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This invention concerns a well logging apparatus adapted to be moved inside
a borehole to measure the properties of geological formations, and more
particularly to a well logging apparatus comprising a pad adapted to
contact the wall of the borehole and carrying sensors. The invention also
concerns a combination well logging device including such an apparatus.
In the technique of well logging, apparatuses are conventionally used which
comprise a measuring pad adapted to contact the wall of the borehole. This
is the case inter alia in density measuring apparatuses such a the LDT
tool of Schlumberger. In this type of apparatus, the pad is connected to
the body of the apparatus by a coupling mechanism comprising parts hinged
to the body in such as way that the pad can space itself radially from the
body until it contacts the wall, and means are provided to apply the pad
against the wall, these means including a counter-arm pivotally mounted on
the side opposite to the pad and adapted to remain applied to the wall.
In techniques of this type, the quality of the measurements requires the
pad to be in contact with the wall of the borehole. This objective is
difficult to achieve to a greater or lesser extent, depending on the
measurement conditions: inclination of the borehole, diameter of the
borehole, state of the borehole (for example, presence of "hollows", state
of roughness of the wall, etc.), nature of the fluid present in the
borehole, speed of displacement of the apparatus, combination with other
logging apparatuses.
A conventional type of coupling mechanism, used in the DRS sonde of
Schlumberger, is described in U.S. Pat. No. 3,356,146 and more recently in
U.S. Pat. No. 5,198,770. This involves a mechanism in form of an actuator
whose cylinder is connected to the body by a pivot and whose piston rod is
connected to the upper end of the pad, as well as to the end of a
counter-arm, and which further comprises a restraining member acting on
the lower end of the pad in order to restrict its radial extension. This
mechanism does not ensure optimum contact, especially in the presence of
"hollows", because of the "effective length" of the pad, which determines
its ability to accommodate the irregularities of the wall and which should
be as small as possible, is increased by the presence of the coupling
actuator, which actuator represents a length of the same order as that of
the pad proper. Moreover, since the pad and the counter-arm are closely
coupled, significant friction against the counter-arm can provoke
disturbances in the pad, in particular a loss of contact with the wall.
Another type of mechanism, described in the U.S. Pat. Nos. 3,654,470,
4,120,353 and 4,432,143, comprises a support arm connected by a pivot to
the upper end of the pad and which exercises the tractive force on the pad
necessary for its displacement, and a retaining arm connected to the pad
by a pivot engaged in an aperture formed in the longitudinal direction.
The mechanisms such as those described above do not give satisfaction
because the traction exerted on the pad by the support arm has a tendency
to distance the pad from the wall in the presence of "hollows".
U.S. Pat. No. 4,120,353 cited above describes another type of mechanism in
which the pad can space itself a limited distance from the body of the
apparatus while always remaining parallel thereto. This design also fails
to allow the pad to remain in contact with the wall in the presence of
"hollows".
An object of the present invention is a well logging apparatus provided
with a measuring pad adapted to contact the wall of the borehole and which
is better than known devices with regard to contact of the pad with the
wall, especially in the presence of "hollows".
According to the invention there is provided a well logging apparatus
adapted to be moved inside a borehole, the apparatus comprising an
elongate body, a measuring pad adapted to contact the wall of the
borehole, a support device for the pad mounted pivotally on one side of
the body, and means for applying the pad against the wall of the borehole,
characterized in that the support device comprises an arm which carries
the weight of the pad and is connected to the pad substantially in the
central part thereof.
An embodiment of the invention is described below with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic overview of an entire combination logging device
constituting an embodiment of the invention, and formed of three sections;
FIG. 2 shows the geometry of the second section of the device of FIG. 1,
and more particularly its joints with the other sections;
FIGS. 3A and 3B are detail views of one embodiment of the above-mentioned
joints;
FIGS. 4A and 4B are schematic diagrams showing one embodiment of the
coupling mechanism for the second section carrying a pad, respectively in
the closed position and in the open position;
FIGS. 5A and 5B illustrate the behavior of the coupling mechanism of FIGS.
4A and 4B under different operating conditions;
FIG. 6 is view of the contact face of the measuring pad, in an embodiment
that includes two types of sensor that are interleaved;
FIG. 7 shows the pad of FIG. 6 in section on the longitudinal plane
VII--VII of FIG. 6; and
FIGS. 8A and 8B are sections of the pad of FIG. 7 on the lines A--A and
B--B respectively.
