Patent Publication Number: US-10320138-B2

Title: System and method for downhole electrical transmission

Description:
This application is a divisional application of co-pending U.S. patent application Ser. No. 14/064,176, filed on Oct. 27, 2013, which is a divisional U.S. Pat. No. 8,602,094, granted on Dec. 10, 2013, the content of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     In a variety of downhole applications, electric signals are transmitted along the wellbore to or from various sensors and tools. For example, electric signals may be transmitted via conductors positioned in or along well strings, e.g. along drill strings. In drilling applications and other downhole applications, electric signals are sometimes transmitted across components which move relative to each other, e.g. electric signals may be transmitted from a rotationally stationary component to a rotating component. Transmission of electrical signals across moving components creates difficulties in many of these applications. 
     In some applications, transmission of electric signals across components which move relative to each other can be avoided by placing the sensor/tool above the moving component. In other applications, the signals may be transmitted across the moving components with an electromagnetic telemetry system, such as a short-hop system. However, existing electromagnetic telemetry systems tend to be relatively expensive and are often more complex than desired for downhole drilling applications and other downhole applications. 
     SUMMARY 
     In general, the present disclosure provides a system and method for enabling transmission of electric signals across well components which move relative to each other in a wellbore environment. The well components are movably, e.g. rotatably, coupled to each other via one or more conductive bearings. Each conductive bearing has a conductive rolling element which enables relative movement, e.g. rotation, between the well components while simultaneously facilitating transmission of electric signals through the bearing. Portions of the bearing are coupled to each of the well components, and those bearing portions may be coupled with electric leads to enable flow of electric signals through the bearing during operation of the system downhole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a schematic illustration of a well system, e.g. a drilling system, deployed in a wellbore and incorporating conductive bearings, according to an embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view of an embodiment of a system for conducting electric signals through bearings from a first well component of a downhole tool to a second well component of the downhole tool, wherein the second well component moves relative to the first well component, according to an embodiment of the present disclosure; 
         FIG. 3  is a schematic view of an alternate embodiment of a conductive bearing which may be used in the well system, according to an alternate embodiment of the present disclosure; 
         FIG. 4  is a side view of an embodiment of a downhole device having components which move relative to each other and through which electric signals may be transferred via conductive bearings, according to an embodiment of the present disclosure; 
         FIG. 5  is a cross-sectional view of an alternate embodiment of a conductive bearing which may be used in the well system, according to an alternate embodiment of the present disclosure; and 
         FIG. 6  is an enlarged view of a portion of the bearing system illustrated in  FIG. 5  which is designed to remove particles so as to maintain conductive contact between bearing portions, according to an alternate embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present disclosure. However, it will be understood by those of ordinary skill in the art that the present disclosure may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     The present disclosure generally relates to a system and methodology to form electrical connections across components in a downhole well assembly. In many applications, the technique facilitates formation of one or more conductive paths along a well system which has components that move relative to one another. The components are movably, e.g. rotatably, coupled to each other via an electrically conductive bearing which has a conductive rolling element. Additionally, electric leads may be coupled to opposing sides of the bearing to enable flow of electric current through the bearing, including through the conductive rolling elements of the bearing, to facilitate communication with and/or transfer of electrical power to/from devices positioned farther downhole from the components that undergo relative motion. 
     A variety of downhole tools have components which move relative to each other during well related operations, such as drilling operations. For example, mud motors and orienter tools have a downhole or bottom component which rotates at different speeds (and typically independent speeds) relative to the uphole or upper component. Other downhole tools, including rotary steerable systems and other bottom hole assemblies, also utilize components which have relative motion with respect to each other. One example of such a tool is the PowerDrive Control unit which is available from Schlumberger Corporation and has a roll-stabilized platform that is geostationary while a collar component rotates at a drill bit rpm. 
     The system and methodology described herein provide a solid, conductive electrical path through downhole components which move relative to each other. For example, a conductive path may be formed between a stationary structure and a rotating component which rotates about the stationary structure in a downhole tool. The continuous electrical connection during the continuous mechanical movement, e.g. rotation, may be created through various types of rolling element bearings. In one example, the conductive paths or wiring associated with the stationary structure are conductively connected to a stationary bearing race ring, and the conductive paths or wiring associated with the rotating structure are conductively connected to a rotating bearing race ring, or vice versa. 
