US3377549A

US3377549A - Coil assembly structure and mounting forming an annular chamber to be mounted and sealed on a drill collar

Info

Publication number: US3377549A
Application number: US420544A
Authority: US
Inventors: James L Newman; William F Osborn; Fletcher H Redwine; Paul J Arnerich
Original assignee: ARPS CORP
Current assignee: ARPS CORP
Priority date: 1964-12-23
Filing date: 1964-12-23
Publication date: 1968-04-09
Anticipated expiration: 1985-04-09

Description

April 9, 1958 J. L. NEWMAN ETAL 3,377,549

COIL ASSEMBLY STRU TURE AND MOUNTING FORMING AN ANNULAR CHAMBER TO BE MOUNTED AND SEALED ON A DRILL COLLAR Filed Dec. 23, 1964 4 Sheets-Sheet 1 W FIG. [A FIG. IB

ANNULAR Aprll 1968 J. NEWMAN ETAL COIL ASSEMBLY STRUCTURE AND MOUNTING FORMING AN CHAMBER TO BE MOUNTED AND SEALED ON A DRILL COLLAR Filed Dec. 25, 1964 4 Sheets-Sheet Z FIG. 3

A ril 9, 1968 J. L. NEWMAN ETAL 3,

COIL ASSEMBLY STRUCTURE AND MOUNTING FORMING AN ANNULAR CHAMBER TOBE MOUNTED AND SEALED ON A DRILL COLLAR Filed Dec. 23, 1964 4 Sheets-Sheet 4 n\ if h 234 5R FIG.8A FIG.8B

United States Patent $377,549 COIL ASSEMBLY STRUCTURE AND MOUNT- ING FORMING AN ANNULAR CHAMBER TO BE MOUNTED AND SEALED ON A DRILL COLLAR James L. Newman, Richardson, William F. Osborn, Dal- Irving, and Paul J. Arnerich,

las, Fletcher H. Redwine, Dallas, Tex., assignors to Arps Corporation, Garland, Tex., a corporation of Delaware Filed Dec. 23, 1964, Ser. No. 420,544 9 Claims. (Cl. 324-6) ABSTRACT OF THE DISCLOSURE This invention relates to well logging and has particular application to systems for determining properties or characteristics of earth formations in a borehole while the borehole is being drilled.

- Various systems have been proposed or have been employed in the past for logging a well bore as it is being drilled to determine one or more characteristics of the formation being penetrated by the bit. In general, such methods of logging while drilling utilize a number of spaced electrodes placed near the lower end of, but insulated from the drill string. In some systems, the :bit itself has been electrically insulated from the rest of the drill pipe so that electric current may be passed between the two electrically-insulated elements through the adjacent formation. The resistivity of the formation can thenbe determined by the flow of current from the drill string to the drill bit. These prior systems have inherent common disadvantages resulting primarily from the necessity to electrically insulate a section of the drill collar from the rest of the drill string. Available insulating materials cannot withstand the violent shocks in tension, compression and shear which must be transmitted mechanically through the drill stem to the bit.

In the type of system for logging resistivity While drilling disclosed in US. Patent No. 2,354,887, a toroid-shaped coil of wire wound on a core of ferromagnetic material encircles the drill collar near the drill bit with the axis of the toroid parallel to and preferably coinciding with the axis of the drill collar. The impedance of such a coil is affected by variations in the impedance of the electric path of its one-turn secondary winding consisting of the drill pipe, the formation near the drill bit and its return path through the earth to produce a measurement of the formation resistance adjacent the drill bit. It was found that the variations in the electrical resistance of the formation near the bit in the normal range of such values made it extremely difiicult to make meaningful measurements. Therefore as described in copending US. application Ser. No. 298,298, entitled, Formation Resistivity Measurement While Drilling, filed July 29, 1963, by Fletcher H. Redwine, et al., a system was described wherein both an input coil and a detector coil are used. The voltage induced in the detector coil as a result of the current induced in the one-turn secondary winding by the input coil is proportional to the conductivity of the formation near the drill bit.