The combination well logging device (or "tool string") shown in FIG. 1 is
adapted to be lowered inside a borehole 10 by means of a transmission
cable 11. The cable is connected in conventional manner to surface
apparatus, not shown, comprising in particular a winch and means for
recording and processing the measurement data produced by the logging
device and transmitted through the cable.
The device comprises three sections mounted end to end: a first section 15
forming the upper part of the device, connected by a coupling 16 to the
cable 11; a second section 17 suspended from the first section 15 and
forming an intermediate part; and a third section 18 suspended from the
second section and forming the lower part of the device.
The first section is provided with a resilient member 19 in the form of a
bow, which bears against the wall of the borehole in its middle part and
exerts a force on the first section 15, pressing it against the wall,
along a generatrix diametrically opposite the member 19. Such an
arrangement is conventional in the case of a neutron logging sonde, which
must necessarily be pressed against the wall, in particular in the region
of the neutron source, in order to provide correct measurements.
In addition to a neutron logging sonde such as a CNT tool of Schlumberger,
the first section may suitably include a sonde for measuring natural gamma
radioactivity and a telemetry cartridge forming the interface between the
elements of the device and the cable. An inclinometer sonde, such as the
GPIT tool of Schlumberger, which comprises accelerometers and
magnetometers for all three axes may also be suitably included in this
section.
The second section 17 comprises a body forming a cradle 20 carrying a
measuring pad 21 adapted to be pressed against the wall, and a counter-arm
22 on the side opposite the pad 21. The measuring pad typically comprises
a gamma-gamma device providing a measure of formation density. It can
also, as described above, comprise a "micro-resistivity" device providing
a measurement of the resistivity in the rear zone of the wall of the
borehole, called the invaded zone.
The second section is connected to the first section 15 by a hinge 23 which
allows the second section 17 to be inclined relative to the first section
15. More particularly, in the embodiment shown in FIG. 1 and more clearly
in FIG. 2, the second section can pivot relative to the first section
about an axis perpendicular to the axis of the first section, in a
longitudinal plane containing the abovementioned contact generatrix. This
pivoting is allowed solely away from said generatrix. It should also be
stressed that the hinge 23 does not allow any twisting between the second
section and the first section.
The angular orientation of the body 20 is preferably such that the median
longitudinal plane of the pad 21 is aligned with the abovementioned
contact generatrix, as shown in FIG. 1.
It is to be noted that the electronics cartridge containing the processing
circuits associated with the sensors in the pad 21 can be located in the
first section 15, rather than being attached to the body 20 in the second
section 17. Such a design has the advantage of reducing the length and the
weight of the second section.
The third section 18 comprises one or more spacers 24 (called stand-offs)
which keep it spaced from the wall of the borehole. The third section
comprises a sonde for measuring resistivity, which can be an electrode
sonde of Laterolog type (DLT or ARI tool of Schlumberger), or a sonde with
induction coils (DIT or AIT tool of Schlumberger).
The third section is connected to the second section 17 by a hinge 25
allowing these sections to be inclined relative to each other. In the
described embodiment, this is a hinge of the same type as the hinge 23,
which provides one degree of freedom between the second and third
sections. The hinge 25 is so located that it allows pivoting in the same
plane as the longitudinal plane defined above for the hinge 23. However,
as shown in FIG. 1, the pivoting is allowed only towards the
abovementioned generatrix--that is to say towards the pad 21--even though
the pivoting is allowed away from the said generatrix in the case of the
hinge 23. Moreover, as in the case of the coupling between the first and
second sections, the third section cannot twist relative to the second
section.
FIG. 2 shows the hinges 23 and 25, symbolized by respective pivots 23a and
25a, with the pivotal angles exaggerated compared with reality.
It is noted that the hinge 23 between the first and second sections has an
axis that is offset from the axis I--I of the first section by a distance
b, the said axis being located on the side opposite to the contact
generatrix 26 of the first section. The maximum pivotal angle allowed by
the hinge 23 is an angle .alpha.. A suitable value for this angle around
2.degree..
The hinge 25 has its axis offset relative to the axis III--III of the third
section by a distance c, the said axis being located on the same side as
the contact generatrix 26 and accordingly on the side opposite to the
pivotal axis of the hinge 23. As in this latter case, the desired maximum
pivotal angle is around 2.degree..
The distances b and c are chosen to minimize the risk of loss of contact
between the first section and wall of the borehole under the action of the
weight of the assembly formed by the second and third sections. For this
purpose it is arranged that the center of gravity of the assembly formed
by the second and third sections to be no further from the wall than the
pivot 23a, or to be closer. Thus, in the embodiment shown, the distance b
is selected to be of the same order as the distance c. Typical values are
for example a distance b of 22 mm and a distance c of 22 mm.
