Abstract:
A downhole tool that is connectable to a line to be run downhole with the tool includes a housing, a sensor and a mechanism that is located inside the housing. The mechanism is coupled to the sensor to, in response to the detection of the feature by the sensor, generate a tension signal in the line without physically contacting a downhole structure.

Description:
BACKGROUND 
     The invention relates to a downhole tool to generate tension signals on a slickline. 
     Certain downhole oilfield applications, such as perforating applications, require the ability to be able to position a tool at a particular and known spot in the well. For example, a slickline service uses a slickline tool assembly that is lowered downhole via a slickline. A depth counter may be used to track the length of the dispensed slickline to approximate the depth of the slickline tool assembly. However, because the depth counter does not precisely indicate the depth, other techniques may be used. 
     For example, a more precise technique may use a depth control log (a gamma ray log, for example), a log that is run while drilling the well and indicates the depths of various casing collars of the well. In this manner, the slickline tool assembly may be run downhole and include a detection device to detect casing collars. When the detection device indicates detection of a casing collar, the coarse depth that is provided by the depth counter may be used to locate the corresponding casing collar on the depth control log. Because the depth control log precisely shows the depth of the detected casing collar, the precise depth of the tool assembly may be determined. From this determination, an error compensation factor may be derived. Then, when a perforating gun is lowered downhole, the error compensation factor is used to compensate the reading of the depth counter to determine the position of the gun. Unfortunately, the error may not be the same, because more or less line may be dispensed than was dispensed when the error compensation factor was derived. Thus, more strain on the line may cause the compensation that is provided by the error compensation factor to be inaccurate. 
     The slickline tool assembly does not have the benefit of electrical communication with the surface of the well. Instead, the slickline tool assembly may generate tension pulses on the slickline to indicate the detection of a casing collar. To accomplish this, the conventional slickline tool assembly may perform some sort of physical interaction with a well casing. For example, the slickline tool assembly may include a mechanical drag device to generate the tension pulses. As an example, the mechanical drag device may be an end of a tubing locator, a device that includes a set of arms that extend to make contact with the well casing when the tool initially passes the end of the tubing of a mule shoe. In this manner, when the slickline operator attempts to pull back into the tubing, the arms catch on the restriction and do not close until a certain amount of tension is applied to the end of the tubing. This catch and release sequence creates a tension pulse on the slickline. This technique may be risky if the end of tubing locator does not release, a condition that may cause the tool assembly to become lodged in the well. Furthermore, the well may not have a suitable profile to permit proper operation of the end of tubing locator. 
     Thus, there is a continuing need for an arrangement that allows real time depth indication at multiple points while running a particular downhole tool (a perforating gun, for example). 
     SUMMARY 
     In an embodiment of the invention, a downhole tool that is connectable to a line to be run downhole with the tool includes a housing, a sensor and a mechanism that is located inside the housing. The mechanism is coupled to the sensor to, in response to the detection of the feature by the sensor, generate a tension signal in the line without physically contacting a downhole structure. 
     Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a slickline system. 
     FIG. 2 is a waveform illustrating tension of a slickline of the system of FIG.  1 . 
     FIGS. 3,  4  and  5  are schematic diagrams illustrating a tool of the system of FIG. 1 according to an embodiment of the invention. 
     FIGS. 6 and 7 are more detailed schematic diagrams of the tool according to an embodiment of the invention. 
     FIG. 8 is a cross-sectional view of a split collar of the tool taken along line  8 — 8  of FIG.  6 . 
     FIG. 9 is a cross-sectional view of the split collar of the tool taken along line  9 — 9  of FIG.  7 . 
     FIG. 10 is a schematic diagram of a tension sensor of the system of FIG. 1 according to an embodiment of the invention. 
     FIG. 11 is a schematic diagram of a hydraulic strain head of the tool according to an embodiment of the invention. 
     FIGS. 12 and 15 are schematic diagrams of tension pulse generators according to different embodiments of the invention. 
     FIG. 13 is a waveform depicting tension of a slickline over time according to an embodiment of the invention. 
     FIG. 14 is a schematic diagram of circuitry to operate the tension pulse generator of FIG. 12 according to an embodiment of the invention. 
     FIG. 16 is a waveform depicting tension of a slickline over time according to an embodiment of the invention. 
