CA1227538A - Electrode structure to measure borehole resistivity - Google Patents
Electrode structure to measure borehole resistivityInfo
- Publication number
- CA1227538A CA1227538A CA000508062A CA508062A CA1227538A CA 1227538 A CA1227538 A CA 1227538A CA 000508062 A CA000508062 A CA 000508062A CA 508062 A CA508062 A CA 508062A CA 1227538 A CA1227538 A CA 1227538A
- Authority
- CA
- Canada
- Prior art keywords
- electrode
- drill string
- electrodes
- current
- measuring
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Landscapes
- Geophysics And Detection Of Objects (AREA)
Abstract
Abstract of the Disclosure An array of electrodes is presented for accurate and improved measurement of borehole resist-ivity. The electrode array comprises five electrodes mounted on a relatively short insulated segment of a drill string. Current flows from the first elect-rode to the second electrode and is sensed by third and fourth electrodes. Any leakage current from the first electrode is collected by a fifth electrode and is kept isolated from the current flowing between the first and second electrodes thus insuring accur-ate determination of apparent formation resistivity.
A current source, preferably a constant current source is used to determine the current in the circuit of the first and fifth electrodes.
A current source, preferably a constant current source is used to determine the current in the circuit of the first and fifth electrodes.
Description
~%~7S38 Field oE the Invention This invention relates to the field of the sensing of borehole parameters, particularly parameters of interest in the drilling of oil well boreholes. More particularly, this invention relates to the measurement of apparent formation resistivity, and more particularly to a novel electrode array and the structure of and methods of forming the insulation and electrode arrays.
Discussion of the Prior Art The desirability of measuring apparent formation resistivity during borehole drilling is well known in the art. This subject has been widely discussed in the literature, including the patent literature, and many proposals have been made for apparatus and systems to measure apparent formation resistivity.
The general concept involves the mounting of electrodes on a segement of the drill string at a downhole location. One typical prior art arrangement is shown in FIGURE 1 and involves an array of four electrodes, A, B, M and N mounted on an insulated segment S of a steel drill string segment D.
Current Il from electrode A is directed through the formation F and is collected at electrode B. The voltage drop across electrodes M-N is measured by 12Z753~3 volt meter V, and the apparent formation resistivity Rf is determined from the values Il and VMN.
However, a serious inaccuracy exists in this typical prior art system because a substantial leakage current path I2 exists between electrode A and drill collar segment D. Thus, while the total value of the current generated at electrode A is a current level I, only part of that current Il, flows to electrode B through the formation F, and substantial leakage current I2 is set up ,in the system. As a result, the voltage drop VMN is a function of only the current component Il (the value of which is unknown even if the total current I generated at electrode A is known) and hence the calculated value Rf of apparent formation resistivity is inaccurate One possible approach to resolving this problem of the prior art is to provide an extremely long insulated section S for the drill string in an attempt to interrupt or minimize the leakage current from electrode A to the drill string segment.
However, the use of an exceptionally long insulated section of drill collar is impracticable because it creates a number of other problems of its own In accordance with the invention there is , provided apparatus for measuring the apparent resis-tivity of a borehole. The apparatus includes an insulated segment of a drill string and an array of five electrode means positioned at predetermined locations in the insulated segment. A current source is connected to a first of the electrode means and a first return circuit means is connected from a second of the electrode means to the current source, the second electrode means being spaced from the first ~L~%7538 electrode means in a flrst direction. Third and fourth electrode means are disposed between the first and second electrode means, and voltage measuring means are connected to the third and fourth electrode means for determining the voltage between the third and fourth electrode means. Current measuring means determine the current in the first return circuit means. A fifth electrode means is spaced from the first electrode means in a second direction, and a second return circuit means is connected from the fifth electrode means to the current source.
Referring to the drawings, wherein like elements are numbered alike in some of the FIGURES:
FIGURE 1 is an illustration of a typical prior art array of four electrodes, as discussed abuve.
FIGURE 2 is a schematic representation of the electrode array of the present invention.
FIGURE 3 shows apparatus and a method for forming the electrode and insulation structure.
FIGURE 4 shows a modification of FIGURE 3 to incorporate expansion partitions in the insulation.
FIGURE 5 shows another modified insulation structure.
FIGURE 6 shows a modification of the FIGURE 5 structure.
FIGURE 7 shows another modified electrode structure.
FIGURES 8 and 9 show further modifications of electrode and insulation structures.
.
