Abstract:
A gap sub comprises at least one, and in a preferred embodiment, two electrically isolating threaded joints in a conductive tubular collar. The threads of the joints are isolated by a non-conductive coating in accordance with the prior art. The shoulders of the joints are separated by an isolating ring. Annular faces on the ring make contact with the shoulders in the joints. The ring is made of conductive material with a non-conductive coating deployed on (1) at least one, and advantageously both, of the ring&#39;s annular faces, and (2) the interior surface of the ring. An annular recess in the collar prevents electrical contact between collar sections nearby the ring either side of each threaded joint.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of, and priority to, both of the following two commonly-invented U.S. Provisional Applications: (1) Ser. No. 61/683,271, filed Aug. 15, 2012; and (2) Ser. No. 61/781,617, filed Mar. 14, 2013. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This disclosure is directed generally to technology useful in measurement-while-drilling (“MWD”) applications in the oil and gas exploration field, and more specifically to isolation technology in electromagnetic (“EM”) telemetry. 
       BACKGROUND OF THE INVENTION 
       [0003]    Ultra-low frequency (ULF) electromagnetic (EM) waves are the preferred transmission mechanism for wireless subterranean telemetry applications due to the ULF wave&#39;s ability to propagate long distances through the Earth&#39;s strata. In a typical subterranean telemetry application, the desired telemetry information is digitally encoded into data packets and sent as modulated “bursts” of ULF carrier waves. Transmission of the carrier waves is physically facilitated by injecting a modulated current into the Earth media using a power amplifier to create a time-varying voltage potential between two transmit electrodes coupled to the Earth media. The electrodes are spaced such that the induced current traverses a section of the Earth media creating associated electric and magnetic field energy which radiates as time-varying wave fronts through the Earth media. 
         [0004]    According to a conventional EM telemetry system, a lower portion of drill string is typically isolated electrically from the upper portion, so that the electrically-isolated lower portion may act as an antenna to transmit or receive ULF carrier waves to or from the surface through the Earth&#39;s strata. Transmission and reception by the antenna is enabled by circuitry within a transceiver located in the lower drill string portion below the point of electrical isolation. The transceiver is conventionally deployed in an antenna sub located just below the point of electrical isolation. In receive mode, the transceiver is connected to the lower drill string portion acting as an antenna that is electrically isolated from the surface. The transceiver may thus receive EM waves propagated from the surface through the Earth&#39;s strata. In transmit mode, the transceiver&#39;s tendency is to want to transmit using the entire drill string as an antenna. However, EM waves propagated by the transceiver are forced to “jump” the point of electrical isolation by passing through the surrounding Earth media. In so doing, the EM waves are thus forced to propagate through the Earth&#39;s media, where they may be received by the surface antennae. The EM system may therefore enable tools on the drill string to intercommunicate with the surface via encoded data packets modulated onto the transceived carrier waves. 
         [0005]    In order for the lower drill string portion configured as an antenna to work well, the lower portion should ideally be electrically isolated from the upper portion as completely as possible. Any loss in complete electrical isolation will cause the lower drill string to start to lose its character as an antenna, reducing the effectiveness of the EM system in communicating via the Earth&#39;s strata. This need for as complete an electrical isolation as possible is magnified in view of the “reality” of the high impedance of the Earth&#39;s strata through which the carrier waves must pass in normal operational mode. In order to encourage robust wave propagation through the Earth&#39;s strata (and deter wave propagation losses to ground via the upper portion of the drill string), the impedance of the electrical isolation must be correspondingly even higher. It will be appreciated, however, that in practice, complete electrical isolation is rarely achievable. Most operational isolations will be “lossy” to some degree. A goal of electrical isolation of the drill string in EM telemetry is thus to minimize “lossiness” to as close to “no losses” as possible. 
         [0006]    A further “reality” is that the EM waves transmitted by the transceiver on the drill string are likely to be weak in comparison to their counterparts transmitted from the surface. Local power available to a transceiver on a tool string is limited. Thus, any wave propagation loss via poor isolation between upper and lower portions of the drill string is likely to cause a magnified reduction in effectiveness of the tool string transceiver&#39;s transmissions, as compared to surface transmissions. 
         [0007]    Electrical isolation of the upper and lower portions of the drill string is frequently enabled by placement of “gap sub” technology in the drill string at the point at which isolation is desired. The gap sub technology provides isolating structure to prevent, as completely as possible, any electrical conductivity through the drill string between the portions of the drill string above and below the gap sub technology. 