     FIG. 17 is a schematic diagram of circuitry to operate the tension pulse generator of FIG. 15 according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 and 2, an embodiment  44  of a slickline downhole tool assembly in accordance with the invention includes a pulse generator tool  20  that is constructed to generate a tension pulse  24  in a slickline  21  when the tool  20  is in close proximity to a casing collar. The tool  20  generates the tension pulse  24  without requiring physical interaction with a well casing  22  that surrounds the tool assembly  44 . Therefore, as a result of this arrangement, no exposed mechanical parts are needed to generate the tension pulse  24 , and the risk of the tool assembly  44  becoming lodged downhole in the well is substantially reduced. 
     Besides the pulse generator tool  20 , the tool assembly  44  may also include a hydraulic strain head  41  to receive command stimuli from a surface of the well (as described below), a jar  40  to aid in disloading the tool assembly  44  if the assembly  44  becomes lodged downhole, a firing head  36  and a perforating gun  38 . In some embodiments, the tool  20  may generate the tension pulses  24  for purposes of precisely positioning the perforating gun  38 . 
     The hydraulic strain head  41  may be used to instruct the tool  20  to start and/or stop generating tension pulses in response to observed casing collars. Because the interval in which the tension pulses are generated may be controlled via the hydraulic strain head  41 , battery power may be conserved, as the number of unnecessary tension pulses is minimized. Furthermore, it is desirable to turn off the generation of the tension pulses before firing the perforating gun  38 . 
     More particularly, FIG. 3 depicts an upper portion  20 A of the tool  20 ; and FIGS. 4 and 5 depict a bottom portion  20 B of the tool  20  for different states of the tool  20 , as described below. As shown, the tool  20  includes a housing  50  that encases the components of the tool  20 , such as a weight  70  (see FIGS. 4 and 5) that resides in an interior chamber  51  of the housing  50 . In a first state (depicted in FIG. 5) of the tool  20 , the weight  70  is held (as described below) in suspension over a contact surface  90  of the housing  50  by a latch, or brake (formed by a split collar  61  and a release shaft  74 ), that are described below. When a magnetic casing collar sensor  98  (see FIG. 3) of the tool  20  detects a casing collar, circuitry of the tool  20  causes the brake to release the weight  70 , an action that causes the weight  70  to travel a distance d (see FIG. 5) and strike the surface  90  (see FIG. 4) to generate the tension pulse  24 . 
     In some embodiments, the brake is designed to selectively grip a threaded shaft  64  (see FIGS. 4 and 5) that is coupled to the weight  70  to control the rise and fall of the weight  70 . In this manner, the brake releases the shaft  64  to allow the weight  70  to fall and strike the surface  90 , and the brake grips the shaft  64  so that the weight  70  may be retracted (as described below) back into its suspended position above the surface  90  in preparation for the generation of another tension pulse  24 . The weight  70  is secured to a connecting rod  66  that couples the weight  70  to the threaded shaft  64 . The threaded shaft  64 , in turn, passes through a split collar  62  that, along with a release shaft  74  and the threaded shaft  64 , form the brake. 
     Referring also to FIGS. 6 and 7 (that do not depict the threaded shaft  64  for purposes of more clearly illustrating operation of the split collar  62 ), in some embodiments, the release shaft  74  is generally cylindrical and includes a release collar  80  near its lower end. The release collar  80  extends in a radial inward direction to operate the split collar  62  (as described below) to selectively grab and release the threaded shaft  64 . In this manner, when the release shaft  74  is at its maximum point of downward travel, the release collar  80  compresses the split collar  62  to cause internal threads of the split collar  62  to grab the thread shaft  64 , as depicted in FIG.  5 . When the release shaft  74  moves in an upward direction, the release collar  80  moves away from the split collar  62 , and due to the spring loaded release (described below) of the split collar  62 , the split collar  62  releases its grip on the threaded shaft  62  and permits the weight  70  to fall and strike the surface  90 . 