~27538 Description of the Preferred Embodiments Referring now to FIGURE 2, a general configuration of the electrode array of the present invention is shown. A drill string segment 10 is shown in borehole 12 which has been drilled in an earth formation F. As is standard in the art, drill string segment 10 is a length of steel pipe which has junction structure at each end for joining to other similar drill string segments to form an elongated drill string. A length of insulating material 14 fills a cylindrical recess or housing 16 on the surface of the drill collar so as to form an annular ring of insulation, the outer surface 18 of which is flush with the outer surface 20 of the drill string segment. Insulation 14 may be any suitable material to meet the requirements of providing electrical insulation and enduring the environment in which the drill string collar must operate.
An array of five annular electrodes A, Bl, B2, M and N are embedded in the insulating material 14. The electrodes may be any suitable conducting material, such as iron, and they are in the form of annular rings the outer surface of which may be flush with or slightly recessed bQlow the outer surface of insulation layer 14. As will be observed, while the electrodes are embedded in the insulating material 14, they are spaced from the outer surface of recess 16, because the electrodes must be insulated from steel drill string segment 10. Electrodes M and N
are positioned between electrodes A and Bl, while electrode B2 is spaced from electrode A on the side opposite to electrode Bl. Electrode Bl i5 the most downhole, i.e., closest to the drill bit, in the array, and electrode B2 is the most uphole, i.e., farthest from the bit, in the array. A constant , _5_ ~27~38 current source 22 is connected to electrode A;
electrode Bl is connected in a return circuit 24 to constant current source 22; electrode B2 is connected in a return circuit 26 to constant current source 22; and a volt meter 2~ is connected between electrodes M and N.
When constant current source 22 delivers a current I to electrode A, a current path Il is set up from electrode A through formation F to electrode Bl. As discussed previously, in the prior art a leakage current path was also set up between electrode A and the drill collar, thus impairing the measurement of apparent formation resistivityO
However, in the present invention, the previous leakage current is collected by the electrode B2 which cooperates with electrode A to define a second current path I2. The relationship then exists that the current I from constant current source 22 is equal to Il plus I2 (I Il I2~- The
Discussion of the Prior Art The desirability of measuring apparent formation resistivity during borehole drilling is well known in the art. This subject has been widely discussed in the literature, including the patent literature, and many proposals have been made for apparatus and systems to measure apparent formation resistivity.
The general concept involves the mounting of electrodes on a segement of the drill string at a downhole location. One typical prior art arrangement is shown in FIGURE 1 and involves an array of four electrodes, A, B, M and N mounted on an insulated segment S of a steel drill string segment D.
Current Il from electrode A is directed through the formation F and is collected at electrode B. The voltage drop across electrodes M-N is measured by 12Z753~3 volt meter V, and the apparent formation resistivity Rf is determined from the values Il and VMN.
However, a serious inaccuracy exists in this typical prior art system because a substantial leakage current path I2 exists between electrode A and drill collar segment D. Thus, while the total value of the current generated at electrode A is a current level I, only part of that current Il, flows to electrode B through the formation F, and substantial leakage current I2 is set up ,in the system. As a result, the voltage drop VMN is a function of only the current component Il (the value of which is unknown even if the total current I generated at electrode A is known) and hence the calculated value Rf of apparent formation resistivity is inaccurate One possible approach to resolving this problem of the prior art is to provide an extremely long insulated section S for the drill string in an attempt to interrupt or minimize the leakage current from electrode A to the drill string segment.
However, the use of an exceptionally long insulated section of drill collar is impracticable because it creates a number of other problems of its own In accordance with the invention there is , provided apparatus for measuring the apparent resis-tivity of a borehole. The apparatus includes an insulated segment of a drill string and an array of five electrode means positioned at predetermined locations in the insulated segment. A current source is connected to a first of the electrode means and a first return circuit means is connected from a second of the electrode means to the current source, the second electrode means being spaced from the first ~L~%7538 electrode means in a flrst direction. Third and fourth electrode means are disposed between the first and second electrode means, and voltage measuring means are connected to the third and fourth electrode means for determining the voltage between the third and fourth electrode means. Current measuring means determine the current in the first return circuit means. A fifth electrode means is spaced from the first electrode means in a second direction, and a second return circuit means is connected from the fifth electrode means to the current source.
Referring to the drawings, wherein like elements are numbered alike in some of the FIGURES:
FIGURE 1 is an illustration of a typical prior art array of four electrodes, as discussed abuve.
FIGURE 2 is a schematic representation of the electrode array of the present invention.
FIGURE 3 shows apparatus and a method for forming the electrode and insulation structure.
FIGURE 4 shows a modification of FIGURE 3 to incorporate expansion partitions in the insulation.
FIGURE 5 shows another modified insulation structure.
FIGURE 6 shows a modification of the FIGURE 5 structure.
FIGURE 7 shows another modified electrode structure.
FIGURES 8 and 9 show further modifications of electrode and insulation structures.
.