Due to the extreme sensitivity required to measure the relatively small variations in the resistivity of the earth formations, considerable difiiculty has been encountered in the physical construction of a sub carrying the two toroid coils which has the necessary sensitivity and also has a useful life of practical duration. The sensitivity problem is complicated by the fact that when the coils are lowered into the well bore they are subjected to a very high pressure. Experience has shown that often this pressure distorts the ferromagnetic coil to such an extent as to materially alter its magnetic properties and distort the measurements of the system. In fact, in some instances the magnetic properties of the system have been changed so radically that the measured values fall completely outside the calibration values previously obtained at the surface. The high pressures also tend toenter the sub and destroy the delicate electrical leads to the coils.

Another very real and difiicult problem lies in the construction of a sub which is sufficiently rugged to withstand the high abrasion and shocks suffered by the sub and coils as the sub is rotated in the well bore, yet which does not interfere with the operation and sensitvity of the coils. The coils cannot be encased in an electrically-conductive material such as steel because the steel would in effect be a short-circuit to the system.

Therefore it is an important object of the present invention to provide a coil sub construction wherein the core material of the coil is protected from pressure differentials which would tend to distort the material.

Another object of the invention is to provide a coil sub of the type described wherein the delicate coils are protected by a case of steel without providing an electricallyconductive path around the coil.

A further object of the invention is to provide a coil sub of the type described which is relatively simple to manufacture and which maybe assembled and overhauled with relative ease.

Another object of the invention is to provide a coil assembly which will not be adversely affected by changes in environmental pressures.

Still another object of the invention is to provide a coil sub which will better withstand the shocks, abrasion, pressures and temperatures of downhole drilling operations.

In accordance with the present invention, a rigid, annular, open-ended structure encircles a drill collar sub and forms an open-ended annular chamber. A toroidal coil is disposed in the chamber and around the sub, and the chamber is filled with a fluid material which will flow to equalize pressure as the sub is subjected to high bottom hold pressures. A rubber insulating ring is disposed around the sub adjacent the open end of the chamber and means are provided for forcing the end of the rubber ring against the structure. More specifically, the structure is formed of inner and outer sleeves which are interconnected at one end. The outer sleeve is longer at the open end than the inner sleeve, and the rubber insulating ring has an annular tongue which extends under the edge of the outer sleeve.

This invention also contemplates a sub assembly comprised of a tubular body having an annular shoulder formed at one end. Upper and lower coil assemblies are disposed around the body, each coil assembly being constructed as described above with the associated rubber insulating rings. A metal sleeve separates the coil assemblies and a means is connected to the tubular body to force the several parts together and against the shoulder as well as hold them in place on the tubular body.

Additional, more detailed aspects of the invention are hereafter pointed out in the appended claims, which, it is to be understood, constitute the sole limitations on the scope of this invention.

Additional objects and advantages of the invention will be apparent from the following detailed description and accompanying drawings, wherein:

FIGURES IA and 1B, in conjunction, are a longitudinal sectional view of a coil sub constructed in accordance with the present invention;

FIGURE 2 is a perspective view of a toroidal coil constructed in accordance with the present invention;

FIGURE 3 is a perspective view of a portion of the coil assembly of FIGURE 2;

FIGURE 4 is a sectional view taken substantially on lines 44 of FIGURE 2;

FIGURE 5 is a sectional view taken substantially on lines 5-5 of FIGURE 4;

FIGURE 6 is an enlarged cross-sectional view of one side of a toroid coil assembly constructed in accordance with this invention which is used in the sub of FIGURES lA-lB;

FIGURE 7 is a sectional view of an alternative toroid coil assembly constructed in accordance with the present invention which may be used in the sub of FIGURES 1A-1B; and,

FIGURES 8A and 8B, in conjunction, are a longitudirial sectional view of another coil sub constructed in accordance with the present invention.

