CA1219039A - Electrode structure to measure borehole resistivity - Google Patents
Electrode structure to measure borehole resistivityInfo
- Publication number
- CA1219039A CA1219039A CA000441023A CA441023A CA1219039A CA 1219039 A CA1219039 A CA 1219039A CA 000441023 A CA000441023 A CA 000441023A CA 441023 A CA441023 A CA 441023A CA 1219039 A CA1219039 A CA 1219039A
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- drill string
- segment
- electrodes
- sleeve
- electrode structure
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Abstract
Abstract of the Disclosure A drill string segment of an electrode structure for a drill string has a recess along an axial length thereof. A sleeve of electrically nonconductive elastomeric material is disposed in the recess, and a plurality of bands of electrically conductive elastomeric material is embedded in the nonconductive elastomeric sleeve.
Description
~2~
Field of 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 arranyement -~ 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 ~ ~, 1:~19Q39 volt meter V, and the apparent formation resistivity Rf is determined from the values Il and ~ VM~. -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 an electrode structure for a drill string.
The structure includes a drill string segment having a recess along an axial length thereof. A sleeve of electrically nonconductive elastomeric material is disposed in the recess and a plurality of bands of electrically conductive elastomeric material is embedded in the nonconductive elastomeric sleeve.
A portion of each of the bands of electrically con-ductive elastomeric material is in contact with the .;~
` "
electrically nonconductive ~lastomeric material. A
resilient electrically conductive material is disposed between a portion of each of the bands of electrically conauc~ive elastomeric material and the electrically nonconductive elastomeric material.
In accordance with the invention, an electrode structure for drill string also includes a drill string segment having a recess along an axial length thereof.
A first sleeve of insulating material is disposed in the recess on the recessed surface of the drill string segment. A second sleeve of insulating material is disposed on the first sleeve of insulating material.
The first and second sleeves of insulating material are of different hardness, the second sleeve being harder than the first sleeve. A plurality of elec-trodes are embedded in the second sleeve.
The invention also relates to a method of formation of an array of electrodes on a segment of a drill string which includes the steps of positioning electrode support means along an axial length of a drill string segment. A plurality of electrodes are positioned on the support means at axially spaced locations along the length of the drill string segment and spaced from the outer surface of the length of drill string segment. An insulating material is molded to the axial length of drill string segment to produce an array of electrodes embedded in a length of insulating material on the drill string segment.
~2~g~39 - 3a -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 abo~e.
5FIGURE 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 10incorporate expansion partitions in the insulation.
FIGURE 5 shows another modified insulation structure.
FIGURE 6 shows a modification of the FIGVRE 5 structure.
15FIGURE 7 shows another modified electrode structure.
FIGURES 8 and 9 show further modifications of electrode and insulation structures.
~. i,~.,, 1219Q3g 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 below 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 B1. Electrode B1 is 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 current source 22 is connected to electrode A;
electrode ~l 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 26 is connected between electrodes M and N.
When constant current source 22 delivers a current I to electrode A, a current path I1 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 resistivity.
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 current Il flows 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 whlch 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 lZ~g~39 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 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.e., 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, tne 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.
~2~ 9 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 segmen~. 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 fill 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 modified 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 lZ190;~9 locations along the leng~h 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, it may be desirable to construct the insula~ion structure out of two different materials, a hard outer sleeve which will be exposed to the hostile drilling environment, and a softer inner sleeve 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, lZ190~9 sleeve 40 will serve to provide abrasion and similar protection, while sle~ve 38 will permit absorbtlon of lateral loads.
Th~ 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 45, 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 40a 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 6 will accommodate lateral deflection of the outer hard sleeve 40 but will reduce or limit the lZ~3~
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 embedding 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 the electrodes is a diamond shaped segment.
If the electrode encounters interference with a rock or other piece o~ debris, the forces will be generated alon~ the inclined surfaces o~ 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 12~9Q39 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 48(a), 48(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 4~ 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 lZ~90;}9 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 ~0 multiple electrical contacts with the conductive elastomer electrode. In this way, electrical continuity from the electrode to the circuit wire is assured.
It is to be understood that while the details of 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.
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. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.
What is claimed is:
Field of 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 arranyement -~ 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 ~ ~, 1:~19Q39 volt meter V, and the apparent formation resistivity Rf is determined from the values Il and ~ VM~. -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 an electrode structure for a drill string.