~27538 Description of the Preferred Embodiments Referring now to FIGURE 2, a general configuration of the electrode array of the present invention is shown. A drill string segment 10 is shown in borehole 12 which has been drilled in an earth formation F. As is standard in the art, drill string segment 10 is a length of steel pipe which has junction structure at each end for joining to other similar drill string segments to form an elongated drill string. A length of insulating material 14 fills a cylindrical recess or housing 16 on the surface of the drill collar so as to form an annular ring of insulation, the outer surface 18 of which is flush with the outer surface 20 of the drill string segment. Insulation 14 may be any suitable material to meet the requirements of providing electrical insulation and enduring the environment in which the drill string collar must operate.
An array of five annular electrodes A, Bl, B2, M and N are embedded in the insulating material 14. The electrodes may be any suitable conducting material, such as iron, and they are in the form of annular rings the outer surface of which may be flush with or slightly recessed bQlow the outer surface of insulation layer 14. As will be observed, while the electrodes are embedded in the insulating material 14, they are spaced from the outer surface of recess 16, because the electrodes must be insulated from steel drill string segment 10. Electrodes M and N
are positioned between electrodes A and Bl, while electrode B2 is spaced from electrode A on the side opposite to electrode Bl. Electrode Bl i5 the most downhole, i.e., closest to the drill bit, in the array, and electrode B2 is the most uphole, i.e., farthest from the bit, in the array. A constant , _5_ ~27~38 current source 22 is connected to electrode A;
electrode Bl is connected in a return circuit 24 to constant current source 22; electrode B2 is connected in a return circuit 26 to constant current source 22; and a volt meter 2~ is connected between electrodes M and N.
When constant current source 22 delivers a current I to electrode A, a current path Il is set up from electrode A through formation F to electrode Bl. As discussed previously, in the prior art a leakage current path was also set up between electrode A and the drill collar, thus impairing the measurement of apparent formation resistivityO
However, in the present invention, the previous leakage current is collected by the electrode B2 which cooperates with electrode A to define a second current path I2. The relationship then exists that the current I from constant current source 22 is equal to Il plus I2 (I Il I2~- The
2~ current Il slows in circuit 24, while the current I2 flows in circuit 26, the leakage current to the drill collar is essentially eliminated or reduced to an insignificant amount. The value of Il can be measured directly by an ammeter 30, or the current Il can be determined indirectly by measuring the current I2 in circuit 26 and subtracting from the total current I. In either event, the current value Il can be accurately determined, and hence the value of the current Il which is responsible for the voltage drop between electrodes M and N
( VMN) is accurately known. The voltage drop VMN is measured by volt meter 26. With the values for Il and VMN accurately known, the apparent formation resistivity Rf can then be accurately calculated. The measured voltage and .
6- 12~:7S~3 3 current values can be transmitted to the surface of a borehole by mud pulse telemetry or any other known transmission technique, or a calculation can be made from the voltage and current values by downhole S equipment to calculate the apparent formation resistivity for transmission to the surface. In either event the valuable information of apparent formation resistivity can be accurately known at the surface of the well.
The insulation material 14 may be selected from among many different available insulating materials as long as certain minimum fundamental requirements are met. The material must, of course, be electrically nonductive. It must be stable (i.er, not decompose, soften or otherwise change its characteristics) for temperatures up to about 150C; it must be compatible with the drilling mud (which will fill the annulus between the drill string segment and the formation) and it must be resistant to oil or gas which may be present in the drilling mud. The insulating material must also be compatible with the steel drill collar to the extent that it can be bonded or otherwise securely adhered to the drill collar; it will not shrink significantly relative to the drill collar; and it should have a thermal coefficient of expansion closely matched to that of the drill collar if the material is not resilient.
Within the boundaries of these requirements, the insulating material may, for example, be selected from material or synthetic rubber, ceramics, thermosetting molding materials such as polyurethanes, thermoplastic molding materials such as polyamides, polystyrenes and polypropylenes or epoxy materials.
_7_ 1 22 7 53 8 FIGURE 3 shows an arrangement in which the array of electrodes embedded in the insulation may be formed. As shown in FIGURE 3, support elements 32 are mounted on drill string segment 10 at positions spaced apart along the axis of the drill string segment corresponding to the desired locations for the five annular electrodes. These support elements 32 may be any suitable electrically insulated elements as long as they are shaped or configured to permit the flow of insulating material in the axial direction along the outside of the drill string segment. The electrodes A, Bl, B2, M and N are then positioned on the respective support elements 32. It will be understood that the ring electrodes may have to be split in segments and then joined together in order to mount them on the drill string.