Referring now to the drawings, and in particular to FIGURES lA-IB, a coil sub constructed in accordance with the present invention is indicated generally by the reference numeral 10. The upper half of the sub is shown in FIGURE 1A and the lower half is shown in FIGURE 1B. The sub 10 is comprised of a tubular body portion 12 having a central fluid passageway 14 extending therethrough, a standard thread box 16 at the lower end, and a standard thread pin 18 at the upper end. The pin of an adapter 20 is threaded into the box 16, and the adapter 20 has a box 22 into which the conventional drill bit 23 may be threaded. The pin 18, on the other hand, is connected to a special sub (illustrated in dotted outline and designated by the reference numeral 21) which contains a telemetering system for transmitting the resistance values measures by the sub 19 to the surface as a series of pressure pulses in the mud stream as described in greater detail in the above-referenced patent application. The body portion 12 has a maximum diameter at 24 and is turned down to a diameter 26 below the shoulder 28 formed by the lower end of a band 30 welded to the body by a seam 32. The band 30 covers bores 34 and 36 which form passageways for conductors which will presently be described. Elongated grooves 38 (only one of which is illustrated) extend downwardly along the turned-down portion 26 of the body 12 to form passageways for electrical lead wires extending from the lower end of the sub 10 as will presently be described.

An upper coil assembly, indicated generally by the reference numeral 40, is disposed around the turned-down portion 26 and abuts against the band 30. The upper end of the coil assembly 40 is keyed to the band 30 and therefore to the body 12 by means of longitudinally-extending shoulders, one of which is indicated in dotted outline at 42. It is necessary to key the coil assembly 40 to the body 12 so that the coil assembly 40 will not rotate and shear the electrical leads to the coil which will hereafter be described. The coil assembly 40 has a toroidally-wound coil 44 which is received within an open-ended chamber formed by an inner sleeve 46 and an outer sleeve 48. The construction of the toroidal coil 44 will hereafter be described in considerably greater detail. The

sleeves

46 and 48 are fabricated from metal and the upper ends of the sleeves are joined together by annular overlapping

shoulder portions

46a and 48a where they are bolted together. The lower end of the outer sleeve 48 is substantially longer than the lower end of the inner sleeve 46, and the lower ends of the two sleeves are separated so as to interrupt the conductive path around the coil. A continuous metal path, in essence, would constitute a short circuit to the system. An electrically non-conductive ring 50, preferably fabricated from rubber, is adherently bonded to a metal or other rigid sleeve 52 for support and is disposed around the body 12. The ring has an upper lip 54 which extends under the lower end of the ring 48 and a lower lip 56 which extends under a metal sleeve 58 which is also disposed around the body portion 12. The sleeve 58 is secured against rotation relative to the body 12 by a key 60 which is fitted in longitudinally-extending grooves in the sleeve and body.

A lower coil assembly indicated generally by the reference numeral 62 of substantially the same construction as the upper coil assembly 40 is abutted against the lower end of the sleeve 58 and is keyed to the sleeve 58 to prevent rotation about the body 12 by key shoulders, one of which isindicated in dotted outline at 64. The lower coil assembly 62 is substantially identical to the upper coil assembly 40 and is also comprised of an inner sleeve 65, an outer sleeve 66, and a toroidal coil 68. A snap ring fits in a groove at the lower end of the body 12 to retain the coil assembly 62, sleeve 58, insulating ring 50 and upper coil assembly 40 on the body 12 merely to facilitate assembly. A lower insulating ring 72, preferably fabricated of rubber, is bonded to a metal backup ring 74 and has a lip portion 76 which extends under the lower end of the outer sleeve 66. The adapter 20 has a shoulder 78 which abuts against the lower end of the body 12 and the insulating ring 72. The total length of the sleeve-like members which are positioned around the body 12 exceeds the distance from the shoulder 28 to the lower end of the body 12 so that the insulating

rings

72 and 50 will be pressed against the open ends of the

coil assemblies

40 and 62, and in particular against the lower ends of the

outer sleeves

48 and 66, and the several sleeve members held in place on the body.

