Information
-
Patent Grant
-
6655458
-
Patent Number
6,655,458
-
Date Filed
Tuesday, November 6, 200122 years ago
-
Date Issued
Tuesday, December 2, 200320 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Salazar; Jennie (JL)
- Jeffery; Briditte L.
- Ryberg; John J.
-
CPC
-
US Classifications
Field of Search
US
- 166 2542
- 166 25001
- 166 2541
- 166 25016
- 166 25017
- 166 206
- 175 94
- 175 97
- 175 106
-
International Classifications
-
Abstract
A well logging instrument is disclosed which includes a housing operatively coupled to a well logging conveyance and movable within the wellbore. The housing has therein a formation testing system and an axial extension mechanism. The axial extension mechanism controllably extends and retracts to allow the formation testing system to perform tests and take samples in an axially fixed position in the wellbore while the housing moves through the wellbore.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of wellbore testing and sample taking instruments. More particularly, the invention relates to designs for such instruments which reduce a possibility of the instrument and/or conveyance device becoming stuck in a wellbore.
2. Background Art
When drilling a wellbore through earth formations for the purpose of producing hydrocarbons, frequently the wellbore operator requires information concerning formation and wellbore parameters, such as fluid pressure and fluid content of the various formations penetrated by the wellbore. Such pressure and fluid content information is used for, among other purposes, determining a depth at which to set casing, determining which formations are likely to be commercially productive of hydrocarbons or whether to set casing at all.
Various instruments are known in the art for taking formation fluid pressure measurements and/or formation fluid samples. Many of these instruments are designed to be conveyed at one end of an armored electrical cable (“wireline” conveyed). Other types of instruments may be conveyed by coiled tubing, drill pipe or similar conveyances. These instruments typically include an elongated instrument housing adapted to traverse the wellbore. The instrument housing includes therein a probe adapted to be extended from the housing and placed in externally sealed engagement with the wall of the wellbore at the position of a formation to be tested. Various flowlines, pressure transducers and sample chambers are disposed in the instrument housing and are adapted to cause fluid to be withdrawn from the selected formation while pressure and fluid composition properties are measured. In some cases a sample of the formation fluid will be directed to a storage tank for ultimate removal from the wellbore and subsequent analysis at the earth's surface. Examples of such formation pressure measuring and sample testing instruments are described in U.S. Pat. No. 6,058,773 issued to Zimmerman et al. and U.S. Pat. No. 4,936,139 issued to Zimmerman et al.
One particular concern associated with substantially all formation pressure measuring and sampling instruments such as the ones described in the above references is that the instrument must be stopped in the wellbore in order to take a sample and/or make a pressure measurement. Stopping the instrument in the wellbore substantially increases the risk of the instrument and/or means of conveyance becoming stuck in the wellbore. Mechanisms for becoming stuck include debris settling out of the drilling fluid and lodging between the instrument and the wellbore wall, differential pressure between the drilling fluid in the wellbore and the formation being tested, and the conveyance becoming “keyseated” in the wall of the wellbore. So called “tractor” devices have been developed to prevent wellbore tools from sticking in the wellbore. Examples of such tractor devices include U.S. Pat. No. 5,954,131 issued to Sallwasser on Sep. 21, 1999 and U.S. Pat. No. 6,179,055 issued to Sallwasser et al. on Jan. 30, 2001, the entire contents of both are hereby incorporated by reference. These tractor devices convey a tool along a wellbore using a cam system to lock against the borehole wall.
What is needed is a device for enabling continued motion of the conveyance and a substantial portion of the instrument while the tool conducts wellbore operations, such as deploying a probe to make a formation pressure measurement and/or fluid test. One such device is described for example in U.S. Pat. No. 4,600,059 issued to Eggleston et al. The device disclosed in this reference includes a telescoping section coupled between a wireline conveyed fluid testing instrument and the armored electrical cable. When the testing instrument is deployed to test a particular earth formation, and is thus stationary, the armored electrical cable may be kept in continuous motion by repeated extension and retraction of the telescoping section. This is known in the art as “yo-yoing” the cable. Yo-yoing the cable requires the cable operator to pay very close attention to a winch control system to avoid too much upward and/or downward motion of the cable for operating the telescoping section. It is desirable to have a telescoping section for a wellbore test instrument which does not require cable yo-yoing.