After the electrodes have been placed in position, a mold 34 is then positioned around the electrode array and fully encloses recess 16 to define recess 16 as a mold cavity. A molding material, such as a thermosetting polyurethane molding material, is then injected at appropriate pressure and temperature into mold 34 to Eill up the mold cavity which corresponds to recess 16. The mold is then removed after appropriate curing, and the resulting structure is an array of annular electrodes embedded in an annular length of insulating material recessed in a segment of the drill string.
Referring to FIGURE 4, a modiEied version of the configuration of FIGURE 3 is shown with an accommodation made to compensate for different coefficients of expansion between the insulating material and the drill string segment. That compensation is accomplished by locating annular expansion blocks or partitions 36 at selected " . .
-8~ 753~
locations along the length of recess 16. Expansion blocks or partitions 36 are resilient and elastic materials (such as synthetic rubber) so that they will compress and expand to absorb differential expansion between the drill collar 10 and the insulating material. The expansion blocks or partitions 36 serve to divide the recess 16 into a series of insulating segments (three in the illustration of FIGURE 4). Therefore, it becomes necessary to modify the mold 34 to provide for injection of the uncured insulating material into each of the partitioned segments of recess 16 in order to properly form the segmented insulating structure which results from the FIGURE 4 arrangement.
The insulating segment of the drill string operates in a relatively hostile environment where it is exposed to mud, sand, cuttings, rocks and other formation elements in the borehole being drilled.
Because of the hostile environment r it may be desirable to construct the insulation structure out of two different materials, a hard outer sleeve which will be exposed to the hostile drilling environment, and a softer inner 51eeve between the hard outer sleeve and the drill collar so that the hard outer sleeve will be able to yield or comply if it encounters a high lateral load, i.e., loading perpendicular to the axis of the drill string segment. A multiple sleeve arrangement of this type is shown in FIGURE 5 wherein the drill string segment 10 has an inner insulating sleeve 38 adjacent the recessed surface of the drill string segment and an outer insulating sleeve 40 adjacent to the inner sleeve 38. Outer sleeve 40 is a relatively hard sleeve of insulating material, while inner sleeve 38 is a sleeve of relatively soft material. Thus, :i ~275313 sleeve 40 will serve to provide abrasion and similar protection, while sleeve 38 will permit absorbtion of lateral loads.
The structure of FIGURE 5 poses two potential problems. One problem is the possibility of angular displacement between outer sleeve 40 and inner sleeve 38 resulting from torsional loads on the outer sleeve. The other problem results from the fact that the electrodes must be contained in the outer sleeve 40. Since the outer sleeve 40 must, of necessity, be thinner than the total thickness of the combined insulating material of sleeves 38 and 40, only a reduced amount of material is available for forming the grooves in sleeve 40 to receive the electrodes, and hence the sleeve 40 is weakened at each of the electrode locations. These problems are addressed and resolved by the structure of FIGURES 6 and 7.
Referring now to FIGURE 6, a modified configuration is shown in a cross sectional view with the section taken perpendicular to the axis of the drill string segment In this configuration, drill collar 10 is formed with spline segments 42 to form alternating axial lengths of thin and thick segments 43 and ~5, respectively. The relatively soft inner insulating sleeve 38 conforms to the spline configuration of the outer surface of drill string segment 10 in a layer of relatively constant thickness or depth, while the outer harder sleeve of insulating material 40 has thin segments 4na in radial alignment with the splines 42 and thicker segments 40b in radial alignment with the recesses between adjacent splines on the drill string. This interlocking sleeve and drill string structure shown in FIGURE will accommodate lateral deflection of the outer hard sleeve 40 but will reduce or limit the , . .
~227538 angular displacement between the hard outer sleeve 40 and the drill string 10.
In the FIGURE 6 arrangement, the electrode shape is also changed so tht each electrode is in the form of a cubic element 44 rather than the annular ring previously described. The cubic shaped electrodes are positioned in the thicker sections 40b of the hard outer insulating sleeve 40, so that the entire sleeve is not weakened by a ring electrode. While only one electrode 44 is shown at an axial station in FIGURE 6, it will be understood that a plurality of such electrodes may be located at two or more of the thicker segments 40b at each axial electrode station.
FIGURE 7 shows still another modification of the structure which is particularly advantageous when employing the arrangement of two sleeves of insulating material as in FIGURE 5 or 6. A problem which may result from the embeddiny of hard iron ring electrodes in a hard outer insulating sleeve is that any axial force imposed on the electrode resulting from interference with rock or other debris in the annulus will be fully transmitted to and imposed on the hard outer insulating sleeve. Such loads could, if sufficiently high, cause serious damage to the electrode or the insulating sleeves and could disable the apparent formation resistivity sensing structure. This problem of axial loading is reduced by the electrode structure shown in FIGURE 7, wherein each of thy electrodes is a diamond shaped segment.