The coil assembly 40, for example, is shown in FIG- URE 2. The coil assembly is comprised of a number of turns of a thin magnetic core material. The core material is commercially available and may be several inches in width and have a thickness like that of a thick metal foil or thin sheet metal. The core material 80, referring to FIGURE 5, is housed within a U-shaped aluminum box 82 having a closed end 86 and an open end 84. The box is then preferably wrapped in a suitable tape 88, such as Mylar tape or the like, in order to facilitate assembly. A circular plastic retainer ring 94 (see FIGURE 3) having a U-shaped cross section (see FIGURES 4 and 5) containing semicircular,

stiff lead wires

92 and 93 is then positioned against the closed end of the box 82 and taped in place by tape 89. The

lead Wires

92 and 93 are enclosed in insulating sleeves and have

portions

92a and 93a which extend through apertures in the retaining ring 94 and extend parallel to the axis of the coil to provide the male halves of bayonet-type couplings to be described. The other ends of the

lead wires

92 and 93 have

flexible pigtail portions

92b and 93b which also extend through openings in the plastic ring 94. Next a number of U- shaped plastic feet or spacers 100 are placed at peripherially spaced points over the opposite ends of the box 82 and glued in place. Then the wire windings 90 of the toroidal coil assembly are connected to one of the pigtail portions 9212 or 93b of one lead wire and wrapped around the core box 82 with uniformly spaced turns, then connected to the pigtail portion of the other lead wire. The entire assembly may then be further wrapped with Mylar tape 95 wound by toroidally and circumferentially to protect the windings 90.

Next the coil assembly is slipped down over the inner sleeve 46 as illustrated in FIGURE 6. The spacers make a snug fit with the sleeve 46. The leads 92 and 93 are passed through bores 95 (only one being illustrated in FIGURE 6), and through a plastic sealing grommet 97 positioned in a counterbore 99. An insert 93 is threaded into the counterbore 99 to swage the sealing grommet around the lead wire 92. The end 92a of the lead wire 92 together with the shank 97a form the male'portion 96 of a bayonet coupling. The outer sleeve 48 is then placed around the inner sleeve 46 with the shoulder 48a abutting the shoulder 46a and bores 101 positioned to receive the male halves 96 of the couplings. The inner and

outer sleeves

46 and 48 may be interconnected by socket head screws (such as the screw 104 in FIGURE 1A) which extend through the inwardly-directed shoulder portion 48a of the outer sleeve into the outwardly-directed shoulder portion 46a of the inner sleeve. An O-ring 106 may be provided between the shoulder portion 46a of the inner sleeve and the outer sleeve 48 to effect a fluid seal. A retaining ring 102 is snapped in place to retain the core within the annular cavity formed between the inner and

outer sleeves

46 and 48.

The annular chamber formed between the inner and

outer sleeves

46 and 48 is filled with a suitable potting fluid material 108. The potting fluid 108 is preferably a silicon potting compound such as General Electrics sili con compound RTV No. 615. The silicon potting corn pound is highly fluid when first placed in the cavity so that it permeates the various layers of Mylar tape wound about the core box 82 and uniformly distributes itself between "the turns of magnetic core material 80. When the potting compound 108 is cured, it has a consistency similar to stiff gelatin and the assembly can be inverted to the position shown in FIGURES 1A and 1B for assembly without any flow of the potting material. However, when the exposed surface of the potting material is subjected 'to bottom hole pressure, the material is sufliciently fluid to exert a uniform and equalized pressure on all exposed surfaces so as to insure that the delicate core material and wire windings are not stressed and deformed to vary their electric or magnetic properties.