It is desirable to have a wellbore instrument, such as a formation fluid pressure and/or sampling instrument, which enables substantially continuous motion of a well logging conveyance in order to prevent sticking and reduce the duration of wellbore operations. This combination of a wellbore instrument in a continuous motion enables economically combining wellbore options, such as a combined pressure/fluid sample test instrument with other types of well logging instruments that make measurements while moving along the wellbore. Typically, such “moving measurements” have not been combined with formation pressure and sampling instruments to operate simultaneously because the former are adapted to make measurements while moving along the wellbore, and the latter, as previously explained, must be stopped. Examples of the former include, without limitation, acoustic devices, resistivity devices and nuclear porosity and lithology measuring devices.
SUMMARY OF INVENTION
One aspect of the invention is a well logging instrument which includes a lower housing having therein a formation testing system adapted to be operated in an axially fixed position in a wellbore. The instrument also includes an upper housing adapted to be operatively coupled to a well logging conveyance. The instrument includes an axial extension mechanism operatively coupled between the lower housing and the upper housing. The extension mechanism is adapted to controllably extend and retract to lengthen and shorten the instrument, respectively.
A method for testing an earth formation according to another aspect of the invention includes moving a logging instrument axially along a wellbore by operating a logging conveyance coupled to an upper end of the instrument. A testing system adapted to test the earth formation at a fixed axial position along the wellbore is deployed, while continuing to move the conveyance along the wellbore. A length of the logging instrument between the conveyance and the testing system is increased by operating an axial extension mechanism disposed between the conveyance and the testing system, while continuing to move the conveyance along the wellbore. The earth formation is tested, the testing system is retracted; and the axial extension mechanism is then retracted. In one embodiment, tension between the instrument and the conveyance is measured, and the axial extension mechanism is extended at a rate adapted to maintain the tension substantially constant.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1
shows an example embodiment of a formation testing instrument according to one aspect of the invention.
FIG. 2
shows another embodiment of an axial extension mechanism according to one aspect of the invention.
FIG. 3
shown another embodiment of an axial extension mechanism according to one aspect of the invention.
FIGS. 4-8
show a well logging/pressure testing operation according to another aspect of the invention.
FIGS. 9-13
show a well logging/pressure testing operation according to another aspect of the invention.
FIGS. 14-19
show a well logging/pressure testing operation according to another aspect of the invention.
FIG. 20
shows an example embodiment of a cable tension measuring device used in another aspect of the invention.
DETAILED DESCRIPTION
An embodiment of a formation testing instrument is shown schematically in FIG.
1
. The instrument
10
in this embodiment is adapted to make formation pressure measurements and/or take fluid samples from an earth formation. Formation fluid pressure measuring/sample taking devices are a principal example, but only one example, of a type of formation testing system which is adapted to perform its testing function while in an axially fixed position within a wellbore.
The instrument
10
includes an upper housing
28
adapted to couple at its upper end to an instrument conveyance, which in this example is an armored electrical cable (not shown). Connection to the cable (not shown) can be either directly, or through other intervening well logging instruments (not shown in
FIG. 1
for clarity). The upper housing
28
is adapted to slidingly, sealingly engage a lower housing
26
.
The lower housing
26
in this embodiment includes therein the various components of a testing and sampling system
12
. The system
12
includes a probe
14
which is adapted to be extended laterally from the lower housing
26
by hydraulic cylinders
16
or the like, and may include a back up pad system
20
located circumferentially opposite the probe
14
about the lower housing
26
. The back up pad system
20
can be of any type well known in the art adapted to provide the probe
14
with adequate ability to be sealingly forced against the wall of a wellbore (not shown) in which the instrument
10
is disposed, particularly when the wellbore has a large diameter as compared with the diameter of the instrument
10
. The back up pad section
20
can be extended and retracted using hydraulic cylinders
18
or the like.