     In some embodiments, the tool  20  includes a solenoid  57  to control the upward and downward travel of the release shaft  74  and thus, control when the release collar  80  compresses the split collar  62  and when the release collar  80  permits the split collar  62  to expand. In this manner, the solenoid  57  is mounted to the housing  50  above the release shaft  74  and includes a shaft  59  (see FIGS. 6 and 7) that is attached to the release shaft  74 . When the solenoid  57  retracts the release shaft  74  to a position above the split collar  62 , the split collar  62  expands and releases the threaded shaft  64 , as depicted in FIGS. 4 and 7. However, as depicted in FIGS. 5 and 6, when the solenoid  57  moves the release shaft  74  so that the release collar  80  compresses the split collar  62 , a threaded connection is formed between the threaded shaft  64  and the split collar  62 . Because the split collar  62  is prevented from rotating with respect to the housing  50 , a screw drive is formed to raise the weight  70  back into its suspended position (see FIGS.  4  and  6 ). In some embodiments of the invention, a motor assembly  56  (that is attached to the housing  50 ) is coupled to the threaded shaft  64  via a drive shaft  60  to retract the weight  70  via the screw drive, as described below. 
     In some embodiments, the tool  20  may include a coiled spring  68  (see FIGS. 4 and 5) that surrounds the threaded shaft  64  and the connecting rod  66  and resides between a top surface of the weight  70  and an inwardly extending shoulder  63  (of the housing  50 ) on which the split collar  62  resides. Due to this arrangement, when the motor assembly  56  raises the weight  70 , the spring  68  is compressed, as depicted in FIGS. 5 and 6. Therefore, when the split collar  62  releases the weight  70 , the potential energy that is associated with the compressed spring  68  is converted into kinetic energy to cause a greater impact of the weight  70  against the surface  90  and thus, generate a more definite signature for the tension pulse  24 . 
     Although, in some embodiments, the weight  70  may have a relatively flat bottom surface  71 , in other embodiments, the surface  90  may be, for example, a conical surface that forms a point for striking the surface  90  of the housing  50 . These variations in the surface  71  as well as the distance d in which the weight  70  travels when released may be varied to define different signatures for the tension signal  24 . 
     In some embodiments, the magnetic sensor  98  generates an electronic pulse signal when the sensor  98  detects a casing collar. In response to the pulse of the electronic signal, the electronics  54  (see FIG. 3) of the tool  20  activate the solenoid  57  and motor assembly  56  accordingly to release the weight  70  and then to retract the weight  70  after the generation of the tension pulse  24 . Among the other features of the tool  20 , the tool  20  may include a battery  52  that provides energy to power, as examples, the electronics  54 , motor assembly  56  and the solenoid  57 . The electronics  54  may be coupled to the hydraulic strain head  41 . 
     In some embodiments, a command may be communicated to the tool  20  (via the hydraulic strain head  41 ) to start generating the tension pulses  24  every time the magnetic sensor  98  detects a casing collar. These commands may be transmitted to the tool  20  via acceleration and deacceleration of the tool assembly  44 , as detected by the hydraulic strain head  41  (see FIG.  1 ). In this manner, the tool  20  may be lowered to the bottom of the well, and then the command may be communicated downhole to instruct the tool  20  to begin generating the tension pulses  24  as the casing collars are detected. However, in other embodiments, an approximation of the tool depth may be performed by, for example, using a depth counter  27  (see FIG. 1) at the surface of the well to measure the length of the deployed slickline  21 . Based on this rough estimation, the position of the tool  20  is coarsely determined, and then a command is sent to the tool  20  to begin the generation of the tension pulses  24  to finely adjust the position of the tool  20 . 
     Referring to FIG. 8, in its closed position, the split collar  62  includes two half sections  102  that when compressed, form a threaded interior cylinder  104  for receiving the threaded shaft  64 . Referring also to FIG. 9, compression springs  99  are formed between the halves  102  to force the split collar  62  apart when the solenoid  57  moves the release shaft  74  to its upper point of travel. 
     Referring back to FIG. 1, the slickline tool assembly  44  is part of a slickline system that may include, as an example, a wellhead  23  that forms a seal with the slickline  21  at the surface of the well. The slickline  21  is attached to a drum  33  of a slickline unit  28  at the surface of the well. In this manner, the drum  33  may be operated to turn to retract or release the slickline  21  to control the depth of the tool assembly  44 . Between the drum  33  and the wellhead  23 , the slickline  21  is threaded through a tension sensor  26  (described below) and upper  32  and lower  35  sheaves. 