An alternative construction for the core assembly 40 is illustrated in FIGURE 7 and is designated generally by the reference character 40a. The coil assembly 40a is substantially identical with the coil assembly 40 except that oil 109, such as hydraulic fluid, is used for the potting compound which fills the chamber formed between the inner and

outer sleeves

46 and 48. Accordingly the core and windings are represented collectively and schematically by the reference character 91. In the assembly 40a, a crush ring 110 is provided at the open end of the annular chamber adjacent the spacers The crush ring is retained in place by a snap ring 112 which may be located in the inner wall of the outer sleeve 48. The oil is sealed in the chamber by an annular elastic seal 114 which is press-fitted between the ends of the inner and

outer sleeves

46 and 48. The sealing ring 114 and'the O- ring seal 106 are adequate to retain the oil within the chamber. Downhole pressure acting on the sealing ring 114 will be transmitted to the oil within the chamber to equalize the pressure around the core and prevent distortion and stress.

The electrical leads to the upper coil assembly 40 are-connected by a female coupling 98 to the male coupling 96 and extend upwardly to a second male coupling 122 through the passageway formed by a cavity 124 in the ring 30, the bore 36, and a bore into which the male half of a coupling half 122 is threaded. The lead wires 126 from the lower coil assembly62 are connected to the wires of the coil 68 by a bayonet-type coupling 128 and extend upwardly'through a passageway formed by'a cavity 130 in the body '12, a cavity 131 formed in the sleeve 58, the groove 38 cut in the body 12, the bore 34, and a bore into which the male half 132 of a second bayonet coupling is threaded. Although only one lead wire for each of the

coils

40 and 62 is illustrated, it will be understood that two such lead wires are provided at peripherally spaced points around thebody'12.

The

electrical lead wires

120 and 126 are'connected to a telemetering system as heretofore described by means of a cross-over assembly indicated generally by the reference numeral 133 and illustrated in FIGURE 1A. The

cross-over assembly 133 is comprised of a body 134 which has an annular flange 135 which abuts a resilient gasket 136 against the end of the pin 18 of the body 12 to absorb vibration. A body 134 has a first sleeve portion 138 which extends into a counterbore 140 in the pin 18 and abuts against the shoulder 142 formed between the counterbore 140 and bore 14. An O-ring seal 144 is provided to seal the upper end of cavities 146 and 148 which receive the male coupling halves 122 and 132. A second sleeve portion 150 extends downwardly into the bore 14 and the space between the sleeve portion 150 and body 12 is sealed by a pair of O-ring seals 152 to seal the lower ends of the cavities 146 and 148. Female coupling halves 154 and 156 for mating with the

male halves

122 and 132 are threaded into bores in the body 134 and are connected to electrical

lead wires

158 and 160 which extend upwardly through passageways to a master coupling 162 which contains axially-spaced contacts for all four electrical leads. A plurality of elongated slots 164 permit fluid to pass back into the passageway 14 after having passed through the fluid annulus formed in the telemetering sub 21. The sub 21 has an annular shoulder 165 which abuts against the shoulder 135 to secure the cross-over assembly 133 in position between the two subs.

When assembling the tool 10, the

coil assemblies

40 and 62 are assembled and potted in the fluid material as previously described. The leads 120 and 126 are placed in the respective passageways and the

couplings

122 and 132 threaded into the respective bores. The coil assembly 40 is slipped over the lower end of the body 12 and the