The probe
14
is in selective hydraulic communication with a pressure testing cylinder
22
having therein a pressure transducer (not shown separately) which makes measurements of the fluid pressure of the selected earth formations adjacent to the wellbore. The pressure testing cylinder
22
may be operatively controlled by a controller/telemetry unit
24
, which operates the pressure testing cylinder
22
and records, formats and/or transmits measurements made by the transducer (not shown) so fluid that pressure of the selected earth formations can be determined. Systems including the system
12
, pressure test cylinder
22
and transducer therein, controller/telemetry unit
24
and the back up pad system
20
may be any one or more of a number of types well known in the art, such as disclosed, for example, in U.S. Pat. No. 6,058,773 issued to Zimmerman et al. and U.S. Pat. No. 4,936,139 issued to Zimmerman et al. The type and structure of the system
12
, pressure test cylinder
22
, controller/telemetry unit
24
and the back up pad system
20
are only provided to help explain the invention, and are not in any way intended to limit the scope of the invention.
The system
12
of
FIG. 1
is depicted as a formation fluid pressure measuring/sample taking system with a probe and hydraulic cylinders. However, the system may be substituted with any downhole instrument capable of performing operations in the apparatus
10
. Example of such downhole instruments include devices, such as a rotary or percussion “core” sampling device, perforation tools, rock testing and sampling tools as well as others instruments usable with downhole tools.
As previously explained, the upper housing
28
and the lower housing
26
are adapted to slidingly, preferably sealingly, engage each other. An axial position of the upper housing
28
with respect to the lower housing
26
is controlled, in various embodiments of the invention, by an axial extension mechanism
38
. One end of the axial extension mechanism
38
is fixedly coupled to a selected position along the lower housing
26
, such as at lower bulkhead
38
A. The other end of the axial extension mechanism
38
is fixedly coupled to a selected position along the upper housing
28
, such as at upper bulkhead
38
B.
The embodiment of the axial extension mechanism
38
shown in
FIG. 1
may include an electric motor
30
the rotary output of which turns an extension screw, or ball screw
32
. The ball screw
32
engages a ball nut
34
fixed to the lower bulkhead
38
A or other element coupled to the lower housing
26
. The motor
30
in this embodiment is controlled by a motor controller
36
, functionality of which will be further explained. To summarize, the motor
30
may be turned to rotate the ball screw
32
to cause the lower housing
26
to slide outward from the upper housing
28
(or as may be described inversely, the upper housing
28
slides outwardly from the lower housing
26
). The outward relative sliding lengthens the instrument
10
. The motor
30
may also be turned in the opposite direction to ultimately cause the housings
26
,
28
to slide together with respect to each other, to shorten the instrument
10
.
As will be appreciated by those skilled in the art, when the pressure testing system
12
is engaged with the wall of a wellbore (not shown) to make a pressure test of an earth formation, its axial position in the wellbore (not shown) is fixed. By causing the motor
30
to operate to lengthen the instrument
10
, the cable (not shown) and any intervening logging instruments (not shown) may continue to move along the wellbore (not shown). After a pressure measurement is made, and the pressure measuring system
12
is retracted, the motor
30
may be operated to cause the instrument
10
to shorten, still while moving the cable and any intervening logging instruments.
Another possible embodiment of the axial extension mechanism
38
is shown in FIG.
2
. In this embodiment, the axial extension mechanism includes an hydraulic cylinder
40
coupled at one end to bulkhead
38
B in the upper housing
28
, and an hydraulic piston
41
coupled at one end to bulkhead
38
A in the lower housing
26
. The piston
41
/cylinder
40
combination may be any conventional type adapted to extend and retract the piston
41
from the cylinder
40
upon application of suitable hydraulic pressure. The piston
41
/cylinder
40
combination should be operatively coupled to a suitably controlled hydraulic pressure source (not shown) to extend and retract the piston
41
from the cylinder
40
to lengthen and shorten the instrument
10
as explained with respect to the previous embodiment of the axial extension mechanism
38
.
Another possible embodiment of the axial extension mechanism
38
is shown in FIG.
3
. This embodiment is a linear electric actuator including a primary winding
43
mechanically coupled to the lower housing
26
and a secondary winding
42
mechanically coupled to the upper housing
28
.