     Referring to FIG. 10, in some embodiments, the tension sensor  26  includes upper rollers  300  and lower rollers  302  (including rollers  302   a  and  302   b ) through which the slickline  21  extends. Sensors  310  are coupled to the lower rollers  302  to detect and indicate (via electrical signals) the downward forces that the slickline  21  exerts on the lower rollers  302   a  and  302   b . In this manner, a sensor  310   a  detects the force that is exerted on the lower roller  302   a , and a sensor  310   b  detects the force that is exerted on the lower roller  302   b . In some embodiments, each sensor  310  produces a signal that is sampled by a separate sample and hold (S/H) circuit  312  that provides the sampled signal to a separate analog-to-digital (A/D) converter  314 . Each A/D converter  314  provides a digital signal that indicates the force exerted on its roller  302 , and thus, indicates the strain on the slickline  21 . Indications of these digital signals are stored in a memory  316 . In some embodiments, a controller  318  of the tension sensor  26  may use the indications that are stored in the memory  316  to detect the signature of the pulse  24 . 
     Besides identifying the signature that is associated with the tension pulse  24 , the controller  318  may also analyze the indications of the waveforms in the memory  316  to determine the direction of the detected tension pulses. More particularly, a tension pulse may be inadvertently generated as the drum  33  retrieves the tool assembly  44 , as the drum  33  may momentarily catch the slickline  21  and generate a tension pulse in the slickline  21 . However, this tension pulse propagates in a direction that is opposite from the tension pulse  24 . Therefore, by detecting the direction of tension pulses that occur on the slickline  21 , the tension sensor  26  may filter out tension pulses that do not originate with the tool  20  and thus, are not tension pulses  24 . To accomplish this, an indication of the tension sensed by each sensor  310  is stored separately in the memory  316  so that the controller  318  may analyze the detected tension signals to determine which sensor  310  detected the tension pulse first. Thus, if the sensor  310   a  experiences a tension pulse first, the detected tension pulse originated with the slickline unit  28 . However, if the sensor  310   b  experiences a tension pulse first, the tension pulse originated downhole and may be a tension pulse  24 . 
     Referring to FIG. 11, in some embodiments, the hydraulic strain head  41  includes a sealed chamber that experiences pressure changes when the tool assembly  44  accelerates either in an upward or downward direction. A pressure-responsive transducer  254  detects the pressure changes and in response, generates electrical signals to indicate the changes and thus, indicate the decoded commands. The hydraulic strain head  41  communicates with the tool  20  through electronics (that are coupled to the electronics  54 ) to communicate the decoded commands. 
     The hydraulic strain head  41  includes a hydraulic power section  222  and a sensor section  224 . The hydraulic power section  222  includes a cylinder  226 . A fishing neck  228  is mounted at the upper end of the cylinder  226  and adapted to be coupled to the drum  33  (see FIG. 1) so that the hydraulic strain head  41  may be lowered into and retrieved from the wellbore via the slickline  21 . With the fishing neck  228  coupled to the slickline  21 , the hydraulic strain head  41  and other attached components may be accelerated or decelerated by the appropriate movement of the drum  33 . The fishing neck  228  may also be coupled to other tools. 
     A mandrel  230  is disposed in and axially movable within a bore  232  in the cylinder  226 . The mandrel  230  has a piston portion  234  and a shaft portion  236 . An upper chamber  238  is defined above the piston portion  234 , and a lower chamber  240  is defined below the piston portion  234  and around the shaft portion  236 . The upper chamber  238  is exposed to the pressure outside the cylinder  226  through a port  242  in the cylinder  226 . A sliding seal  244  between the piston portion  234  and the cylinder  226  isolates the upper chamber  238  from the lower chamber  240 , and a sliding seal  246  between the shaft portion  234  and the cylinder  226  isolates the lower chamber  240  from the exterior of the cylinder  226 . The sliding seal  244  is retained on the piston portion  234  by a seal retaining plug  248 , and the sliding seal  246  is secured to a lower end of the cylinder  226  by a seal retaining ring  250 . 
     The sensor section  224  includes a first sleeve  252  which encloses and supports a pressure transducer  254  and a second sleeve  256  that includes an electrical connector  258 . The first sleeve  252  is attached to the lower end of a connecting body  262  with a portion of the pressure transducer  254  protruding into a bore  264  in the connecting body  262 . An end  266  of the shaft portion  236  extends out of the cylinder  226  into the bore  264  in the connecting body  262 . The end  266  of the shaft portion  226  is secured to the connecting body  262  so as to allow the connecting body  262  to move with the mandrel  230 . Static seals, e.g., o-ring seals  276  and  278 , are arranged between the connecting body  262  and the shaft portion  236  and pressure transducer  254  to contain fluid within the bore  264 . 