couplings

96 and 98 connected. The key shoulders 42 are then mated and the excess length of the lead 120 folded in the cavity formed by the bore 36. Next the insulating ring 50 is slipped over the end of the body 12 into place. The key 60 is positioned in the mating groove in the body 12 and the metal sleeve 58 slipped into position. Then the lower coil assembly 62 is slipped over the body 12 and the electrical leads interconnected by the couplings 128. The excess length of the lead 126 may then be crimped into the cavities 34 or 130131 as the coil assembly is abutted against the lower end of the sleeves 58 and the keying shoulders 64 engaged. The snap ring 70 is then inserted to hold these members in place on the body 12. Next the lower insulating ring 72 is positioned around the body 12 and the adapter 20 threaded tightly into the box 16. A suitable potting material is pumped under relatively high pressure into all cavities formed between the various members. For example, the potting material may be pumped inwardly through the bores to fill the cavities formed by the bores 34 and 36. A number of additional bores (not illustrated) may be provided at spaced points around the periphery and over the length of the tool for the injection of the potting fluid. It is particularly important that the potting fluid completely fill all cavities associated with the electrical leads. Otherwise, the bottom hole pressures will force the potting materials, along with the lead wires, into any unfilled cavities and thereby sever the electrical leads.

Next the cross-over assembly 133 is inserted in the pin 18 and the

couplings

154 and 156 aligned and mated with the

couplings

122 and 132. It will be noted that the cavities containing these couplings are not filled with a potting material but are completely sealed by the O-

rings

144 and 152 and the threaded connections between the coupling halves and the sub body 12 and cross-over 134. The telemetering equipment is connected through the coupling 162 and the sub 21 threaded onto the pin 18 so as to bear on the annular shoulder 135 and securely retain the cross-over assembly 133 in place between the two subs.

When the sub 10 is lowered into the well bore, the high downhole pressure will enter through the various abutting joints and apertures and place the potting material under uniform compression. If all the apertures and voids within the assembly are properly filled with the potting material, the potting material will not migrate and the various electrical leads and various other delicate members of the assembly will not be harmed.

In accordance with an important aspect of this invention, the downhole pressure acting on the cavity formed between the inner and

outer rings

46 and 48 will be transmitted uniformly throughout the

fluid potting material

108 or 109, as the case may be, which has permeated the box 82 containing the magnetic core material 80. This is important because if the magnetic core material 80 is bent, stretched, or significantly deformed, its magnetic characteristics are changed such that previous calibrations of the core assembly become inaccurate and often are changed to such an extent as to exceed the range of the recording instruments. The toroidal coils are protected by the steel sleeve 48 such that the normal abuse suffered by the sub in the drilling operation does not adversely affect the delicate coils. The

rubber insulating rings

50 and 72 are partially protected by the adjacent steel sleeve 48 and sleeve 58 and the adapter 20 and therefore have a relatively long life.

Another coil sub constructed in accordance with the present invention is indicated generally by the reference numeral 200. The upper half of the sub is illustrated in FIGURE 8A and the lower half is illustrated in FIG- URE 8B. The sub 200 employs upper and lower

toroidal coil assemblies

202 and 204 both of which are of the same construction as the

coil assemblies

40 or 62 heretofore described. However, the sub 200 has a body 206 having a maximum diameter portion 208 which forms a box for receiving the pin of a conventional drill bit. A pin 209 is formed at the other end of the body. The body 206 has an upwardly-facing shoulder 210 formed between the maximum diameter of the portion 208 and a minimum diameter portion 212. The minimum diameter portion 212 is of uniform diameter except for a keyway 214 and a pair of grooves 216 (only one of which is illustrated) for the electrical lead wire 218 to the lower coil assembly 204. A lower insulating ring 220 is bonded around a metal reinforcing band 222 and the lower end is abutted against the shoulder 210. The outer sleeve of the lower coil assembly 204 extends down over the lip portion 224 of the insulating ring 220. A sleeve226 is splined to the body 206 by a key 208 received in the groove 214. The upper end of the coil assembly 204 is keyed to the sleeve 226 by mating offset semicircular extensions which abut along opposite shoulders 230 shown in dotted outline. An upper insulating ring 231 is bonded to a metal band 232 and has a tongue portion 234 on the lower end which extends under an annular lip formed at the upper end of the sleeve 226, and a tongue portion 236 which extends under the lower end of the outer band 2380f the upper coil assembly 202. The lower end of a retaining sleeve 240 abuts against the upper end of the coil assembly 202 and the two parts are keyed against relative rotation by opposite vertical shoulders 242 indicated in dotted outline. The retaining sleeve 240 is secured on the body 206 and is also secured against rotation relative to the body 206 by a plurality of set screws 241 spaced about its periphery which are threaded into recesses formed in the body 206.