For any embodiment of the axial extension mechanism, such as the ones described above, it is understood that the positions of the various elements of any embodiment of the mechanism
38
described above within either of the upper housing
28
and lower housing
26
are only to illustrate the general principle of an instrument made according to this aspect of the invention. Accordingly, the relative positions of the various components of the axial extension mechanism shown herein are not meant to limit the invention. For example, the motor
30
and balls crew
32
of
FIG. 1
could as easily and effectively be located in an coupled to the lower housing
26
. It is also understood that having the lower housing
26
be adapted to slide within the upper housing
28
as shown in
FIGS. 1
,
2
and
3
is also meant only to illustrate the principle. The upper housing
28
could as easily be adapted to slide within the lower housing
26
while performing as intended within the scope of the invention.
A method of performing wellbore operations according to the invention is illustrated in
FIGS. 4 through 8
. Specifically,
FIGS. 4 through 8
show a method of taking pressure measurements and/or fluid samples.
FIGS. 4 through 8
also show how the overall length of the instrument
10
extends and retracts over time.
In
FIG. 4
, an instrument
10
according to the invention is coupled to a well logging cable
50
. The instrument
10
has an upper housing
28
coupled to the cable
50
through an intervening logging instrument
51
adapted to make one or more types of petrophysical measurements while the intervening instrument
51
is moved along the wellbore
52
. The instrument
10
also has a lower housing
26
coupled to the upper housing
28
as previously described. Other embodiments of a method according to the invention may exclude the intervening logging instrument
51
.
FIG. 4
shows the instrument
10
at initial time to with the axial extension mechanism fully retracted. The test system
12
is deployed in an earth formation which is intended to be tested, and the various components of the test system
12
which are adapted to contact the wellbore
52
are placed in contact therewith.
FIG. 5
shows the tool as it has advanced up the wellbore at a time t
1
. The logging cable
50
continues to be withdrawn from the wellbore
52
, in some embodiments, at substantially the same rate as prior to deployment of the test system
12
. As the cable
50
continues to be withdrawn, the axial extension mechanism
38
is operated to enable the upper housing (
28
in
FIG. 1
) to continue to move at the same rate as the cable
50
. The lower housing (
26
in
FIG. 1
) remains axially fixed within the wellbore
52
.
FIG. 6
show the tool at time t
3
. The lower housing
26
of the instrument
10
is stopped with the system
12
deployed to perform wellbore operations. Upper housing
28
continues to advance uphole thereby increasing the overall instrument
10
length as shown in
FIG. 6
as the pressure test system
12
is operated to make at least one fluid pressure test from the surrounding earth formation.
FIG. 7
shows the instrument
10
at time t
4
. In
FIG. 7
, the pressure test is completed, and the pressure test system is retracted to enable resumed upward motion of the lower housing
26
having the pressure test system
12
therein. The lower housing
26
of the instrument
10
is released from the wellbore and begins to retract into the upper housing and the overall length of the tool
10
begins to decrease. The lower housing is retracted by operating the axial extension mechanism
38
to shorten the instrument length as discussed previously.
FIG. 8
shows the instrument
10
at time t
4
. In
FIG. 8
, the lower housing
26
is fully retracted and the overall instrument length is returned to its original, retracted length at t
1
. The tool may then be moved into another position within the wellbore to take additional tests, or be withdrawn from the borehole.
Referring now to
FIG. 9
, the instrument
10
is shown in combination with an extender
100
. The extender
100
has an upper portion
28
a
coupled to the lower housing
26
of instrument
10
, and a lower portion
26
a.
The upper portion
28
a
and the lower portion
26
a
having an axial extension mechanism
38
a
adapted to axially extend and retract upper portion
28
a
and lower portion
26
a
as previously described with respect to axial extension mechanism
38
of
FIGS. 1 through 3
. The lower portion
26
a
of the extender
100
may optionally be provided with additional instruments to perform tests.
A method of performing wellbore operations using the instruments
10
with the extender
100
is illustrated in
FIGS. 9 through 13
.
FIGS. 9 through 13
show the instrument
10
advance up the wellbore as previously described with respect to
FIGS. 4 through 8
. At time t
0
, the instrument
10
is in the fully retracted position and the extender
100
is in the fully extended position. As shown in FIG.