FIGS. 3A and 3B show one possible implementation of the hinges 23, 25. If
the case of the hinge 23 is considered, this comprises a tubular part 30
with an axis IV--IV, comprising an end part 31 adapted to be fixed to the
adjacent section, in this case the first section 15. The part 30 has an
annular transverse face 32 at the end opposite the part 31 and two
parallel, longitudinal clevis plates 33, 33' extending from the face 32.
These plates have respective holes 34, 34' with the same transverse axis
B, which axis is spaced from the axis IV--IV by a distance b in accordance
with the preceding remarks. The part of the hinge which is connected to
the second section 17 comprises two clevis plates. 35, 35' associated with
the plates 33, 33' respectively and provided with respective pivots 36,
36' engaged in the holes 34, 34'. The plates 35, 35' are shown in FIG. 3B
as being located outside the plates 33, 33', but they could equally be
located between the plates 33, 33'. Each of the plates 35,35' has an end
surface with two facets which cooperate with the end face 32 of the
tubular part 30 in order to allow the limited pivoting, and that solely in
one sense, of the second section 17 relative to the first section 15. This
surface thus comprises a transverse facet 37 which is accordingly parallel
to the end face 32, and an oblique facet 38 meeting the transverse facet
at an edge which intersects the axis IV--IV. The oblique facet 38 is
located on the side of the pivots 36, as FIG. 3A shows. The angle between
the oblique facet and the transverse facet corresponds to the pivotal
angle .alpha. mentioned above. Moreover, although the transverse facet 37
has been shown for the sake of clarity spaced from the face 32, it should
be noted that the transverse facet abuts the face 32, thus preventing any
pivoting of the plates 35, 35' in counterclockwise sense (in FIG. 3A). The
plates 35, 35' can only pivot clockwise until the oblique facet 38 comes
into abutment with the end face 32.
One implementation of the coupling mechanism of the second section 17
including a measuring pad 21 is now described with reference to the
schematic views of FIGS. 4A and 4B.
The face of the pad 21 for contacting the wall of the borehole is
referenced 40. The pad 21 is supported by an arm 41, one end of which is
connected to the rear face 42 of the pad by a hinge 43, which allows
pivoting between the pad and the arm 41. The arm 41 is extended at its end
remote from the hinge 43 by a bent part or crank 44, whose end is
connected to the body 20 by a fixed pivot 45. The counter-arm 22, whose
outer end 46 is adapted to contact the wall of the borehole on the side
opposite the pad 21, is similarly extended by a bent part or crank 47,
whose end is connected by a pivot 48 to the end of a connecting rod 48'
pivoted to the end of the rod 49 of a piston 50. This piston is mounted in
a cylinder 51 fixed to the body 20 and can be displaced by application of
hydraulic pressure overcoming the action of a spring 52, which acts on the
pivot 48 in the sense opening out the counter-arm 22. The arm 41 and the
counter-arm 22 are connected by a floating pivot 53 located at their
junctions with the crank 44 and the crank 47 respectively but not
connected to the body 20. This arrangement has the effect that, when the
counter-arm 22 comes into contact with the wall of the borehole under the
action of the spring 52, as in the position of FIG. 4B, the arm 41 pivots
until the pad 21 contacts the wall of the borehole. The piston 50 is
actuated to restore the mechanism to the closed position shown in FIG. 4A.
The embodiment shown in FIGS. 4A and 4B also comprises an upper link 55
having its upper end connected to the body 20 by a pivot 56 and having a
pivot pin 57 at its lower end engaging in a slot 58 formed in the upper
part of the pad 21. The link 55 acts to limit the inclination which the
pad 21 can assume relative to the axis of the body 20 but, with the play
of the pivot pin 57 along the slot 58, does not exert any significant pull
on the pad 21 during the displacement of the body. A similar arrangement,
comprising a lower link, can be provided in a symmetrical manner at the
lower part of the pad.
It is to be noted that, in this implementation, the pad 21 is supported by
the single arm 41 and, as a result, the whole of the force exerted on the
pad 21 to move it along the wall of the borehole is transmitted by the
pivot 43. This results in particular in the force being applied without
giving rise to a couple tending to tilt the pad. As shown in FIGS. 4A and
4B, the pivot 43 is preferably located in the central part of the pad 21
(considered in the longitudinal direction), near to the central plane M-M'
of the pad transverse to the axis of the body. The location considered the
most advantageous for the pivot 43 is in the lower half of the pad,
slightly below the central plane M-M', as is shown in FIGS. 4A and 4B.