     The shaft portion  236  includes a fluid channel  290  that is in communication with the bore  264  in the connecting body  262 . The fluid channel  290  opens to a bore  292  in the piston portion  234 , and the bore  292  in turn communicates with the lower chamber  240  through ports  294  in the piston portion  234 . The bore  292  and ports  294  in the piston portion  234 , the fluid channel  290  in the shaft portion  236 , and the bore  264  in the connecting body  262  define a pressure path from the lower chamber  240  to the pressure transducer  254 . The lower chamber  240  and the pressure path are filled with a pressure-transmitting medium (oil or other incompressible fluid, as examples) through fill ports  296  and  298  in the seal retaining plug  248  and the connecting body  262 , respectively. By using both fill ports  296  and  298  to fill the lower chamber  240  and the pressure path, the volume of air trapped in the lower chamber and the pressure path can be minimized. Plugs are provided in the fill ports  296  and  298  to contain fluid in the pressure path and the lower chamber  240 . 
     In operation, the tool assembly  44  is lowered into the wellbore with the lower chamber  240  and pressure path filled with a pressure-transmitting medium. When the tool assembly  44  is accelerated in the upward direction, the total force, F total , that is applied to the piston portion  234  by the tool assembly  44  increases and results in a corresponding increase in the pressure, P lc , in the lower chamber  240 . When the downhole tool assembly  44  is accelerated in the downward direction, the force, F total , which is applied to the piston portion  234  by the downhole tool assembly  44  decreases and results in a corresponding decrease in the pressure, P lc , in the lower chamber  240 . The tool assembly  44  may also be decelerated in either the upward or downward direction to effect similar pressure changes in the lower chamber  240 . The pressure changes in the lower chamber  240  are detected by the pressure transducer  254  as pressure pulses. Moving the tool assembly  44  in prescribed patterns will produce pressure pulses which are converted to electrical signals. 
     When the hydraulic strain head  41  is coupled to the tool  20 , the net force, F net , resulting from the pressure differential across the piston portion  234  supports the weight of the rest of the tool assembly  44 . The net force resulting from the pressure differential across the piston portion  234  can be expressed as: 
     
       
           F   net =( P   lc   −P   uc )· A   1c   (1) 
       
     
     where P 1c  is the pressure in the lower chamber  240 , P uc  is the pressure in the upper chamber  238  or the wellbore pressure outside the cylinder  226 , A lc  is the cross-sectional area of the lower chamber  240 . 
     The total force, F total , that is applied to the piston portion  234  by the tool assembly  44  may be expressed as: 
     
       
         F total   =m   tool ( g−a )+ F   drag   (2) 
       
     
     where “m tool ” is the mass of the tool assembly  44 , “g” is the acceleration due to gravity, “a” is the acceleration of the downhole tool  218 , and F drag  is the drag force acting on the tool assembly  44 . Drag force and acceleration are considered to be positive when acting in the same direction as gravity. 
     Assuming that the weight of the sensor section  224  and the weight of the connecting body  262  is negligibly small compared to the weight of the tool assembly  44 , then the net force, F net , resulting from the pressure differential across the piston portion  234  can be equated to the total force, F total , applied to the piston portion  234  by the tool assembly  44 , and the pressure, P lc , in the lower chamber  240  can then be expressed as:                P   lc     =       1     A   lc            [         m   tool     ·     (     g   -   a     )       +     F   drag     +       P   uc     ·     A   lc         ]               (   3   )                                
     The expression above demonstrates that the pressure, P lc , in the lower chamber  240  changes as the tool assembly  44  is accelerated or decelerated. These pressure changes are transmitted to the pressure transducer  254  through the fluid in the lower chamber  240  and the pressure path. The pressure transducer  254  responds to the pressure changes in the lower chamber  240  and converts them to electrical signals. For a given acceleration or deceleration, the size of a pressure change or pulse can be increased by reducing the cross-sectional area, A lc , of the lower chamber  240 . 