The structure includes a drill string segment having a recess along an axial length thereof. A sleeve of electrically nonconductive elastomeric material is disposed in the recess and a plurality of bands of electrically conductive elastomeric material is embedded in the nonconductive elastomeric sleeve.
A portion of each of the bands of electrically con-ductive elastomeric material is in contact with the .;~
` "
electrically nonconductive ~lastomeric material. A
resilient electrically conductive material is disposed between a portion of each of the bands of electrically conauc~ive elastomeric material and the electrically nonconductive elastomeric material.
In accordance with the invention, an electrode structure for drill string also includes a drill string segment having a recess along an axial length thereof.
A first sleeve of insulating material is disposed in the recess on the recessed surface of the drill string segment. A second sleeve of insulating material is disposed on the first sleeve of insulating material.
The first and second sleeves of insulating material are of different hardness, the second sleeve being harder than the first sleeve. A plurality of elec-trodes are embedded in the second sleeve.
The invention also relates to a method of formation of an array of electrodes on a segment of a drill string which includes the steps of positioning electrode support means along an axial length of a drill string segment. A plurality of electrodes are positioned on the support means at axially spaced locations along the length of the drill string segment and spaced from the outer surface of the length of drill string segment. An insulating material is molded to the axial length of drill string segment to produce an array of electrodes embedded in a length of insulating material on the drill string segment.
~2~g~39 - 3a -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 abo~e.
5FIGURE 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 10incorporate expansion partitions in the insulation.
FIGURE 5 shows another modified insulation structure.
FIGURE 6 shows a modification of the FIGVRE 5 structure.
15FIGURE 7 shows another modified electrode structure.
FIGURES 8 and 9 show further modifications of electrode and insulation structures.
~. i,~.,, 1219Q3g 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 below 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 B1. Electrode B1 is 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 current source 22 is connected to electrode A;
electrode ~l 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 26 is connected between electrodes M and N.
When constant current source 22 delivers a current I to electrode A, a current path I1 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 resistivity.
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 current Il flows 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 whlch 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 lZ~g~39 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 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.e., 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, tne 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.
~2~ 9 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 segmen~. 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 fill 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 modified 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 lZ190;~9 locations along the leng~h 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, it may be desirable to construct the insula~ion structure out of two different materials, a hard outer sleeve which will be exposed to the hostile drilling environment, and a softer inner sleeve 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, lZ190~9 sleeve 40 will serve to provide abrasion and similar protection, while sle~ve 38 will permit absorbtlon of lateral loads.
Th~ 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 45, 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 40a 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 6 will accommodate lateral deflection of the outer hard sleeve 40 but will reduce or limit the lZ~3~
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 embedding 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 the electrodes is a diamond shaped segment.
If the electrode encounters interference with a rock or other piece o~ debris, the forces will be generated alon~ the inclined surfaces o~ 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 12~9Q39 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 48(a), 48(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 4~ 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 lZ~90;}9 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 ~0 multiple electrical contacts with the conductive elastomer electrode. In this way, electrical continuity from the electrode to the circuit wire is assured.
It is to be understood that while the details of 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.
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. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.
What is claimed is:
Claims (22)
1. An electrode structure for a drill string, including:
a drill string segment having a recess along an axial length thereof;
a sleeve of electrically nonconductive elasto-meric material in said recess;
a plurality of bands of electrically con-ductive elastomeric material embedded in said non-conductive elastomeric sleeve, a portion of each of said bands of electrically conductive elastomeric material being in contact with said electrically non-conductive elastomeric material; and resilient electrically conductive material between a portion of each of said bands of electrically conductive elastomeric material and said electrically nonconductive elastomeric material.
a drill string segment having a recess along an axial length thereof;
a sleeve of electrically nonconductive elasto-meric material in said recess;
a plurality of bands of electrically con-ductive elastomeric material embedded in said non-conductive elastomeric sleeve, a portion of each of said bands of electrically conductive elastomeric material being in contact with said electrically non-conductive elastomeric material; and resilient electrically conductive material between a portion of each of said bands of electrically conductive elastomeric material and said electrically nonconductive elastomeric material.
