole gravimeter 105 described
for the first embodiment of the tool 100. For the embodiment of the
tool 310 shown in FIG. 2, like numbers may be used for components
that are the same or similar to components for the first embodiment
of a logging tool 100.
[0041] Each borehole gravimeter 300, 305 may have its own tilt
measuring device 115a,b as shown in FIG. 2, or may share a tilt
measuring device. In one embodiment, the vacuum chambers 125a,b and
sets of prisms for the gravimeters 300, 305 are about one meter
apart. In other embodiments, the distance between the vacuum
chambers 125a,b and sets of prisms are more or less than one meter
apart. The first and second borehole gravimeters 300, 305 may each
use the same processor 220 and timing device 120 as shown in FIG.
2, or may use separate processors and timing devices.
[0042] The borehole gravimeters 300, 305 may each use the same
light source 135 as shown in FIG. 2, or may use different light
sources. When using the same light source 135, the light source 135
may emit a light beam 155 that travels along a first pathway 315 to
a semi-transparent mirror 320 for the first interferometer 130a of
the first borehole gravimeter 300. At the semi-transparent mirror
320, a first portion of the light beam 155 may be redirected for
use in the first borehole gravimeter 300 along a second pathway 325
and a second portion of the light beam 155 may continue to travel
along the first pathway 315 towards a fully reflective mirror 330
for the second interferometer 130b. At the fully reflective mirror
330, the second portion of the light beam 155 may be redirected for
use in the second borehole gravimeter 305 along a third pathway.
Once redirected into the first and second borehole gravimeters 300,
305, the first and second portions of light beam 155 may travel
along light paths substantially similar to the light paths
described above with respect to the embodiment of the borehole
gravimeter 105 depicted in FIG. 1 to create light interference
patterns for determining the distances traveled by the sliding
weights in each gravimeter 300, 305 as described in more detail
above.
[0043] Each borehole gravimeter 300, 305 is operated as described
above to obtain measurements for determining a gravitational
acceleration. Using equation 7, the determined density represents
an average density of the underground rock layer adjacent to the
borehole tool 310 between the first borehole gravimeter 300 and the
second borehole gravimeter 305. The difference between the
gravitational accelerations determined from the measurements made
using each borehole gravimeter 300, 305 indicates the gravitational
gradient .DELTA.g. The distance .DELTA.z between the two sensors is
known with the required accuracy of 1 mm (0.1%). However, since the
exact distance between the tops of the trajectories of the two
sliding weights is not known with the required accuracy,
calibration on the surface with absolute gravimeters may be
necessary.
[0044] FIG. 3 depicts a third embodiment of a borehole tool 400
containing a borehole gravimeter 405. Like numbers may be used for
the components that are the same as or similar to the components of
the first embodiment. The third embodiment is similar to the first
embodiment except the induction coil is replaced with a lifting
mechanism 410 for returning two sliding weights 140a,b to a drop
position and the tool 400 includes two interferometers 130a,b.
Also, one or more light sources may emit a separate light beam for
each sliding weight 140a,b. For example, a single light source 135
as shown in FIG. 3 may emit a first light beam 415 for the first
sliding weight 140a and a second light beam 420 for the second
sliding weight 140b. Each light beam may be directed by a
reflective mirror 425 to first and second semi-transparent mirrors
430, 435 respectively. As described above for the first embodiment,
a first portion of each light beam 415, 420 passes through its
respective semi-transparent mirror 430, 435 to proceed directly to
an interference area in its interferometer 130a,b and a second
portion is redirected along a pathway that includes its respective
sliding weight 140a,b and stationary prism 160a,b. Also as
described above with respect to the first embodiment, the first and
second portions for each light beam 415, 420 interfere in an
interference area within the beam's respective interferometers
130a,b. The interference of the portions of each light beam 415,
420 provide an indication of the distance traveled by the sliding
weight 140a,b associated with the light beam 415, 420. Each
interferometer 130a,b may use the same processor 220 and timing
device 120 as shown in FIG. 3, or may have its own processor and/or
timing device.
[0045] In operation, the sliding weights 140a,b are moved to their
drop position using the lift mechanism 410, which can be a moving
electromagnetic coil, a magnetic elevator or other suitable device.
When the weights 140a,b are released, they travel downward in the
vacuum chamber 125 until their movements are stopped by the
catchers 145a,b. The two weights 140a,b have approximately an equal
mass, but the contact area A.sub.1 of the first weight 140a with
the chamber surface of the first weight is approximately twice the
contact area A.sub.2 of the second weight 140b. The contact areas
can be controlled to a high degree of accuracy by using a sledge or
other suitable design. Assuming that the start velocities V.sub.o
of the two sliding weights 140a,b are equal, the equations that
describe the incremental movements .DELTA.X.sub.k and
.DELTA.X.sub.l of the two weights over the same time interval t
are:
.DELTA.X.sub.k=V.sub.ot+1/2g.sub.0(cos .theta.-A.sub.1.eta.' sin
.theta.)t.sup.2 (8)
and
.DELTA.X.sub.l=V.sub.ot+1/2g.sub.0(cos .theta.-.DELTA..sub.2.eta.
sin .theta.)t.sup.2 (9)
[0046] where g.sub.o is gravitational acceleration; [0047] A.sub.1
is the contact area of the first sliding weight; [0048] A.sub.2 is
the contact area of the second sliding weight; [0049] .theta. is
the tilt angle of the borehole gravimeter relative to vertical; and
[0050] .eta.' is the coefficient of friction per surface unit
between the sliding weight and the vacuum chamber.
Taking the difference between .DELTA.X.sub.k and .DELTA.X.sub.l
yields:
[0051]
.DELTA.X.sub.k-.DELTA.X.sub.l=(A.sub.1-A.sub.2)g.sub.o.eta.'(sin
.theta.)t.sup.2 (10)
In a manner similar to the one described above for the first
embodiment of a borehole gravimeter 100, .DELTA.X.sub.k and
.DELTA.X.sub.l are determined by counting fringes using the
interferometers 130a,b and two light beams 415, 420 with different
wavelengths. Time t is measured with a timing device 120 (e.g. an
atomic clock), the angle .theta. with a tilt-meter 115, and areas
A.sub.1 and A.sub.2 from calibration at the surface. With this
information, the product of g.sub.0 .eta.' may be determined. The
gravitational acceleration g.sub.0 may then be determined by
inserting this calculated product for g.sub.0 .eta.' in either
equation 8 or 9.
[0052] Although the present invention has been described with
reference to example embodiments, persons skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention. The invention
is limited only by the scope of the following claims.
* * * * *