     The second sleeve  256  is mounted on the first sleeve  252  and includes slots  280  which are adapted to ride on projecting members  282  on the first sleeve  252 . When the slots  280  ride on the projecting members  282 , the hydraulic strain head  41  moves relative to the tool assembly  44 . A spring  283  connects and biases an upper end  284  of the second sleeve  256  to an outer shoulder  286  on the first sleeve  252 . The electrical connector  258  on the second sleeve  252  is connected to the pressure transducer  254  by electrical wires  288 . The electrical connector  258  forms a power and communications interface between the pressure transducer  254  and electronic circuitry (not shown) to decode the commands. 
     If the tool assembly  44  becomes stuck and jars are used to try and free the assembly, the pressure differential across the piston portion  234  can become very high. If the bottom-hole pressure, i.e., the wellbore pressure at the exterior of the tool assembly  44 , is close to the pressure rating of the tool assembly  44 , then the pressure transducer  254  can potentially be subjected to pressures that are well over its rated operating value. To prevent damage to the pressure transducer  254 , the fill plug may be provided with a rupture disc which bursts when the pressure in the lower chamber  240  is above the pressure rating of the pressure transducer  254 . When the rupture disc bursts, fluid drains out of the lower chamber  240  and the pressure path, through the fill port  296 , and out of the cylinder  226 . As the fluid drains out of the lower chamber  240  and the pressure path, the piston portion  234  will move to the lower end of the cylinder  226  until it reaches the end of travel, at which time the hydraulic strain head  41  becomes solid and the highest pressure the pressure transducer  254  will be subjected to is the bottom-hole pressure. Instead of using a rupture disc, a check valve or other pressure responsive member may also be arranged in the fill port  296  to allow fluid to drain out of the lower chamber  240  when necessary. If the tool assembly  44  becomes unstuck, commands can no longer be generated using acceleration or deceleration of the tool assembly  44 . However, traditional methods, such as manipulation of surface wellhead controls or movement of the tool assembly  44  over fixed vertical distances in a column of liquid can still be used. 
     Other embodiments are within the scope of the following claims. For example, other arrangements (hydraulic or electrical, as examples) may be used to generate the tension pulses. As an example, FIG. 12 depicts an embodiment of a tension pulse generator  400  that may be used to at least partially replace the pulse generator that is described above for purpose of generating tension signals on a slickline. The tension pulse generator  400  includes an electromagnetic stator coil  404  to move a shaft  402  that is attached to the slickline  12  and is circumscribed by the coil  404 . In some embodiments of the invention, the shaft  402  is formed from stack of permanent magnets. Thus, by controlling the magnetic field that is generated by the coil  404 , the shaft  402  may be moved to generate a desired tension signal on the slickline  21 . Thus, the coil  404  and shaft  402  form at least part of a linear actuator. 
     More specifically, in some embodiments of the invention, the electromagnetic stator coil  404  may be aligned with the longitudinal axis of the tool assembly and may be circumscribed by a housing  408  of the tool assembly. As an example, the coil  404  may be encapsulated in a non-ferromagnetic material  409  that forms an inner space  411  in which the shaft  402  slides. The pulse generator  400  includes a coil spring  410  that is located in the space  411  and is compressed between the top surface of the shaft  402  and the lower surface of a cap  413  that extends radially inwardly from the sidewall of the housing  408 . The cap  413  includes an opening  406  that receives the slickline  21 . The slickline  21  extends along the longitudinal axis of the spring  410  and is attached to the top of the shaft  402 . 
     Thus, due to this arrangement, the polarity and magnitude of current that is received by the coil  404  may be controlled to move the shaft  402  up and down to generate a tension signature on the slickline  21 . As an example, FIG. 13 depicts two tension signals  420  that are generated on the slickline  21 . Each signal  420  represents a modulated tension on the slickline  21  and is the result of a modulated (frequency modulated, for example) current flowing through the coil  404 . In this manner, a terminal voltage of the coil  404  may be modulated to produce the modulated current, that in turns, moves the shaft  402  to generate the tension signal  420 . The two signal  420  may be separated in time by a predetermined time interval (an interval of two seconds, for example). Any combination of signatures may be used to indicate detection of a collar. 
     FIG. 14 depicts circuitry  440  that may be used to control the current in the coil  404 . The circuitry  440  includes a controller  442  that receives indications of any detected collars from the magnetic collar sensor  98 . The controller  442  also communicates with the hydraulic strain head  41  for purposes of determining when to generate a tension signal in response to a detected collar. When detection of a collar is to be communicated to the surface of the well, the controller  442  momentarily activates a modulator  444  (an frequency modulated (FM) modulator, for example) to generate a modulated voltage that is applied (via a driver  446 ) to the coil  404  for some predefined time interval. If more than one tension signal is to be generated, the controller  442  may delay for a predefined time interval before activating the modulator  444  to generate the next tension signal on the slickline  21 . 