     In some embodiments, the rolling element bearings may be preloaded to avoid separation of race rings and rolling elements, thus preventing disruptions to current flow due to disconnection of contact between the bearing elements while in a downhole environment susceptible to shock and vibration. Additionally, the rolling element bearings may be packaged with appropriate insulation so that the electric signals, e.g. power signals and/or data signals, follow the intended path along separate, electrically independent poles. The bearing system also may comprise a plurality of rolling element bearings selectively placed to create a rugged, multi-conductor electrical transmission assembly. The conductive bearings can be utilized in a variety of downhole tools, including wired mud motors, wired coiled tubing orienter tools, rotary steerable systems, and other downhole tools having components which undergo relative motion. 
     The style of conductive bearing may vary from one downhole application to another depending on the environment and on the specific parameters of a given wellbore operation. By way of example, the conductive bearings may comprise ball bearings, e.g. deep groove ball bearings or axial ball bearings, in which the conductive rolling element comprises a plurality of conductive balls. However, the conductive bearings also may comprise other types of bearings, including roller style bearings, in which the conductive rolling element comprises a plurality of conductive rollers. Examples of roller bearings and conductive rollers include angular contact roller bearings, crossed roller bearings, tapered roller bearings, cylindrical roller bearings and needle bearings. The specific type of bearing may be selected according to desired parameters, such as a desired preload on the rolling elements to avoid separation of conductive contact in a high shock and vibration environment. Furthermore, the rolling element bearings used for transmission of electrical communication and/or for electrical power transfer may be used simultaneously for the structural support of the mechanical components which rotate relative to each other, i.e. the bearings may have both electrical and mechanical characteristics. 
     Many types of downhole applications and downhole tools may benefit from the conductive bearing systems described herein which provide a relatively simple, dependable system for transmitting electric signals, e.g. power or data signals, downhole and/or uphole. Referring generally to  FIG. 1 , an example of a well system  20  is illustrated as incorporating a downhole tool  22  having a first component  24  and a second component  26  which may be moved relative to first component  24 . For example, the second component  26  may be rotated with respect to first component  24 . In some embodiments, the second component  26  rotates while the first component  24  is rotationally stationary, however other embodiments may utilize rotation of both the second component  26  and the first component  24  but at different rotational speeds. 
     The second component  26  is movably, e.g. rotatably, coupled to the first component  24  via one or more conductive bearings  28 . The conductive bearings  28  may be individually or collectively coupled with one or more electrically conductive communication lines  30  which carry electric signals, e.g. electric power signals and/or electric data signals. The signals are passed through the downhole tool  22  to enable communication with a device  32  located on the downhole side of tool  22 . The device  32  may comprise one or more devices in the form of sensors, gauges, measurement-while-drilling systems, logging-while-drilling systems, or a variety of other downhole devices which output or receive electric signals and/or require electrical power. Each electrically conductive communication line  30  may be divided into a downhole electric lead  34  coupled to a downhole side of the corresponding bearing  28  and an uphole electric lead  36  coupled to an uphole side of the same corresponding bearing  28 . The leads  34 ,  36  may be connected to bearings  28  by soldering, connectors, or other suitable fasteners. 
     As discussed above, the downhole tool  22  may have a variety of forms depending on the specific wellbore operation being conducted. If, for example, the wellbore operation is a drilling operation utilizing a drill bit  38  to drill a desired wellbore, the downhole tool  22  may comprise a mud motor assembly  40 . In drilling operations, as well as other downhole applications, the downhole tool  22  also may comprise an orienter tool assembly  42  (shown in dashed lines). The orienter tool assembly  42  may be combined with coiled tubing in coiled tubing drilling applications or other downhole applications. In both drilling operations and other downhole applications, the downhole tool  22  also may comprise a variety of bottom hole assemblies, such as a bottom hole assembly having a rotary steerable system  44  (shown in dashed lines) to enable, for example, directional drilling. It should be noted that the various downhole tools  22  have been illustrated and described as examples of downhole tools having components which undergo relative movement while performing downhole operations. Depending on the specific downhole application, the various downhole tools  40 ,  42 ,  44  may be used alone or in various combinations. When used in combination, sequential assemblies of the conductive bearings  28  may be employed in the sequential downhole tools  22  to enable transmission of electric signals through the various movable components. 