If the electrode encounters interference with a rock or other piece of debris, the forces will be generated along the inclined surfaces of the electrode, and the electrode will tend to displace slightly in either an axial or lateral direction in response to the force. Such displacement will reduce ,~ .
. ..
~7~i38 the likelihood of damage to the electrode or its boundary connections with the insulating material, thus extending the life of the structure.
While wiring of the electrodes has not been discussed, it will, of course, be understood that the electrodes must be connected by wire to an electronic package in the drill string (which will also house the voltmeter and ammeter). That wiring may be accomplished in any convenient manner, such as by a protective tube running along the inner surface of recess 16 or by a groove, such as groove 46 in spline segment 42 of the FIGURE 6 arrangement. Connection from those protected wires to the electrodes may, for example, be made by means of helicoidally shaped wires to accommodate relative movement between the electrodes and the drill string segment 10.
FIGURE 8 shows still another alternate configuration for the insulation and electrode structure In the FIGURE 8 arrangement, the insulation sleeve is made up of a band 48 of elastomeric nonconductive material. Band 48 could be formed from a plurality of interlocking rings or segments ~8~a), 4~(b) and 48(c), for example. The electrodes are composed of rings or bands of conductive elastomeric material 49 in appropriately located recesses in the band of nonconductive material. Elastomeric materials 48 and 49 may be the same or similar base materials to match coefficients of expansion, with the conductive bands 49 being filled with silver, carbon or other conductive material to make the bands conductive electrodes.
The band of nonconductive and conductive elastomeric material may be mounted on the drill collar segment by being molded in place, or, if desired, by being stretched and slipped over the thicker section of the -12- 12X~S38 drill string segment and then released to contract into place in the recess 16. A tube 50 is shown in FIGURE 8, the tube 50 housing appropriate wiring which is connected by helical segments to each of the electrodes. Only three of the electrodes are shown in FIGURE 8, but it will be understood that two additional electrodes would be employed to complete the array of FIGURE 2.
Referring to FIGURE 9, an improved detail of effecting electrical connection to the elastomeric electrode of FIGURE 8 is shown. In the FIGURE 9 arrangement, a recess or groove 51 in the nonconductive elastomer 48(b) is coated with a resilient conductive material, such as a band of resilient metalic turnings (as in the nature of steel wool) or woven conductive cable indicated at 52. The helical circuit wire is physically and electrically connected to this ring of conductive turnings or steel wool material 52, and the material 52 forms multiple electrical contacts with the conductive elastomer electrode. In this way, electrical continuity prom the electrode to the circuit wire is assured.
It is to be understood that while the details of 2$ electrode construction disclosed herein are considered to be important, the basic concept of the five electrode array and its improved results are not limited to the details of electrode construction set forth herein.
( VMN) is accurately known. The voltage drop VMN is measured by volt meter 26. With the values for Il and VMN accurately known, the apparent formation resistivity Rf can then be accurately calculated. The measured voltage and .
6- 12~:7S~3 3 current values can be transmitted to the surface of a borehole by mud pulse telemetry or any other known transmission technique, or a calculation can be made from the voltage and current values by downhole S equipment to calculate the apparent formation resistivity for transmission to the surface. In either event the valuable information of apparent formation resistivity can be accurately known at the surface of the well.
The insulation material 14 may be selected from among many different available insulating materials as long as certain minimum fundamental requirements are met. The material must, of course, be electrically nonductive. It must be stable (i.er, not decompose, soften or otherwise change its characteristics) for temperatures up to about 150C; it must be compatible with the drilling mud (which will fill the annulus between the drill string segment and the formation) and it must be resistant to oil or gas which may be present in the drilling mud. The insulating material must also be compatible with the steel drill collar to the extent that it can be bonded or otherwise securely adhered to the drill collar; it will not shrink significantly relative to the drill collar; and it should have a thermal coefficient of expansion closely matched to that of the drill collar if the material is not resilient.
Within the boundaries of these requirements, the insulating material may, for example, be selected from material or synthetic rubber, ceramics, thermosetting molding materials such as polyurethanes, thermoplastic molding materials such as polyamides, polystyrenes and polypropylenes or epoxy materials.
_7_ 1 22 7 53 8 FIGURE 3 shows an arrangement in which the array of electrodes embedded in the insulation may be formed. As shown in FIGURE 3, support elements 32 are mounted on drill string segment 10 at positions spaced apart along the axis of the drill string segment corresponding to the desired locations for the five annular electrodes. These support elements 32 may be any suitable electrically insulated elements as long as they are shaped or configured to permit the flow of insulating material in the axial direction along the outside of the drill string segment. The electrodes A, Bl, B2, M and N are then positioned on the respective support elements 32. It will be understood that the ring electrodes may have to be split in segments and then joined together in order to mount them on the drill string.