10
and at time t
1
, the upper housing
28
of the instrument
10
and the lower portion
26
a
of the extender
100
have begun to move uphole. At time t
2
of
FIG. 11
, the instrument
10
is in the fully extended position, and the extender
100
is in the fully retracted position.
FIG. 12
shows the instrument
10
at time t
3
with the sampling probe
14
having completed its test. The instrument
10
begins to retract while the extender
100
begins to extend. At time t
4
shown in
FIG. 13
, the instrument
10
has fully retracted and the extender
100
has fully extended. The cycle may then begin again at another position in the wellbore.
FIGS. 9 through 13
depict the instrument
10
extending while the extender
100
retracts and the extender
100
extending as the instrument
10
retracts. This depiction of the instrument
10
operating at alternate intervals with the extender is one example of an operation with multiple extension mechanisms. The instrument and extender may be timed to operate simultaneously, out of sync, or at any desired interval.
Another embodiment of the present invention is depicted in FIG.
14
. The instrument
200
is provided with a slotted housing
130
having an upper end
140
and a lower end
150
. An axial mechanism
180
having an upper portion
32
a
and a lower portion
32
b
is disposed within the housing. A mechanical stop
160
is disposed between the upper portion
32
a
and the lower portion
32
b.
An axially movable testing systems
12
a
is positioned on upper portion
32
a,
and an axially movable testing system
12
b
is positioned on lower portion
32
b.
Each testing system is provided with a probe
14
and opposing back up pad section
18
extendable through slots (not shown) in the housing
130
. The testing systems
12
a
and
12
b
are axially movable along their respective portion of the axial mechanism
180
.
A method of performing wellbore operations using the instrument
200
in accordance with the invention is illustrated in
FIGS. 14 through 19
. The instrument
200
is shown progressing uphole in the wellbore
52
from time t
0
of
FIG. 15
to time t
5
of FIG.
20
. As shown in
FIGS. 14 through 16
at times t
0
through t
2
, the testing system
12
a
extends through the slot (not shown) in the housing and engages the wellbore
52
to perform a testing function. Testing system
12
a
advances toward mechanical stop
160
along the upper portion
32
a
of the axial mechanism
180
, and the testing system
12
b
advances toward mechanical stop
160
along the lower portion
32
b
of the axial mechanism
180
.
Referring now to
FIG. 17
at time t
3
, testing system
12
a
retracts back into the housing, and testing system
12
b
extends through the slotted housing to perform a test. As the instrument
200
continues uphole as shown in
FIG. 18
at time t
4
, instrument
12
b
advances towards the lower end
150
of the instrument, and testing system
12
a
advances toward the upper end
140
of the instrument
200
. As shown in
FIGS. 14 through 19
, testing systems
12
a
and
12
b
test at alternate intervals, but could be timed at alternate, simultaneous or random intervals to perform a variety of tests.
It is understood that reference to a well logging cable as explained with respect to
FIGS. 4-19
are merely examples of a well logging conveyance which may be used in various embodiments of an instrument and method according to the invention. Coiled tubing and drill pipe logging conveyances may also be used in other embodiments.
Another aspect of the invention can be better understood by referring to FIG.
20
.
FIG. 20
shows a typical cable head
53
which is used to make electrical and mechanical connection between the logging cable
50
and the instrument (
10
in FIGS.
4
-
9
). This embodiment of the cable head
53
includes therein a sensor
54
having an output related to the amount of tension between the logging cable
50
and the cable head
53
. As will be appreciated by those skilled in the art, the instrument upper housing (
28
in
FIG. 1
) synchronously moves with the cable even when the lower housing (
26
in
FIG. 1
) is axially fixed in the wellbore, as explained with respect to
FIGS. 4-9
. If the cable motion matches the rate at which the axial extension mechanism (
38
in
FIGS. 4-9
) increases the instrument length, the tension between the cable head
53
and the cable should remain substantially constant. In this embodiment of the invention, the sensor
54
is operatively coupled to the motor controller (
36
in FIG.