Thus it will be noted that the lower coil assembly 204 is secured against rotation by the keyed connection to the metal sleeve 226 which is in turn splined to the body 206, and the upper coil assembly 202 is secured against rotation by the keyed connection to the retaining sleeve 240 which is in turn secured to the body 206 by the set screws 241. Of course the set screws 241 also secure the insulating ring 220, coil assembly 204, sleeve 226, insulating ring 231, and coil assembly 202 in place on the body 206 in close, abutting, end-to-end relationship. The electrical lead wire 218 to the lower coil assembly 204 and the lead wire 244 to the upper coil assembly 202 may extend through passageways formed by bores 246 and 243, respectively, to a cross-over assembly 250.

The cross-over assembly 250 has a tubular body 251 which is inserted in a counterbore 252 in the pin 209. O-

ring seals

253 and 254 seal the pressure within the drill string from the pressure outside the drill string and also isolate the passageway for the lead wires. The upper portion of the cross-over assembly 250 may be constructed similar to that of the cross-over assembly 133 and accordingly is not illustrated.

The principal difference between the sub 200 and the sub 10 is the manner in which the various sleeve com ponents are assembled and held in place on the

respective bodies

12 and 206. In the sub 10, the adapter 20 is used to secure the various slipon components in place, and the components are slipped on from the lower end of the body 12. In the sub 200, both the box 208 and the pin 209 are integral with the sub, and the components are slipped on from the top and screwed in place by the retaining sleeve 240 and the set screws 241. The sub 200 is preferably potted as described in connection with the sub 10 so that all voids in the structure are uniformly filled with a fluid potting material. However, the crossover assembly 250 must first be placed in the position illustrated before the potting material is introduced under pressure so that the passageways for the

electrical lead wires

218 and 244 will be filled.

From the above detailed description of preferred embodiments of the invention, it will be noted that an improved coil sub has been described wherein the coils are well protected from abrasion and impact by the steel rings forming the open-ended annular chambers in which the coils are disposed. Further, the construction is such that the pressure around the individual turns of the magnetic core material is equalized at all times to prevent distortion. Thus as the tool is subjected to the great bottom hole pressures, the delicate core material is not warped, bent, or stretched and continues to have the same magnetic properties. All voids in the coil sub are filled with fluid potting material so as to prevent damage to the delicate electrical leads. The strong metallic housing for the toroidal coils does not, however,-form a complete electrical path around the coil which would constitute a shorted turn and prohibit making the measurements which are required.

Although preferred embodiments of the invention have been described in detail, it is to be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

In a system for logging formation resistivity while drilling in a borehole, a coil subcombination comprising:

a tubular body adapted to be connected into a drill string having a central bore extending therethrough and an annular shoulder formed adjacent one end,

upper and lower coil assemblies disposed around the tubular body, each coil assembly being comprised of a rigid, annular, open-ended housing forming an open-ended chamber, a toroidal coil disposed in the open-ended chamber, and a fluid material filling the voids in the annular chamber,

a rubber insulating ring disposed around the tubular body adjacent the open end of each coil assembly and engaging the responsive assemblies,

a metal sleeve disposed around the tubular body and separating the coil assemblies, and

means connected to the end of the tubular body opposite from said shoulder for forcing the coil assemblies, the rubber insulating rings, and the metal sleeve into end-to-end abutting relationship between said means and said shoulder and for retaining the assemblies, rings and sleeves in place on the tubular body.