2. An electrode structure for a drill string as in claim 1 wherein:
said electrically nonconductive elastomeric material and said electrically conductive elastomeric material are the same base materials.
said electrically nonconductive elastomeric material and said electrically conductive elastomeric material are the same base materials.
3. An electrode structure for a drill string as in claim 2 wherein:
said conductive elastomeric material is filled with electrically conductive material.
said conductive elastomeric material is filled with electrically conductive material.
4. An electrode structure for a drill string as in claim 1 wherein:
said resilient electrically conductive material is selected from the group consisting of resilient metallic turnings, steel wool or woven con-ductive cable.
said resilient electrically conductive material is selected from the group consisting of resilient metallic turnings, steel wool or woven con-ductive cable.
5. An electrode structure for a drill string as in claim 1 wherein:
said sleeve of nonconductive elastomeric material includes a plurality of interlocking rings of elastomeric material.
said sleeve of nonconductive elastomeric material includes a plurality of interlocking rings of elastomeric material.
6. An electrode structure for a drill string as in claim 5 wherein:
said electrically nonconductive elastomeric material and said electrically conductive elastomeric material are the same base materials.
said electrically nonconductive elastomeric material and said electrically conductive elastomeric material are the same base materials.
7. An electrode structure for a drill string as in claim 6 wherein;
said conductive elastomeric material is filled with electrically conductive material.
said conductive elastomeric material is filled with electrically conductive material.
8. An electrode structure for a drill string as in claim 1 including:
helical circuit wire being attached at one end of said resilient electrically conductive material and leading to a power source.
helical circuit wire being attached at one end of said resilient electrically conductive material and leading to a power source.
CLAIM 9. An electrode structure for a drill string, including:
a drill string segment having a recess along an axial length thereof;
a first sleeve of insulating material in said recess on the recessed surface of said drill string segment;
a second sleeve of insulating material on said first sleeve of insulating material;
said first and second sleeves of insulating material being of different hardness, with said second sleeve being harder than said first sleeve; and a plurality of electrodes embedded in said second sleeve.
a drill string segment having a recess along an axial length thereof;
a first sleeve of insulating material in said recess on the recessed surface of said drill string segment;
a second sleeve of insulating material on said first sleeve of insulating material;
said first and second sleeves of insulating material being of different hardness, with said second sleeve being harder than said first sleeve; and a plurality of electrodes embedded in said second sleeve.
CLAIM 10. An electrode structure for a drill string as in claim 9 wherein:
said second sleeve of insulating material provides protection against abrasion and said first sleeve permits absorption of lateral leads.
said second sleeve of insulating material provides protection against abrasion and said first sleeve permits absorption of lateral leads.
CLAIM 11. An electrode structure from a drill string as in claim 9 including:
spline elements formed on the outer surface of said drill string segment in said recess to form alternating axial lengths of thinner and thicker segments;
said first sleeve conforming to said drill string segment and splines in a coating of relatively uniform thickness; and said second sleeve having alternating axial lengths of thicker and thinner segments in alignment with the thinner and thicker segment, respectively, of said drill string segment.
spline elements formed on the outer surface of said drill string segment in said recess to form alternating axial lengths of thinner and thicker segments;
said first sleeve conforming to said drill string segment and splines in a coating of relatively uniform thickness; and said second sleeve having alternating axial lengths of thicker and thinner segments in alignment with the thinner and thicker segment, respectively, of said drill string segment.
CLAIM 12. An electrode structure for a drill string as in claim 11 wherein:
said electrode means are electrodes embedded in said thicker segments of said thicker segments of said second sleeve.
said electrode means are electrodes embedded in said thicker segments of said thicker segments of said second sleeve.
CLAIM 13. An electrode structure for a drill string as in claim 12 wherein:
said electrodes are generally cubic elements.
said electrodes are generally cubic elements.
CLAIM 14. An electrode structure for a drill string as in claim 12 wherein:
said electrodes are generally diamond shaped.
said electrodes are generally diamond shaped.
CLAIM 15. An electrode structure for a drill string as in claim 12 including:
channel means in at least one of said thicker segments of said drill string segment; and electric conductor means in said channel means for connection to said electrodes.
channel means in at least one of said thicker segments of said drill string segment; and electric conductor means in said channel means for connection to said electrodes.