         [0008]    This disclosure uses the term “gap sub technology” in the previous paragraph because in alternative deployments, the electrical isolation of the upper and lower portions of the drill string may be achieved using differing arrangements. For example, electrical isolation may be enabled by deploying a single integrated electrical break in one or more locations on the drill string, where such electrical break(s) are integrated and continuous across the tubular drill collar and the tooling within the drill collar. In other arrangements, electrical isolation may be enabled via separate but cooperating electrical breaks: one (or more) electrical break(s) on the tubular drill collar, plus one (or more) separate electrical break(s) within the tooling structure deployed inside the collar. This disclosure pertains to the latter (separate but cooperating) arrangement, and specifically to electrical isolation of the drill collar itself. 
         [0009]    By way of further explanation, the drill string often, at and around the desired point of isolation, comprises operational downhole tool structure deployed inside a hollow cylindrical outer collar. The collar generally refers to a string of concatenated hollow tubulars made from non-magnetic material, usually stainless steel. In such a deployment, it is often advantageous to make separate but cooperating physical electrical breaks in both the tooling and in the collar itself in order to achieve overall electrical isolation of the entire drill string. 
         [0010]    Inside the collar, an “internal gap” is provided, usually positioned just above the transceiver tooling. The internal gap electrically isolates the drill collar internals below the internal gap from the drill collar internals above the internal gap. As noted, this disclosure is not directed to the internal gap. 
         [0011]    On the collar itself, a “gap sub” is provided, comprising a hollow tubular inserted in the concatenation of hollow tubulars that comprise the collar. The concatenated connections of the collar are conventionally pin and box threaded connections, and the collar itself is conventionally a non-magnetic material (usually stainless steel). The gap sub is thus conventionally a non-magnetic tubular with pin and box connections at either end, configured to be inserted at a desired position in a concatenated string of similarly-connected non-magnetic drill collar tubulars. It will be appreciated that the collar, in and of itself, is a portion of the overall drill string. Functionally, therefore, the gap sub electrically isolates the portions of the drill collar (and therefore, by extension, the entire drill string) above and below the gap sub. 
         [0012]    This disclosure is directed to an improved gap sub, providing excellent (almost complete) electrical isolation of the non-magnetic collar above and below the gap sub. The improved gap sub has further demonstrated excellent performance in operating conditions historically known to cause the isolating structure of prior art gap subs to break down or fail, causing unacceptable loss of isolation (and corresponding loss in EM telemetry) during live drilling operations. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention addresses one or more of the above-described drawbacks of the prior art. In preferred embodiments, a gap sub is provided in which the improvement comprises generally an isolating ring made out of conductive material. In a preferred embodiment, the conductive material is a non-magnetic material such as stainless steel. The gap sub comprises at least one, and in a preferred embodiment, two electrically-isolating threaded joints in a non-magnetic tubular collar. The threads of the joints are isolated by a non-conductive coating, which may be deployed on the outside of the pin threads in accordance with the prior art. The shoulders of the joints are separated by the disclosed new isolating ring, wherein the ring is coated with non-conductive material on (1) at least one, and advantageously both, of its upper and lower faces, and (2) its interior surface. An annular recess in the collar prevents electrical contact between collar sections nearby the ring either side of each threaded joint. In the preferred embodiment, the non-conductive coating is a ceramic coating. The coating itself and its method of deployment may be in accordance with the prior art. The combination of (1) the non-conductive coating ring isolating the shoulders of the joint, and (2) the non-conductive coating between mating threads on the joint enables a robust electrical isolation either side of each threaded joint. 
         [0014]    It is therefore a technical advantage of the disclosed gap sub to provide excellent (almost complete) drill collar isolation either side of the above-described electrically isolating threaded joints. As noted, when one, and advantageously two, of the above described threaded joints are deployed, the combination of (1) the non-conductive coating ring isolating the shoulders of the joint, and (2) the non-conductive coating between mating threads on the joint, enables a robust electrical isolation either side of the joint. As a result, optimized EM wave propagation is provided back and forth through the Earth&#39;s strata between the lower drill string (i.e. below the gap sub) and the surface. 
         [0015]    A further technical advantage of the disclosed gap sub is to provide sustained electrical isolation either side of the above-disclosed threaded joints in operating conditions. Modern directional drilling operations require the drill string to undergo bending loads as the borehole changes direction. Historically, such bending loads have been known to crack or fracture electrically isolating coatings deployed on previous gap subs, causing loss in isolation. However, the non-conductive coatings as configured on the new gap sub disclosed herein (and particularly on the coated non-magnetic ring) have been shown to be very robust, even when the gap sub is undergoing high operational bending loads. 