2. The coil sub defined in claim 1 wherein said means comprises:

an adapter structure threadedly connected at one end to said tubular body and abutting the rubber insulating ring adjacent the open end of said lower coil assembly.

3. The coil sub defined in claim 2 wherein:

one end of the upper coil assembly is disposed adjacent the annular shoulder and is keyed therewith to prevent rotation,

one rubber insulating ring is disposed adjacent to the other end of the upper coil assembly,

one end of the metal sleeve is disposed adjacent said one rubber insulating ring and is keyed to the tubular body to prevent rotation,

one end of the lower coil assembly is disposed adjacent the other end of the metal sleeve and is keyed therewith to prevent rotation,

the other rubber insulating ring is disposed adjacent the other end of the lower coil, and

the adapter structure abuts against said other rubber insulating ring.

4. The coil sub defined in claim 1 wherein:

the annular shoulder is disposed in the region below said lower coil assembly, and

the means connected to the end of the tubular body is a second sleeve disposed around the tubular body and secured thereto by set screws.

5. The coil sub defined in claim 4 wherein:

one end of one of the rubber insulating rings is disposed adjacent to the annular shoulder,

the open end of one of the coil assemblies is disposed adjacent the other end of said one rubber insulating ring,

one end of the metal sleeve is disposed adjacent the other end of said one coil assembly and is keyed to the coil assembly and to the tubular body to prevent rotation,

one end of the other rubber insulating ring is disposed adjacent to the other end of the metal sleeve,

the open end of the other coil assembly is disposed adjacentthe other end of the other rubber insulating ring, and

the other end of the other coil assembly is disposed. adjacent the second sleeve and is keyed therewith to prevent rotation of the coil assembly. 6. The coil sub defined in claim 1 further characterized by:

passageways for electrical lead wires extending through the sub from each of the coil assemblies to one end of the sub,

electrical lead wires extending through the passageways and connected to the respective coils and to electrical couplings at said one end of the sub, and

a fluid potting material filling the passageways and all voids in fluid communication therewith to prevent migration of the lead wire as a result of unequalized down hole pressures.

ized by:

a counterbore in one end of the tubular body forming an outwardly-facing annular shoulder, the electrical lead wires being connected to bayonet coupling halves disposed at peripherally spaced points around the annular shoulder and aligned parallel to the axis of the sub,

a cross-over assembly including a tubular portion inserted in the counterbore and having mating coupling halves in the end of the whole portions connected to said bayonet coupling halves around the annular shoulder, lead wires connected to said mating coupling halves extending through passageways to the other end for connection to additional circuitry, fluid ports in the sides of the cross-over assembly for passing drilling fluid from the outside to the inside of the tubular portions, and

annular sealing means between the cross-over assembly and the tubular body on each side of the mating couplings for forming a fluid-tight chamber around the couplings.

8. The coil sub defined in claim 7 wherein:

the coupling halves are threadedly connected to the respective tubular body and cross-over assembly to provide a fluid-tight seal.

9. The coil sub defined in claim 6 further characterized by a counterbore in one end of the tubular body forming an outwardly-facing annular shoulder,

a crossover assembly housing a tubular portion disposed in the counterbore,

passageways in the cross-over assembly forming continuations of the passageways in the tubular body of the sub and extending to the outer end of the cross-over assembly,

annular sealing means between the tubular portion and the tubular body disposed on opposite sides of the passageways to seal the passageways from fluid within and without the sub, and wherein the electrical leads extend through the passageways in the cross-over assembly to coupling halves at the upper end of the cross-over assembly.

References Cited UNITED STATES PATENTS 2,411,696 11/1946 Silverman et al.- 324-1 X 2,964,698 12/1960 Lehmberg 32410 3,079,549 2/1963 Martin 324--6 X 3,249,857 5/1966 Simon-Suisse 324-6 RUDOLPH V. ROLINEC, Primary Examiner. WALTER L. CARLSON, Examiner.

55 G. R. STREC'KER, Assistant Examiner.