More specifically, the distance p between the pivot 43 and the central
plane M-M' preferably lies between 0% and 15% of the length of the part 40
of the pad which is adapted to make contact with the wall. With such a
design, the reaction force of the wall of the borehole on the pad acts in
the central zone of the pad, as is desirable for optimum application of
the pad against the wall.
The described implementation ensures satisfactory contact of the pad 21
with the wall of the borehole under diverse working conditions illustrated
in FIGS. 5A and 5B.
FIG. 5A illustrates the simultaneous presence of a "hollow" 60, consisting
of a cavity formed in the wall of the borehole beside the pad 21, and of a
projection or cornice 61 on the opposite side of the wall. FIG. 5A shows
that the coupling mechanism described above is able to absorb such abrupt
variations in the diameter of the borehole: the respective angular
divergences of the counter-arm 22 and of the arm 41 relative to the axis
of the body can vary significantly without affecting the force applied to
the pad 21 by the arm 41 and, on the other hand, the arrangement of arms
connected to the pad 21, namely the arm 41 and the link 55 in the
illustrated embodiment, allows the pad 21 to rest against the wall,
despite the presence of a hollow, in an optimum manner given the
longitudinal dimension of the pad.
FIG. 5B shows an inclined borehole with a ramp 62. The pad 21 is capable of
remaining in contact with the wall along this ramp because of the
inclination which it can assume relative to the body of the sonde.
Equally, the mounting of the body 20 between two hinges, as described
above, is a favorable factor which avoids the pad becoming "caked" against
the wall by the body 20.
It should be noted that the pivotal connection between the arm 41 and the
pad 21 can be implemented in a different way from that described above.
Thus a connection could be provided between two lateral pivots located on
one side and the other of the pad 21, engaging in a fork extending the arm
41.
A preferred embodiment of the pad 41 will now be described with reference
to FIGS. 6, 7 and 8A-8B. As indicated above, it is arranged to combine in
a single pad both gamma-gamma transducers or sensors providing a measure
of the density of the formations and transducers of different type
providing a measure with high vertical resolution of another magnitude,
preferably resistivity, in the zone adjoining the wall of the borehole.
This combination is realized as shown in FIG. 6 by interleaving members of
different types: the gamma-gamma sensors comprise a source of gamma rays
and at least one near detector and one far detector, represented in FIG. 6
by the windows 70, 71 and 72 associated with these components
respectively; the electrodes A0, A1, M, etc., which make up the device for
measuring resistivity are interleaved in the space between the window of
the near detector 71 and that of the far detector 72. This design
minimizes the length of the contact part 40 of the pad, which is
advantageous for good application of the pad against the wall and is
equally favorable from the point of view of combining the density
measurements with the micro-resistivity measurements respectively provided
by these sensors, since errors in the depths attributed to the
measurements arising in particular from variations in the instantaneous
speed of the pad, are avoided in large measure.
In the embodiment shown in the figures, the gamma-gamma device comprises,
apart from the near and far detectors, a backscatter detector located in
the immediate vicinity of the source, in accordance with the teaching of
U.S. Pat. No. 4,958,073. This detector is characterized by a non-negative
response to an increase in the density of the formation, in contrast with
the near and far detectors which are attenuation detectors and exhibit a
negative response to an increase in the density. The window associated
with the backscatter detector is shown at 73 in FIG. 6.
As to the design of the electrodes of the micro-resistivity sensor, this is
generally in conformity with the teaching of U.S. Pat. No. 5,036,283,
especially FIG. 5 of this patent, or of U.S. Pat. No. 5,198,770 and
reference is made to these documents for more information. The design of
the electrodes shown comprises firstly a generally I-shaped guard
electrode A0 providing passive focusing, and an active focusing system
comprising focusing electrodes A1 located on either side of the electrode
A0 relative to the longitudinal direction of the pad, and control
electrodes M located between the guard electrode A0 and the focusing
electrodes A1. The illustrated design also comprises a measuring electrode
in the form of a button B0 fitted within the central electrode A0, being
insulated therefrom, being centered in its median plane, and being
sensitive to the resistivity Rxo of the invaded zone, together with two
buttons B1, B2 aligned with the button B0 and of smaller diameter, which
provide a measure of resistivity with a smaller depth of investigation, of
the type known as Microlog. In addition a button B3 identical with the
button B0 is provided in alignment therewith in the longitudinal
direction. The buttons B0 and B3, being aligned in the longitudinal
direction (i.e. in the direction of movement of the pad) provide two
measurements from the same zone of the formation at offset instants, which
makes it possible to determine the speed of movement of the pad by
correlation. The resulting information is used to determine a depth
correction. It is advantageously combined with an indication of the speed
obtained from acceleration measurements provided by the above-mentioned
inclinometer sonde of the first section, in order to improve the accuracy
of the correction, in accordance with a method described in detail in
French patent application 93 04229 filed 9, Apr. 1993.