     The overall well system  20  also may have a variety of configurations. For purposes of explanation, however, the well system  20  has been illustrated as comprising a drill string  46  deployed in a wellbore  48 . Depending on the drilling application, the drill string  46  may comprise mud motor assembly  40 , orienter tool assembly  42 , and/or rotary steerable system  44  which may each or all be used to control the drill bit  38 . The drill string  46  may include many additional and/or other types of components depending on the specific design of the well system  20 . 
     In a drilling application, a drilling fluid, e.g. drilling mud, is pumped down through an interior of the drill string  46 , through drill bit  38 , and then up through an annulus  50  between the drill string  46  and the surrounding wellbore wall  52 . The flowing drilling fluid or drilling fluid flow path is represented by arrows  54  in  FIG. 1 . The drilling fluid is pumped down through drill string  46  under pressure to remove drill cuttings up through the annulus  50 . As described in greater detail below, the bearings  28  may be isolated from the drilling mud to facilitate long-term, dependable conductive contact and transmission of electric signals along the drill string  46 . 
     Referring generally to  FIG. 2 , an embodiment of downhole tool  22  is illustrated with a conductive bearing assembly  56  having a plurality of conductive bearings  28 , e.g. two conductive bearings  28 , designed to provide a conductive path for the flow of electric signals. In this embodiment, each conductive bearing  28  comprises first and second conductive race rings  58  and a conductive rolling element  60  positioned between and in contact with both conductive race rings  58 . In this particular example, the conductive rolling element  60  comprises a plurality of conductive balls  62  which cooperate with the conductive race rings  58  to form conductive ball bearings. Electric leads  36  may be connected to first conductive race rings  58 , e.g. outer conductive race rings, and electric leads  34  may be connected to second conductive race rings  58 , e.g. inner conductive race rings. The electric leads  36  are routed through first component  24  of downhole tool  22 , and the electric leads  34  are routed through second component  26  of downhole tool  22 . 
     In the embodiment illustrated, the bearings  28  are located in an isolated cavity  64  which may contain an isolating fluid  66 , such as an incompressible oil or other fluid having suitable insulating/dielectric qualities. The isolated cavity  64  is located in an internal housing  68  which forms part of first component  24 . The internal housing  68  comprises an opening  70  through which a shaft  72  of second component  26  is received. The shaft  72  extends into cavity  64  and is rotatably received by bearings  28 . Additionally, a seal  74  may be located about shaft  72  within the opening  70 . By way of example, the leads  34 ,  36  may be routed along passages formed in shaft  72  and housing  68 . 
     As illustrated, the first or outer conductive race rings  58  may be mounted to, e.g. affixed to, the first component  24 . By way of example, the outer conductive race rings  58  are mounted to an interior of internal housing  68 . Similarly, the second or inner conductive race rings  58  may be mounted to, e.g. affixed to, the second component  26 . By way of example, the inner conductive race rings are mounted to the shaft  72  so that the rolling element  60  of each bearing  28  is secured between the outer and inner conductive race rings  58 . To ensure the bearings  28  are isolated, appropriate electrical insulation  76 , e.g. layers/pads of insulation, may be positioned between the conductive race rings  58  and the corresponding structure to which the race rings  58  are mounted. For example, an electrical insulation layer/pad  76  may be positioned between the outer conductive race rings  58  and an interior wall of internal housing  68 , and another electrical insulation layer/pad  76  may be positioned between the internal conductive race rings  58  and shaft  72 . In some applications, it can be beneficial to preload the bearings  28  by applying a suitable preload force, as indicated by arrows  78 , to ensure firm, conductive contact between bearing components. The preload may be established by providing appropriate shoulders, spring washers, and/or bearing nuts on housing  68  and/or shaft  72 . 