After the electrodes have been placed in position, a mold 34 is then positioned around the electrode array and fully encloses recess 16 to define recess 16 as a mold cavity. A molding material, such as a thermosetting polyurethane molding material, is then injected at appropriate pressure and temperature into mold 34 to Eill up the mold cavity which corresponds to recess 16. The mold is then removed after appropriate curing, and the resulting structure is an array of annular electrodes embedded in an annular length of insulating material recessed in a segment of the drill string.
Referring to FIGURE 4, a modiEied version of the configuration of FIGURE 3 is shown with an accommodation made to compensate for different coefficients of expansion between the insulating material and the drill string segment. That compensation is accomplished by locating annular expansion blocks or partitions 36 at selected " . .
-8~ 753~
locations along the length of recess 16. Expansion blocks or partitions 36 are resilient and elastic materials (such as synthetic rubber) so that they will compress and expand to absorb differential expansion between the drill collar 10 and the insulating material. The expansion blocks or partitions 36 serve to divide the recess 16 into a series of insulating segments (three in the illustration of FIGURE 4). Therefore, it becomes necessary to modify the mold 34 to provide for injection of the uncured insulating material into each of the partitioned segments of recess 16 in order to properly form the segmented insulating structure which results from the FIGURE 4 arrangement.
The insulating segment of the drill string operates in a relatively hostile environment where it is exposed to mud, sand, cuttings, rocks and other formation elements in the borehole being drilled.
Because of the hostile environment r it may be desirable to construct the insulation structure out of two different materials, a hard outer sleeve which will be exposed to the hostile drilling environment, and a softer inner 51eeve between the hard outer sleeve and the drill collar so that the hard outer sleeve will be able to yield or comply if it encounters a high lateral load, i.e., loading perpendicular to the axis of the drill string segment. A multiple sleeve arrangement of this type is shown in FIGURE 5 wherein the drill string segment 10 has an inner insulating sleeve 38 adjacent the recessed surface of the drill string segment and an outer insulating sleeve 40 adjacent to the inner sleeve 38. Outer sleeve 40 is a relatively hard sleeve of insulating material, while inner sleeve 38 is a sleeve of relatively soft material. Thus, :i ~275313 sleeve 40 will serve to provide abrasion and similar protection, while sleeve 38 will permit absorbtion of lateral loads.
The structure of FIGURE 5 poses two potential problems. One problem is the possibility of angular displacement between outer sleeve 40 and inner sleeve 38 resulting from torsional loads on the outer sleeve. The other problem results from the fact that the electrodes must be contained in the outer sleeve 40. Since the outer sleeve 40 must, of necessity, be thinner than the total thickness of the combined insulating material of sleeves 38 and 40, only a reduced amount of material is available for forming the grooves in sleeve 40 to receive the electrodes, and hence the sleeve 40 is weakened at each of the electrode locations. These problems are addressed and resolved by the structure of FIGURES 6 and 7.
Referring now to FIGURE 6, a modified configuration is shown in a cross sectional view with the section taken perpendicular to the axis of the drill string segment In this configuration, drill collar 10 is formed with spline segments 42 to form alternating axial lengths of thin and thick segments 43 and ~5, respectively. The relatively soft inner insulating sleeve 38 conforms to the spline configuration of the outer surface of drill string segment 10 in a layer of relatively constant thickness or depth, while the outer harder sleeve of insulating material 40 has thin segments 4na in radial alignment with the splines 42 and thicker segments 40b in radial alignment with the recesses between adjacent splines on the drill string. This interlocking sleeve and drill string structure shown in FIGURE will accommodate lateral deflection of the outer hard sleeve 40 but will reduce or limit the , . .
~227538 angular displacement between the hard outer sleeve 40 and the drill string 10.
In the FIGURE 6 arrangement, the electrode shape is also changed so tht each electrode is in the form of a cubic element 44 rather than the annular ring previously described. The cubic shaped electrodes are positioned in the thicker sections 40b of the hard outer insulating sleeve 40, so that the entire sleeve is not weakened by a ring electrode. While only one electrode 44 is shown at an axial station in FIGURE 6, it will be understood that a plurality of such electrodes may be located at two or more of the thicker segments 40b at each axial electrode station.