Referring more particularly to FIGS. 8A, 8B, note firstly the face 40
adapted to contact the wall of the borehole and having to this end the
shape of a sector of a cylinder. On the opposite side is seen the pivot 43
for making the connection to the support arm, such as the arm 41 shown in
FIG. 4B. At its upper end, the pad is extended by two clevis plates 75,
each with a slot referenced 58 as in FIG. 4B, for the connection with a
link such as the link 55 shown in FIG. 4B.
As its main structural part, the pad comprises a pressure-resistant casing
78 of generally cylindrical shape, made of stainless steel for example.
Inside the casing 78 are disposed gamma ray detectors 81, 82, 83, the
detectors 81 and 82 "near" and "far" attenuation detectors respectively
and the detector 83 being a backscatter detector. In order to allow the
gamma rays to reach the detectors, the casing 78 has openings provided
with collars 91, 92, 93 on which rest respective domed parts 91', 92', 93'
made of a pressure-resistant material with low gamma ray absorption, such
as beryllium or titanium. Protective parts 101, 102, 103 of synthetic
material of the PEEK type for example are fixed on the domes 91', 92', 93'
respectively, these parts forming the windows 71, 72, 73 shown in FIG. 6.
The detectors are appropriately photo-scintillators of GSO type (gadolinium
orthosilicate) or NaI. Assemblies 111, 112, 113 respectively are
associated with the detectors 81, 82, 83, each being composed of a
photo-multiplier, a high voltage supply with its control circuit and a
preamplifier circuit.
The gamma ray source, typically a caesium 137 source is a unit 80 located
at the lower end of the pad on the outside of the casing 78 and which has
its own pressure-resistant envelope 100.
The electrodes A0, M, A1 which form the micro-resistivity sensor are metal
parts located in respective recesses formed in a sector of a collar 110 of
insulating material, for example PEEK. The detectors 81-83 are screened
from unwanted gamma rays (i.e. those which have not interacted with the
formation), in particular from direct arrivals, by shields with high gamma
ray absorbing power, for example made of material such as tungsten or
depleted uranium.
Thus, one shield member 119 with the shape of a sector of a collar is shown
in FIGS. 8A, 8B covering the casing 78 in the lower part of the pad, the
member 119 having openings which form the windows 70 (for the source), 71
(near detector) and 73 (backscatter detector). This member has an end part
120 which beyond the source 80, thus forming the lower end of the pad, in
such a way as to minimize the gamma ray flux towards the borehole fluid.
At the opposite end the member 119 is continued by a part 121 extending
well beyond the window 71 of the near detector 81, the part 121 being
located between the casing 78 and the inside of the sector 110 of
insulating material. This arrangement of the shield member minimizes the
risk of gamma rays escaping through the insulating material, which has a
low absorption to gamma rays. In like manner, a shield member 122 is
located on the outside of the casing 78 in the region of the far detector
82 and has an opening forming the window 72.
Shield members of generally cylindrical shape are also provided on the
inside of the casing 78. A first inner member 125 surrounds the
backscatter detector 83. The member 125 has an oblique hole 126 which
ensures collimation of the gamma rays towards the detector 83. A second
inner member 127 is fixed to the member 125 and surrounds the near
detector 81. The member 127 has the shape of a half-cylinder, as FIG. 7
shows, in its upper part, leaving the lower part of the space inside the
casing 78 free, which can thus receive circuit boards or electronics
components 128. In like manner, an inner shield member 129 surrounds the
far detector 82.
It is also noted that the casing 78 has openings receiving sealed
feed-throughs 130, 131 to provide passage for conductors, such as
conductors 132 which connect the electrodes (A0, etc.) to circuit boards
or components located inside the casing 78 and conductors 133 which
connect the boards or associated components to the gamma detectors or to
the electrodes on the body of the sonde.
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