     Referring to FIG. 15, in some embodiments of the invention, a pulse generator  460  may be used in place of the pulse generators described above. The pulse generator  460  includes a generally cylindrical shaft  462  that extends along the longitudinal axis of the pulse generator  460  and is circumscribed by an electromagnetic coil  474 . The electromagnetic coil  474  is encapsulated by a non-ferromagnetic material, such as plastic, that is part of a generally cylindrical housing  472 . The housing  472  is attached to the tool assembly and thus, is attached to the slickline  21 . The shaft  462  includes a stack  464  of permanent magnets that is enclosed by a non-ferromagnetic (plastic, for example) cylindrical housing  465  of the shaft  462 . Upper  468  and lower  470  metal spears are located at the top and bottom, respectively, of the housing  465  and are used to strike upper  471  and lower  473  end caps of the housing  472 . The pulse generator  460  may include rollers  475  (plastic rollers, for example) that are located between the outer surface of the housing  465  of the shaft  462  and the inner surface of the housing  472 . 
     Thus, due to the above-described arrangement, the current through the coil  474  may be controlled to move the shaft  462  in an upward direction to strike the upper cap  471  to momentarily reduce tension on the slickline  21  in the form of a negative pressure pulse  490  that is depicted in FIG.  16 . The current in the coil  474  may also be controlled to move the shaft  462  in a downward direction to strike the lower cap  473  to momentarily increase tension on the slickline  21  in the form of a positive pressure pulse  491 . Thus, the coil  474  and shaft  462  form at least part of a linear actuator. 
     In some embodiments of the invention, the occurrence of a negative pressure pulse  490  that is followed in time by a positive pressure pulse  491  may form a signature to indicate detection of a casing collar. Any combination of the of pulses or signatures may be used to indicate detection of a casing collar. 
     In some embodiments of the invention, circuitry  500  that is depicted in FIG. 17 may be used to control the current through the coil  474  for purposes of generating the positive and negative tension pulses in the slickline  21 . The circuitry  500  may include a controller  502  that receives indications of any detected collars from the magnetic collar sensor  98 . The controller  502  also communicates with the hydraulic strain head  41  for purposes of determining when to generate a tension signature in response to a detected collar. 
     The circuitry  500  may include two capacitor banks  504  and  506  to generate two successive tension pulses. In this manner, each capacitor bank  504 ,  506  stores energy that is converted into a large current for purposes of producing a large magnetic force (via the coil  474 ) to propel the shaft  462  into the cap  471  or  473 . Because each capacitor bank  504 ,  506  stores energy at a slower rate than which the capacitor bank  504 ,  506  delivers the energy to the coil  474 , two capacitor banks may be needed to produce two successive tension pulses. 
     The controller  502  generates a particular tension pulse by controlling a switch circuit  508  (that is coupled to the capacitor banks  504  and  506 ) to discharge one of the capacitor banks  504  and  506  into the coil  474  via a driver  510 . The switch circuit  508  not only selects one of the capacitor banks  504  and  506 , the switch circuit  508  also selects the polarity of the voltage that is applied to the coil  474 , in some embodiments of the invention. 
     For example, to generate a negative tension pulse that is followed by a positive tension pulse, the controller  502  may communicate with the switch circuit  508  to select the capacitor bank  504  and set the polarity of the voltage that is applied to the terminal voltage to cause the shaft  462  to slam into the cap  471 . In this manner, the capacitor bank  504  discharges to produce the negative tension pulse in the slickline  21 . Next, after waiting for some predefined time, the controller  502  communicates with the switch circuit  508  to select the capacitor bank  506  and set the polarity of the voltage that is applied to the terminal voltage to cause the shaft  462  to slam into the cap  473 . In this manner, the capacitor bank  506  discharges to produce the positive tension pulse in the slickline  21 . The controller  502  may control the movement of the shaft  462  to produce other signatures in other embodiments of the invention. 
     Other embodiments are within the scope of the following claims. For example, the electromagnetic coil and magnet stack of the pulse generator  400 ,  406  may be replaced by a tubular linear motor, in some embodiments of the invention. Other variations are possible. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.