         [0016]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should be also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1A  illustrates, in perspective view, a presently preferred embodiment of an assembled drill collar gap sub in accordance with the present disclosure; 
           [0019]      FIG. 1B  illustrates, again in perspective view, the gap sub of  FIG. 1A  in disassembled form; and 
           [0020]      FIG. 2  illustrates, in cross-section view, isolating pin connection  107  (as indicated on  FIG. 1B ) in detail, in further cooperation with isolating ring  106 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIGS. 1A and 1B  illustrate, in assembled and disassembled views respectively, a presently preferred embodiment of a tubular drill collar gap sub  100 . In  FIG. 1A , gap sub  100  comprises pin end portion  101  and box end portion  102  separating isolating portion  105 . Isolating rings  106  are provided at either end of isolating portion  105 . One isolating ring  106  is included in each of two threaded isolating joints  120  (one threaded isolating joint  120  between pin end portion  101  and isolating portion  105 , the other threaded isolating joint  120  between isolating portion  105  and box end portion  102 ). All the components illustrated on  FIG. 1A  (pin end portion  101 , isolating portion  105 , box end portion  102  and isolating rings  106 ) are made from a non-magnetic material such as stainless steel. 
         [0022]    It will be appreciated that gap sub  100 , fully assembled as depicted in  FIG. 1A , is disposed to be inserted into the drill string. More, precisely, as described earlier in this disclosure, fully assembled gap sub  100  may be inserted into a concatenated string of non-magnetic drill collar tubulars. With further reference to  FIG. 1A , conventional pin connection  103  and box connection  104  (hidden from view on  FIG. 1A ) at either end of fully assembled gap sub  100  enable such insertion into the concatenated string of drill collar tubulars. When inserted into the drill collar string, gap sub  100  provides electrical isolation within the drill collar either side of gap sub  100 . The concatenated string of non-magnetic drill collar tubulars is in turn connected at either end to other (upper and lower) portions of the entire drill string. Gap sub  100  thus provides electrical isolation in the drill collar between the upper and lower portions of the entire drill string. 
         [0023]    As noted,  FIG. 1B  illustrates gap sub  100  from  FIG. 1A  in disassembled form. All the items called out by part number on  FIG. 1A  are illustrated on  FIG. 1B  by the same part number. Disassembly of gap sub  100  as shown in  FIG. 1B  allows further illustration of the threaded isolating joints  120  (identified on  FIG. 1A ) at each end of isolating portion  105 .  FIG. 1B  shows isolating portion  105  including isolating pin connections  107  at each end, each isolating pin connection  107  configured to be received into a mating isolating box connection  108  on pin end portion  101  and box end portion  102  respectively.  FIG. 1B  also depicts isolating rings  106  inserted into the threaded isolating joints between isolating portion  105  and pin end portion  101 , and between isolating portion  105  and box end portion  102 . 
         [0024]      FIG. 2  illustrates isolating pin connection  107  from  FIG. 1B  in more detail, per the view indicated on  FIG. 1B . It will be understood, however, that although  FIG. 2  illustrates isolating pin connection  107  near box end portion  102  (per  FIG. 1B ), the details illustrated on  FIG. 2  are functionally and structurally the same for isolating pin connection  107  near pin end portion  101  (also per  FIG. 1B ). 
         [0025]    Referring now to  FIG. 2 , isolating pin connection  107  comprises male threads  112  at the end thereof. Male threads  112  are coated (functionally depicted as item C on  FIG. 2 ) with a non-conductive coating as is know in the art. In a presently preferred embodiment, the non-conductive coating is a ceramic coating. Further, as is known in the art, male threads  112  are sized, shaped and configured so that once coated with the non-conductive coating, the coated male threads  12  mate with corresponding female threads on isolating box connection  108  on box end portion  102  (items  108  and  102  not shown on  FIG. 2  for clarity, but depicted on  FIG. 1B ). 
         [0026]      FIG. 2  also depicts isolating ring  106  disposed to be received into ring cutout  110  on isolation portion  105  (item  110  described in more detail immediately below). Although not illustrated on  FIG. 2 , it will be understood that isolating ring  106  receives a non-conductive coating on face surfaces  106   F1  and  106   F2 , and on interior surface  106   I . In a preferred embodiment, the non-conductive coating on isolating ring  106  is a ceramic coating. It should be noted that testing has revealed that coating the entire isolating ring (i.e. on the exterior ring surface as well as on face surfaces  106   F1  and  106   F2 , and interior surface  106   I ) is not particularly advantageous. It has been found that under operating conditions, the coating on the exterior ring surface chips and cracks quite quickly. These chips and cracks propagate and extend into the coating on the face surfaces  106   F1  and  106   F2 , causing premature breakdown in electrical isolation. It has been observed that the coating on face surfaces  106   F1  and  106   F2  performs much better when there is no coating on the exterior ring surface. 