     The isolating fluid  66  may undergo volume changes due to pressure and temperature changes downhole. Accordingly, a pressure compensator  80  may be connected to internal housing  68  in communication with isolated, internal cavity  64  to compensate for changes in volume (and thus changes in pressure) of the isolating fluid  66  as the isolating fluid provides electrical insulation for bearings  28 . A variety of compensators  80  may be employed, but one example utilizes a spring-loaded piston  80  sealably mounted within an opening  84  extending through internal housing  68 . 
     The bearing assembly  56  and internal housing  68  may be employed in a variety of downhole applications and in a variety of downhole tools  22 . For example, the bearing assembly  56  and internal housing  68  may be employed in drilling applications. In one example, the internal housing  68  is mounted within an external housing  86  of drill string  46  to create fluid flow paths, as indicated by arrows  88 . As indicated, the fluid flow paths  88  may be routed externally of internal housing  68  to conduct, for example, a flow of drilling fluid e.g. drilling mud, through the downhole tool  22 . The bearings  28  and the interior of cavity  64  remain isolated from the flow of drilling fluid along fluid flow paths  88 . The internal housing  68  may be held at a desired position within external housing  86  by a centralizer  90  or other suitable mechanism. 
     It should be noted that bearings  28  may have a variety of shapes, sizes and configurations depending on the parameters of a specific downhole application. As illustrated in  FIG. 3 , for example, the conductive bearings  28  may utilize roller style bearings in which the conductive rolling element  60  comprises a plurality of conductive rollers  92 . Examples of roller bearings and conductive rollers  92  include angular contact roller bearings, crossed roller bearings, tapered roller bearings, cylindrical roller bearings and needle bearings. The specific type of bearing may be selected according to desired parameters, e.g. the desired preload  78  on the rolling elements for avoiding separation of conductive contact in a high shock and vibration environment 
     By way of example, the downhole tool  22  illustrated in  FIG. 2  may comprise a wired mud motor for use in mud motor assembly  40 . In this example, the first component  24  may comprise a stationery collar or housing and the second component  26  may comprise a rotating output shaft, such as shaft  72 . When downhole tool  22  comprises a mud motor, the bearing assembly  56  may be positioned above the rotor or rotor catcher of the mud motor assembly  40 . A flexible connection element may be provided to carry the wires/electric leads  34  from the rotating shaft  72  to a top of the rotor. The electric leads  34  may be routed through a bore in the center of the rotor all the way down to an electrical connector in a bit box of the drilling assembly. 
     In another application, the downhole tool  22  illustrated in  FIG. 2  comprises the wired, coiled tubing orienter tool assembly  42 . Electricity is supplied through bearings  28  of bearing assembly  56  to tools, e.g. logging-while-drilling tools, running below the orienter tool assembly  42 . In some applications, orienter tool assembly  42  may be positioned above a mud motor assembly, e.g. mud motor assembly  40 , which may also employ conductive bearings  28 . 
     In another application, the downhole tool  22  comprises a rotary steerable system  44 , e.g., a push-the-bit-type rotary steerable system (such as is shown, for example, in U.S. Pat. Nos. 5,265,682; 5,582,678; 5,603,385; 7,188,685; and 2010-0139980), to enable transfer of electrical power and/or electrical data signals into a roll stabilized control unit without the need for wireless transmission systems. An example of the rotary steerable system  44  is illustrated in  FIG. 4  and is designed to enable exchange of electric power and/or data with a control unit  94  without the use of wireless transmissions via short hop receivers. In this example, control unit  94  is a rotating control unit mounted in a pair of hangers  96  which also support torquers  98 . The bearing assembly  56  may be located in a connector box  100  disposed within external housing  86 . The collar side electric leads  36  may be terminated in a standard LTB connector  102 . By way of example, the conductive bearings  28  and bearing assembly  56  may be coupled with wired drill pipe to provide a high data transmission rate. The application of rolling element bearings for electrical power and/or signal transmission also may be used on the downhole end of a push-the-bit-type control unit, for example, to communicate with and/or to power electronic components situated in the bias unit. 