1
), and the controller is adapted so that the rate of extension of the axial extension mechanism
38
may be substantially matched to the rate of motion of the logging cable
50
. If the cable
50
moves faster than the extension mechanism
38
lengthens, it would be expected that the tension indicated by the sensor
54
will increase, and vice versa.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
Claims
- 1. A well logging instrument, comprising:a lower housing having therein a wellbore testing system adapted to be operated in an axially fixed position in a wellbore; an upper housing adapted to be operatively coupled to a well logging conveyance; and an axial extension mechanism operatively coupled between the lower housing and the upper housing, the extension mechanism adapted to controllably extend and retract so as to controllably lengthen and shorten the instrument, respectively.
- 2. The instrument as defined in claim 1 wherein the axial extension mechanism comprises a motor having a ball screw operatively coupled thereto, and a ball nut operatively coupled to the ball screw.
- 3. The instrument as defined in claim 1 wherein the axial extension mechanism comprises an hydraulic cylinder and piston combination.
- 4. The instrument as defined in claim 1 wherein the axial extension mechanism comprises an electric linear actuator.
- 5. The instrument as defined in claim 1 further comprising a controller operatively coupled to the axial extension mechanism and a sensor having an output related to a tension on the well logging conveyance, the controller adapted to operate the extension mechanism so as to maintain a substantially constant tension on the well logging conveyance.
- 6. The instrument as defined in claim 1 wherein the conveyance comprises a well logging cable.
- 7. The instrument as defined in claim 1 wherein the testing system is selected from the group of a formation pressure testing system, a formation sampling system, a rock indentation system and a perforating system.
- 8. The instrument as defined in claim 1 further comprising a well logging device coupled between the conveyance and the upper housing, the well logging device adapted to make measurements while moving along the wellbore.
- 9. The instrument as defined in claim 1 further comprising an extender.
- 10. The instrument as defined in claim 9 wherein the extender comprises an upper portion, a lower portion, and a second axial extension mechanism operatively coupled therebetween, the upper portion coupled to the lower housing; the extension mechanism adapted to controllably extend and retract so as to controllably lengthen and shorten the extender, respectively.
- 11. The instrument as defined in claim 10 having therein a second wellbore testing system.
- 12. A well logging instrument, comprising:a housing disposable in a wellbore, the housing having axial slots therethrough; an axial mechanism positioned within the housing; and a wellbore testing system movably positionable along the axial mechanism, the testing system adapted to extend through the slots of the housing and perform wellbore tests as the housing advances through the wellbore.
- 13. A method for testing an earth formation, comprising:(a) moving a logging instrument axially along a wellbore by operating a logging conveyance coupled to an upper end of the instrument; (b) deploying a testing system adapted to test formation parameters at a fixed axial position along the wellbore; (c) extending a length of the logging instrument by operating an axial extension mechanism disposed between the conveyance and the testing system; (d) testing the formation; (c) retracting the testing system; and (f) retracting the axial extension mechanism, wherein the deploying, extending the length, testing the formation and retracting the testing system and extension mechanism are performed while continuing to move the conveyance along the wellbore.
- 14. The method as defined in claim 13 wherein the extending comprises operating a motor and ball screw.
- 15. The method as defined in claim 13 wherein the extending comprises operating an hydraulic cylinder and piston.
- 16. The method as defined in claim 13 wherein the extending comprises operating an electric linear actuator.
- 17. The method as defined in claim 13 further comprising measuring a tension between the conveyance and the instrument, and controlling a rate of the extending to maintain the tension substantially constant.
- 18. The method as defined in claim 13 further comprising making a measurement of at least one formation property from an instrument adapted to move synchronously with the conveyance, the making the measurement continuing while extending the length of the logging instrument.
- 19. The method as defined in claim 13 wherein the moving the conveyance comprises withdrawing a well logging cable from the wellbore.
- 20. The method as defined in claim 13 wherein the step of moving a logging instrument comprises moving a wellbore tool comprising multiple logging instruments axially coupled together axially along a wellbore by operating a logging conveyance coupled to an upper end of the tool, and wherein steps (b)-(f) are performed for each instrument.
- 21. The method as defined in claim 20 wherein the multiple instruments perform steps (b)-(f) alternately with adjacent instruments.
- 22. The method as defined in claim 20 wherein the multiple instruments perform steps (b), (d) and (c) simultaneously.
US Referenced Citations (18)
Foreign Referenced Citations (1)
Number |
Date |
Country |
0 346 229 |
Dec 1989 |
EP |