         [0027]    With further reference to  FIG. 2 , isolating pin connection  107  also comprises a shoulder  109  at the point at which isolating portion  105  transitions into male threads  112 . Shoulder  109  is separated from male threads  112  by ring cutout  110  and recess  111 . Ring cutout  110  is sized and shaped to receive isolating ring  106  after a non-conductive coating has been applied to isolating ring  106 , as described immediately above. Recess  111  is positioned, sized and contoured so that when isolating ring  106  is received into ring cutout  110 , and when male threads  112  are operationally engaged on corresponding female threads on isolating box connection  108  (item  108  omitted for clarity on  FIG. 2 ), there is no physical (or electrical) contact between isolating portion  105  and box end portion  102  (item  102  omitted for clarity) except via threaded engagement of male threads  112  or via contact through isolating ring  106 . 
         [0028]    Functionally complete isolation is thus achieved. Since male threads  112  have a non-conductive coating, electrical isolation across threaded engagement of male threads  112  is enabled. Moreover, since isolating ring  106  also has a non-conductive coating, electrical isolation across shoulder  109  is also enabled. As noted above, recess  111  prevents any other physical (or electrical) contact between isolating portion  105  and box end portion  102  (item  102  shown on  FIG. 1B  but omitted for clarity on  FIG. 2 ). 
         [0029]    Field testing has shown operational drill collar gap subs designed in accordance with this disclosure have shown excellent (almost complete) isolation in normal directional drilling service. Isolating ring  106  (per  FIG. 1B ) has the compressive strength and durability of a non-magnetic metal while still providing electrical isolation via its coating. As such, it is theorized that this hybrid nature of isolating ring  106 &#39;s properties enables superior performance. 
         [0030]    Field testing has further shown that gap subs designed in accordance with this disclosure also show good isolation performance (and limited damage to non-conductive coatings) even when placed under high bending loads associated with elevated build rates. For example, one test was designed to simulate placing the gap sub downhole and subjecting it to approximately 140 continuous hours at a maximum operating bending stress that might be expected during directional drilling. In this test, bending stress was exerted on the gap sub equivalent to a simulated build of a 12° dogleg. This bending stress was imparted to the gap sub 25.2 times per second over a 6 hour period. Electrical isolation remained substantially complete and unchanged throughout the entire test. 
         [0031]    The following paragraphs describe further alternative embodiments which, although not illustrated, are considered within the scope of this disclosure and the inventive material described herein. 
         [0032]    It will be seen on  FIG. 1B  that an isolating pin connection  107  is provided on each end of isolating portion  105 . This is to simplify manufacturing. It is understood in the art that it is easier to deploy robust and precise non-conductive thread coatings on male threads on a pin end than on female threads in a box end. Thus, by providing an isolating pin connection on each end of isolating portion  105 , only one tubular (isolation portion  105 ) need be handled to receive a non-conductive coating for two isolating thread joints. Absent manufacturing economy and convenience, however, other embodiments may deploy isolating pin connection  107  on pin end portion  101  and/or box end portion  102  with equivalent enabling effect. 
         [0033]    As noted, the presently preferred non-conductive coating on male threads  112  and isolating ring  106  (per  FIG. 2 ) is a ceramic coating. This is because a ceramic coating has demonstrated good performance in gap subs designed in accordance with this disclosure. However, the disclosure is not limited in this regard. Other non-conductive coatings (such as, for example, plastics, fiberglass or composites) could be used with equivalent enabling effect. 
         [0034]    The presently preferred embodiment described in this disclosure illustrates gap sub  100  with two threaded isolation joints  120  (per  FIG. 1A ). Nothing in this disclosure should be interpreted to limit the disclosure to two isolation joints, however. Other embodiments of gap sub  100  may deploy only one isolation joint, or more than two isolation joints, depending on user and service requirements. In embodiments where there is to be only threaded one isolation joint  120 , it will be appreciated that, with further reference to  FIGS. 1A and 1B , box end portion  102  may be eliminated, and isolating pin connection  107  near box end portion  102  may be substituted for a conventional box connection. In embodiments where there are to be more than two threaded isolation joints  120 , additional isolating portions  105  may be deployed between pin end portion  101  and box end portion  102 , with associated additional structure per  FIG. 2  deployed on such additional isolating portions  105 . 
         [0035]    It will be appreciated that throughout this disclosure, pin and box connections have been called out and identified according to a presently preferred embodiment. Nothing herein should be interpreted, however, to limit this disclosure to require a pin connection or a box connection at a particular location. It will be understood that pin connections and box connections may be deployed interchangeably on parts that thread together. 
         [0036]    This disclosure has described a gap sub that is made entirely of a non-magnetic material (such as stainless steel) in a presently preferred embodiment. However, the scope of this disclosure is not limited to non-magnetic material. It will be appreciated that parts (or all) of the gap sub may alternatively be made of other serviceable materials (including magnetic materials such as carbon steel) with equivalent enabling effect. 
         [0037]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.