FIGURE 7 shows still another modification of the structure which is particularly advantageous when employing the arrangement of two sleeves of insulating material as in FIGURE 5 or 6. A problem which may result from the embeddiny of hard iron ring electrodes in a hard outer insulating sleeve is that any axial force imposed on the electrode resulting from interference with rock or other debris in the annulus will be fully transmitted to and imposed on the hard outer insulating sleeve. Such loads could, if sufficiently high, cause serious damage to the electrode or the insulating sleeves and could disable the apparent formation resistivity sensing structure. This problem of axial loading is reduced by the electrode structure shown in FIGURE 7, wherein each of thy electrodes is a diamond shaped segment.
If the electrode encounters interference with a rock or other piece of debris, the forces will be generated along the inclined surfaces of the electrode, and the electrode will tend to displace slightly in either an axial or lateral direction in response to the force. Such displacement will reduce ,~ .
. ..
~7~i38 the likelihood of damage to the electrode or its boundary connections with the insulating material, thus extending the life of the structure.
While wiring of the electrodes has not been discussed, it will, of course, be understood that the electrodes must be connected by wire to an electronic package in the drill string (which will also house the voltmeter and ammeter). That wiring may be accomplished in any convenient manner, such as by a protective tube running along the inner surface of recess 16 or by a groove, such as groove 46 in spline segment 42 of the FIGURE 6 arrangement. Connection from those protected wires to the electrodes may, for example, be made by means of helicoidally shaped wires to accommodate relative movement between the electrodes and the drill string segment 10.
FIGURE 8 shows still another alternate configuration for the insulation and electrode structure In the FIGURE 8 arrangement, the insulation sleeve is made up of a band 48 of elastomeric nonconductive material. Band 48 could be formed from a plurality of interlocking rings or segments ~8~a), 4~(b) and 48(c), for example. The electrodes are composed of rings or bands of conductive elastomeric material 49 in appropriately located recesses in the band of nonconductive material. Elastomeric materials 48 and 49 may be the same or similar base materials to match coefficients of expansion, with the conductive bands 49 being filled with silver, carbon or other conductive material to make the bands conductive electrodes.
The band of nonconductive and conductive elastomeric material may be mounted on the drill collar segment by being molded in place, or, if desired, by being stretched and slipped over the thicker section of the -12- 12X~S38 drill string segment and then released to contract into place in the recess 16. A tube 50 is shown in FIGURE 8, the tube 50 housing appropriate wiring which is connected by helical segments to each of the electrodes. Only three of the electrodes are shown in FIGURE 8, but it will be understood that two additional electrodes would be employed to complete the array of FIGURE 2.
Referring to FIGURE 9, an improved detail of effecting electrical connection to the elastomeric electrode of FIGURE 8 is shown. In the FIGURE 9 arrangement, a recess or groove 51 in the nonconductive elastomer 48(b) is coated with a resilient conductive material, such as a band of resilient metalic turnings (as in the nature of steel wool) or woven conductive cable indicated at 52. The helical circuit wire is physically and electrically connected to this ring of conductive turnings or steel wool material 52, and the material 52 forms multiple electrical contacts with the conductive elastomer electrode. In this way, electrical continuity prom the electrode to the circuit wire is assured.
It is to be understood that while the details of 2$ electrode construction disclosed herein are considered to be important, the basic concept of the five electrode array and its improved results are not limited to the details of electrode construction set forth herein.
3~ While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordinglyl it is to be understood that the present invention has been described by way of illustrations and not limitation.
Claims (7)
1. Apparatus for measuring the apparent resist-ivity of a borehole, including:
an insualted segment of a drill string;
an array of five electrode means positioned at predetermined locations in said insulated segment;
a current source connected to a first of said electrode means;
first return circuit means connected from a second of said electrode means to said current source, said second electrode means being spaced from said first electrode means in a first direction;
third and fourth electrode means between said first and second electrode means;
voltage measuring means connected to said third and fourth electrode means for determining the voltage between said third and fourth electrode means;
current measuring means for determining the current in said first return circuit means;
fifth electrode means spaced from said first electrode means in a second direction; and second return circuit means connected from said fifth electrode means to said current source.
an insualted segment of a drill string;
an array of five electrode means positioned at predetermined locations in said insulated segment;
a current source connected to a first of said electrode means;
first return circuit means connected from a second of said electrode means to said current source, said second electrode means being spaced from said first electrode means in a first direction;
third and fourth electrode means between said first and second electrode means;
voltage measuring means connected to said third and fourth electrode means for determining the voltage between said third and fourth electrode means;
current measuring means for determining the current in said first return circuit means;
fifth electrode means spaced from said first electrode means in a second direction; and second return circuit means connected from said fifth electrode means to said current source.
2. Apparatus for measuring the apparent resist-ivity of a borehole as in claim 1 wherein:
said second electrode is the most downhole electrode in the array; and said fifth electrode is the most uphole electrode in the array.
said second electrode is the most downhole electrode in the array; and said fifth electrode is the most uphole electrode in the array.