CLAIM 16. An electrode structure for a drill string as in claim 9 wherein:
said electrodes are generally diamond shaped.
said electrodes are generally diamond shaped.
17. A method of formation of an array of elect-rodes on a segment of a drill string, including the steps of:
positioning electrode support means along an axial length of a drill string segment;
positioning a plurality of electrodes on said support means at axially spaced locations along said length of drill string segment and spaced from the outer surface of said length of drill string seg-ment; and molding an insulating material to said axial length of drill string segment to produce an array of electrodes embedded in a length of insulating material on said drill string segment.
positioning electrode support means along an axial length of a drill string segment;
positioning a plurality of electrodes on said support means at axially spaced locations along said length of drill string segment and spaced from the outer surface of said length of drill string seg-ment; and molding an insulating material to said axial length of drill string segment to produce an array of electrodes embedded in a length of insulating material on said drill string segment.
18. The method of formation of an array of elect-rodes on a segment of a drill string as in claim 17 wherein:
said axial length of drill string segment is recessed, and said support means, said electrodes and said insulating material are located in said recessed length.
said axial length of drill string segment is recessed, and said support means, said electrodes and said insulating material are located in said recessed length.
19. The method of formation of an array of electrodes on a segment of a drill string as in claim 18 wherein said step of molding includes:
positioning mold structure about said length of drill string segment to define a mold cavity;
introducing a molding material into said mold cavity, and curing the molding material to form said array of electrodes embedded in a length of molding material.
positioning mold structure about said length of drill string segment to define a mold cavity;
introducing a molding material into said mold cavity, and curing the molding material to form said array of electrodes embedded in a length of molding material.
20. The method of formation of an array of electrodes on a segment of a drill string as in claim 19 wherein:
said step of positioning support means includes locating a plurality of support elements along said recessed length of the drill string segment in an array which permits flow of insulating material along the recessed length; and said step of positioning electrodes includes positioning ring electrode elements on each of said support elements.
said step of positioning support means includes locating a plurality of support elements along said recessed length of the drill string segment in an array which permits flow of insulating material along the recessed length; and said step of positioning electrodes includes positioning ring electrode elements on each of said support elements.
21. The method of formation of an array of electrodes on a segment of a drill string as in claim 18 including:
positioning compensating means in said recess to compensate for differential expansion or contrac-tion between said drill string segment and said in-sulating material.
positioning compensating means in said recess to compensate for differential expansion or contrac-tion between said drill string segment and said in-sulating material.
22. The method of formation of an array of electrodes on a segment of a drill string as in claim 21 wherein:
said compensating means divides said recessed length into a plurality of recessed seg-ments; and wherein the step of molding includes:
positioning mold structure about the length of each recessed segment to define a plurality of mold cavities;
introducing a molding material into each mold cavity; and curing the molding material to form said array of electrodes embedded in a length of molding material with expansion compensating means.
said compensating means divides said recessed length into a plurality of recessed seg-ments; and wherein the step of molding includes:
positioning mold structure about the length of each recessed segment to define a plurality of mold cavities;
introducing a molding material into each mold cavity; and curing the molding material to form said array of electrodes embedded in a length of molding material with expansion compensating 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 (8)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US44110782A | 1982-11-12 | 1982-11-12 | |
| US440,755 | 1982-11-12 | ||
| US441,107 | 1982-11-12 | ||
| US440,754 | 1982-11-12 | ||
| US06/440,754 US4570123A (en) | 1982-11-12 | 1982-11-12 | Electrode array for measurement of borehole resistivity |
| US440,753 | 1982-11-12 | ||
| US06/440,753 US4618828A (en) | 1982-11-12 | 1982-11-12 | Insulating segment for a drill string electrode structure |
| US06/440,755 US4738812A (en) | 1982-11-12 | 1982-11-12 | Method of forming an electrode structure |
Related Child Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000508062A Division CA1227538A (en) | 1982-11-12 | 1986-04-30 | Electrode structure to measure borehole resistivity |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1219039A true CA1219039A (en) | 1987-03-10 |
Family
ID=27503877
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000441023A Expired CA1219039A (en) | 1982-11-12 | 1983-11-14 | Electrode structure to measure borehole resistivity |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1219039A (en) |
-
1983
- 1983-11-14 CA CA000441023A patent/CA1219039A/en not_active Expired
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