     In any of the embodiments described herein, the bearings  28  may be employed not only for transmitting electricity but also to provide mechanical support. By enabling electric transmission while simultaneously providing mechanical support via bearings  28 , the overall downhole tool  22  and the overall well system  20  can be substantially simplified for a variety of well related applications. 
     Depending on the application and environment in which downhole tool  22  is utilized, additional measures may be implemented to prevent mud invasion into cavity  64 . If mud or other environmental fluids enter cavity  64 , the fluid  66  or other features within cavity  64  can potentially become conductive and create short-circuits between the independent electrical poles. This risk can be mitigated by applying thin gap insulation principles (see also insulation layers  76 ) such as utilizing a thin and long gap between the poles to limit leakage current and to prevent electrical shorting. In this example, the insulation layers for the stationery and the rotating side may be formed into a geometry where they come in very close contact without touching, thus forming a very thin gap, separating electrically conductive elements from each other (e.g., electrical power from electrical ground or similar). This will increase the electrical resistance of any unwanted, invading conductive fluid intruding into the gap and thus limit the short circuit current, thereby protecting the electrical equipment on both sides of the rotating assembly. 
     Another risk associated with mud invasion is the interference of solid particles moving between the rolling element  60  and the corresponding contact surfaces within conductive race rings  58  of bearing  28 . If sufficient particles move between the respective running contact surfaces of the rolling element  60  and the corresponding conductive race rings  58 , the rolling elements  60  can be lifted from the running surfaces and cause an interruption in electrical conductance. An example of one system and methodology for mitigating this risk is illustrated in  FIG. 5  as utilizing point contacts formed between the conductive rolling element  60  and the conductive race rings  58 . 
     In the specific example illustrated in  FIG. 5 , the conductive race rings  58  are formed with convex surfaces  104  or other suitable surfaces able to form a more focused contact  106 , referred to as a point contact, with the rolling element  60 . By way of example, the rolling element  60  may comprise a plurality of conductive balls  62  or conductive rollers  92 . In one embodiment, each conductive race ring  58  comprises a pair of conductive rings  108 , such as conductive O-rings, with each pair of conductive rings  108  being held by a corresponding race ring holder  110 . By way of example, the conductive rings  108  may comprise metal O-rings. The conductive rings  108  cooperate to secure the rolling element  60  therebetween, and preload forces  78  may be applied to race ring holders  110  to help maintain constant conductive contact between the conductive race rings  58  and the conductive rolling element  60  while helping force out any undesirable particles. 
     As better illustrated in  FIG. 6 , the resulting point contact  106  between the two convex radii of rolling element  60  and conductive rings  108  forces particles  112  away from the point contact  106 . The design causes the particles  112  to be rejected by creating a squeezing effect which forces the particles  112  out of the way, as indicated by arrows  114 , rather than trapping them between the rolling element  60  and the interior surfaces of conductive race rings  58 . In some applications, the radii of the rolling elements  60  and the metal O-ring  108  may be made small to increase this effect further. In a more extreme example, the radius of contact on the metal O-ring may be reduced such that a knife edge contact is created to further improve particle rejection. 
     In the embodiments described herein, the conductive bearings  28  provide a simple, reliable approach to transmitting electrical signals between components which move relative to each other. In some applications, one component may be rotationally stationary while the other component rotates. In other applications, however, both components may rotate or otherwise move at different speeds relative to each other. Single or multiple bearings  28  may be employed in a variety of bearing assemblies and may be arranged sequentially or in other patterns according to the design of a given downhole tool  22 . For example, multiple conductive rolling element bearings  28  may be used in a tool to provide a rugged, multi-conductor, electrical transmission assembly. The size and configuration of internal housing  68  and cavity  64  may be adjusted and may be designed for cooperation with a variety of compensators, electrical leads and/or electrical lead connection mechanisms. Additionally, the conductive bearing system may be incorporated into a variety of downhole tools for use in many types of downhole, well related applications. Individual or multiple downhole tools  22  incorporating conductive bearings  28  may be employed in individual well systems  20 . 
     Although only a few embodiments of the present disclosure have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.