3. Apparatus for measuring the apparent resist-ivity of a borehole as in claim 1 wherein said insul-ated drill string segment includes:
a cylindrical recess on the outer surface of the drill string segment; and insulating material in said cylindrical recess.
a cylindrical recess on the outer surface of the drill string segment; and insulating material in said cylindrical recess.
4. Apparatus for measuring the apparent resist-ivity of a borehole as in claim 3 wherein:
said insulating material is flush with the outer surface of said drill string segment.
said insulating material is flush with the outer surface of said drill string segment.
5. Apparatus for measuring the apparent resist-ivity of a borehole as in claim 4 wherein:
said electrode means are annular rings in said insulating material and spaced from the outer surface of said drill string segment.
said electrode means are annular rings in said insulating material and spaced from the outer surface of said drill string segment.
6. Apparatus for measuring the apparent resist-ivity of a borehole as in claim 3 wherein:
said insulation is adhered to said drill string and has a thermal coefficient approximately equal to that of the drill string segment.
said insulation is adhered to said drill string and has a thermal coefficient approximately equal to that of the drill string segment.
7. Apparatus for measuring the apparent resist-ivity of a borehole as in claim 3 wherein said current measuring means includes:
current measuring means in said first return circuit means.
current measuring means in said first return circuit means.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000508062A CA1227538A (en) | 1982-11-12 | 1986-04-30 | Electrode structure to measure borehole resistivity |
Applications Claiming Priority (10)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US44110782A | 1982-11-12 | 1982-11-12 | |
| US440,753 | 1982-11-12 | ||
| US06/440,755 US4738812A (en) | 1982-11-12 | 1982-11-12 | Method of forming an electrode structure |
| US440,755 | 1982-11-12 | ||
| US06/440,753 US4618828A (en) | 1982-11-12 | 1982-11-12 | Insulating segment for a drill string electrode structure |
| US441,107 | 1982-11-12 | ||
| US06/440,754 US4570123A (en) | 1982-11-12 | 1982-11-12 | Electrode array for measurement of borehole resistivity |
| CA000441023A CA1219039A (en) | 1982-11-12 | 1983-11-14 | Electrode structure to measure borehole resistivity |
| CA000508062A CA1227538A (en) | 1982-11-12 | 1986-04-30 | Electrode structure to measure borehole resistivity |
| US440,754 | 1995-05-15 |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000441023A Division CA1219039A (en) | 1982-11-12 | 1983-11-14 | Electrode structure to measure borehole resistivity |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1227538A true CA1227538A (en) | 1987-09-29 |
Family
ID=27508228
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000508062A Expired CA1227538A (en) | 1982-11-12 | 1986-04-30 | Electrode structure to measure borehole resistivity |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1227538A (en) |
-
1986
- 1986-04-30 CA CA000508062A patent/CA1227538A/en not_active Expired
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4575681A (en) | Insulating and electrode structure for a drill string | |
| US4738812A (en) | Method of forming an electrode structure | |
| US4570123A (en) | Electrode array for measurement of borehole resistivity | |
| US4618828A (en) | Insulating segment for a drill string electrode structure | |
| CN101592031B (en) | A combined propagation and lateral resistivity downhole tool | |
| CA2711853C (en) | Electromagnetic telemetry assembly with protected antenna | |
| US4684946A (en) | Device for transmitting to the surface the signal from a transmitter located at a great depth | |
| US4766442A (en) | Antenna structure for use with a transmitter located at a great depth | |
| US3518608A (en) | Telemetry drill pipe with thread electrode | |
| US7071697B2 (en) | Centralizer including measurement means | |
| CA2890603C (en) | Downhole electromagnetic telemetry apparatus | |
| US7986145B2 (en) | Logging tool sonde sleeve | |
| US4601100A (en) | Method for making logging sleeve | |
| CA2549541C (en) | Mass isolation joint for electrically isolating a downhole tool | |
| US5157331A (en) | Enhanced wide aperture groove for antenna of downhole resistivity tool | |
| US3057409A (en) | Well casing | |
| JP2007518906A (en) | Drill column for deep well and drill pipe and bush for the drill column | |
| GB2337546A (en) | Drillpipe structures to accomodate downhole testing | |
| CA1227538A (en) | Electrode structure to measure borehole resistivity | |
| GB2129946A (en) | Electrode structure for drill string | |
| EP0198985A1 (en) | Induction logging sonde with metallic support | |
| CA1219039A (en) | Electrode structure to measure borehole resistivity | |
| US20040186665A1 (en) | Measurement device and support for use in a well | |
| GB2344127A (en) | Drillpipe with tubular metallic structure | |
| JPS59225500A (en) | Signal transmitter |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |