Patent Publication Number: US-8118094-B2

Title: Tracer injector tool for well investigation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims the benefit as a divisional from co-pending U.S. application Ser. No. 10/596,814, filed on Jun. 26, 2006; which is related to and claims the benefit of International Application No. PCT/EP2004/013681, filed on Dec. 1, 2004; which is related to and claims the benefit of European Application No. 03293353.3, now European Patent No. EP 1 550 790 B1, filed on Dec. 31, 2003; the entire contents of each are hereby incorporated by reference. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates generally to downhole injection of one or more tracers or marker materials in a well. 
     2. Background Art 
     When a well, specifically an oil or gas well, has been completed and is yielding a desired product, it is necessary to monitor the well&#39;s performance to ensure that it is behaving as expected. In particular, it is desirable to measure a velocity of a fluid along a borehole and up to the surface. In an oil well, the fluid may be oil, water, gas or a combination, even a mixture, of all three. It is generally desirable to monitor the velocities of the fluids actually down the well itself rather than merely when they reach the surface. 
     Many types of method and apparatus have been proposed for this purpose. A first method involves the use of a mechanical “spinner&gt;&gt;: a wireline-supported tool carrying a small propeller- (or turbine-) driven dynamo is placed in a flowing fluid so that the propeller is turned around by it, and the dynamo&#39;s output indicates the velocity of the flowing fluid. The first method provides satisfactory results in borehole sections that are vertical, but not in sections which are horizontal. A horizontal section may indeed comprise several layers, e.g. an oil layer above a water layer. 
     A second method involves an injector/detector tool that injects a small amount of tracer e.g., a detectable chemical or a radioactive substance, at a first location and detects the injected tracer at a second location. The flow velocity is calculated by a simple distance over time calculation. An example of injector/detector tool is the Tracer Injector tool of Schlumberger which is described in U.S. Pat. Nos. 4,166,215 and 4,166,216. The U.S. Pat. No. 6,125,934 describes an injector tool adapted for a horizontal section. For a determined tracer intended to be ejected into a corresponding layer, the injector tool comprises an ejection port that is oriented such that the determined tracer may be ejected directly into the corresponding layer. Hence an oil miscible tracer is injected into an oil layer and a water miscible tracer is injected into a water layer. 
       FIG. 1  illustrates a typical injector/detector tool known from prior art. An injector tool  101  ejects a tracer into a borehole  102  at a first location (not shown in  FIG. 1 ), and a detector tool  103  detects the tracer at a second location (not shown in  FIG. 1 ). The ejection of the tracer is commanded via a command wire  106  that controls from a surface system  111  located at the surface a solenoid valve  105  of the injector tool. When the switch is on, the solenoid valve  105  is open and a relatively constant force acting on a piston  107  due to a spring  104  expels an amount of the tracer through a thin tube  112 . 
       FIG. 1  illustrates a system as it is being used in a vertical well. However, such a system may also be used in a horizontal well. For this purpose, it may further comprise an ejection port that is judicially oriented to aim a determined layer. 
     A dedicated wire  108  transmits an analog signal to activate the detector tool  103 , thus allowing to detect the tracer. When activated, the detector tool transmits results to the surface system  111  via a results wire  115 . The electrical wires ( 106 ,  108 ,  115 ) are located within a central chamber  113  so as to be protected from any liquid. The central chamber  113  may also contain one or more additional wires that are used for a communicating between the surface system  111  and one or more additional devices, e.g. a logging device  109 . 
     In the system illustrated in  FIG. 1 , the tracer is directly ejected from a reservoir  114 . The quantity of tracer that is discharged into the borehole  102  is determined by a duration of the opening of the solenoid valve  105 , the duration being controlled at surface. However, it is not possible to insure that the quantity of tracer corresponding to the duration has effectively been ejected. 
     In another system from prior art, the quantity of tracer that is periodically discharged into the borehole space is determined by a size of a syringe filled with the tracer before ejection. The tracer may be stored in a reservoir that communicates with the syringe so as to provide a regular filling of the syringe. A Hall Effect sensor may detect the end of stroke of the piston so as to confirm that the quantity of tracer has been ejected. 
     When a piece of the injector tool has to be replaced, e.g., the reservoir  114 , the injector tool needs to be completely stripped out. It is indeed necessary to cut electrical wires located in the central chamber  113  to remove the piece. 
     Similarly, the electro-valve  105  comprises a solenoid coil  117  communicating with the command wire  106 , and a solenoid seat  118  through which the tracer is expelled. The solenoid coil  117  is used to control a movement of a plunger  119  that allows the tracer to be expelled. When the solenoid seat  118  is replaced, the solenoid coil also needs to be removed. 
     SUMMARY OF INVENTION 
     In a first aspect the invention provides a tool system for monitoring a flow of liquid within a borehole. The tool system comprises a plurality of tools disposed on a longitudinal axis of the tool system. The plurality of tools comprises at least a first injector tool for ejecting in the borehole a tracer and a detector tool to detect the ejected tracer. The tool system further comprises a standard digital bus traversing at least a portion of each tool of the plurality of tools. The standard digital bus allows a communication between each tool of the plurality of tools. 
     In a first preferred embodiment, the plurality of tools comprises a control tool to manage data exchanges through the standard digital bus. 
     In a second preferred embodiment, the plurality of tools comprises a second injector tool located on an opposite side of the detector tool of the tool system as compared to the first injector tool so as to allow to detect a possible reverse flow in the borehole. 
     In a third preferred embodiment, plurality of tools also comprises a third injector tool distinct from the first injector tool. The third injector tool is located on the same side of the detector tool in the tool system as the first injector tool. 
     In a fourth preferred embodiment, the borehole has a longitudinal direction that is substantially horizontal. The plurality of tools also comprises an orientating tool to measure an orientation of at least an ejection port of the first injector tool. 
     In a fifth preferred embodiment, the first injector tool comprises a first group of electrical wires corresponding to the standard digital bus and at least one standard connector. The at least one standard connector allows to removably connect the first group of electrical wires to a second group of electrical wires corresponding to the standard digital bus within a distinct tool from the plurality of tools. 
     In a sixth preferred embodiment, both the first group of electrical wires and the second group of electrical wires comprise two power wires dedicated to power transportation and two signal wires dedicated to signal transportation. 
     In a second aspect, the invention provides a method for monitoring a flow of liquid within a borehole. The method comprises providing a plurality of tools on a longitudinal axis of the borehole. The tools are linked with a standard digital bus allowing a communication between each tool of the plurality of tools. A quantity of tracer is ejected using a first injector tool among the plurality of tools. The ejected tracer is detected using a detector tool among the plurality of tools. 
     In a seventh preferred embodiment, the method further comprises managing data exchanges through the standard digital bus using a control tool among the plurality of tools. 
     In an eighth preferred embodiment, a possible reverse flow in the borehole is detected using a second injector tool among the plurality of tools. The second injector tool is located on an opposite side of the detector tool as compared to the first injector tool. The second injector tool communicates with the detector tool using the standard digital bus. 
     In a ninth preferred embodiment, an orientation of at least an ejection port of the first injector tool is measured with an orientating tool among the plurality of tools. 
     In a third aspect, the invention provides an injector tool for ejecting a tracer in a system for monitoring a flow of liquid within a borehole. The injector tool comprises measuring means to measure an ejected quantity of the ejected tracer. 
     In a tenth preferred embodiment, the injector tool further comprises a body and a piston to expel the tracer. The measuring means measure a displacement of the piston relative to the body. 
     In an eleventh preferred embodiment, the measuring means comprise at least one magnetic ring mounted on the piston, and a plurality of Hall effect switches mounted on the body. 
     In a twelfth preferred embodiment, three magnetic rings are mounted on the piston. The Hall Effect switches are organized into four independent arrays. The Hall Effect switches belonging to a determined array are tied to a single determined wire. 
     In a thirteenth preferred embodiment, the injector tool further comprises a reservoir into which the tracer is stored, an opening through which the tracer may be ejected from the injector tool and an electro-valve to control the opening. The injector tool further comprises actuating means. The actuating means allow to move the piston such that the piston moves when the electro-valve opens the opening and the tracer is ejected. 
     In a fourth aspect, the invention provides a tool system for monitoring a flow of liquid within a borehole comprising an injector tool according to the second aspect of the invention. 
     In a fifth aspect, the invention provides a method for monitoring a flow of liquid within the borehole. The method comprises ejecting a tracer with an injector tool located within the borehole. The method further comprises measuring an ejected quantity of the ejected tracer. 
     In a fourteenth preferred embodiment, the ejected tracer is detected with a detector tool located within the borehole. 
     In a fifteenth preferred embodiment, a value of a desired quantity of tracer is received downhole. The method further comprises starting the ejecting of the tracer, comparing the measured ejected quantity of tracer with the value of the desired quantity and interrupting the ejecting if measured ejected quantity substantially equals the value of the desired quantity. 
     In a sixteenth preferred embodiment, a counter is initialized at the starting of the ejecting. The counter is incremented while the tracer is being ejected. A value of the counter is transmitted to a surface system at the interrupting of the ejection. The value of the counter is a function of a duration of the ejecting. 
     In a sixth aspect, the invention provides an injector tool for ejecting a tracer in a system for monitoring a flow of liquid within a borehole. The injector tool comprises a first group of hydraulic parts intended to be in contact with the tracer and a second group of electrical elements. The hydraulic parts of the first group may be accessed and replaced during a maintenance operation. The electrical elements of the second group remain protected during the maintenance operation. 
     In a seventeenth preferred embodiment, the injector tool further comprises an electro-valve. The electro-valve comprises an electrical portion belonging to the second group, a solenoid seat belonging to the first group and a high pressure barrier. The high pressure barrier allows to isolate the electrical portion of the electro-valve from the solenoid seat. The electro-valve is mounted in the injector tool such that the solenoid seat may be accessed without removing the electrical portion. 
     In a eighteenth preferred embodiment, the injector tool further comprises electrical wires belonging to the second group, and a connector. The connector allows to connect the electrical elements of the second group with other electrical elements of a distinct tool. The connector comprises a first portion and a second portion. The first portion may be removed during the maintenance operation. The second portion continues to protect the electrical wires during the maintenance operation. 
     In a nineteenth preferred embodiment, an injection counter is activated at the ejecting of the tracer. The injection counter is incremented at an acquisition frequency that is independent from a communication frequency of the communication between the plurality of tools. A value of the injection counter may be read to evaluate a time duration between the ejecting and the detecting. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a schematic of an injector/detector tool within a borehole from Prior Art. 
         FIG. 2  illustrates an example of a tool system according to the present invention. 
         FIG. 3  illustrates an example of a portion of an injector tool according to the present invention. 
         FIG. 4  illustrates measuring means of an injector tool according to a preferred embodiment of the present invention. 
         FIG. 5  illustrates an example of an algorithm to be executed by an electronic card of a tool system according to the present invention. 
         FIG. 6  illustrates an example of a portion of an injector tool according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Prior Art 
     The ejector/detector tool from prior art encloses a dedicated wire to insure a synchronization between the injector tool and the detector tool at the ejecting of the tracer. An analog signal activates the detector for a possible detecting of the tracer. If intermediate tools are located between the injector tool and the detector tool, the dedicated wire has to run through all the intermediate tools, which adds an additional cabling to an intermediate cabling of the intermediate tools. Hence, the injector tool may not be easily set anywhere. 
     It may happen that a casing of the borehole is damaged by a hole, thus causing losses of fluids through the hole. A water producing zone above the hole may generate a reverse flow. A possible way to detect the presence of the hole is to measure a velocity of the reverse flow in the borehole. The reverse flow may also be caused by a recirculation of water in a horizontal well. In both cases, a production rate of the well is affected. The reverse flow may be monitored by ejecting a tracer at a first location with an injector tool and by providing a detector tool at a second location, the second location being located deeper within the borehole than the first location. In a case of a lateral hole, the second location is located further from a main well than the first location. 
     However, the detector tools as known in the art are not able to manage several analog signals from several dedicated wires: a multiple injector tools—single detector tool configuration is not possible. Therefore, the detecting of the reverse flow requires providing a dedicated detector tool. 
     A system allowing a synchronization of an injector tool with a detector tool without such a dedicated wire may render the multiple injector tools—single detector tool configuration possible. There is thus a need such a synchronization without the dedicated wire so as to allow to detect the reverse flow without providing a dedicated detector tool. 
     Furthermore, the quantity of tracer that is periodically discharged into the borehole is predetermined. It may occur that the predetermined quantity is too small to allow a proper detection of the tracer. Or, on the contrary, the predetermined quantity may be so high that a recharging of the reservoir is frequently required. There is a need for a system in which the ejected quantity of tracer may be commanded and changed from the surface. 
     When a piece of the injector tool has to be replaced, it is necessary to intervene on an electrical element thereof. For example, if a solenoid seat of the electro-valve needs to be replaced, it is necessary to intervene on a solenoid coil. Electrical wires located in a central chamber also have to be cut when a piston is replaced. Most maintenance operations on the injector tool may hence be relatively long. For example, it may take two hours for replacing the electro-valve. There is a need for a system in which the pieces may be replaced more rapidly. 
     Standard Digital Bus Tracer Injector Tool 
       FIG. 2  illustrates an example of a tool system for monitoring a flow of liquid within a borehole according to the invention. A plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ) is disposed on a longitudinal axis of a tool system  212  within a borehole  202 . In this example, the tools are assembled to form a tube. The plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ) comprises at least a first injector tool  201  for ejecting in the borehole  202  a tracer and a detector tool  203  to detect the ejected tracer. A standard digital bus  206  traverses longitudinally at least a portion of each tool of the plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ). The standard digital bus  206  allows a communication between each tool of the plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ). 
     In the example illustrated in  FIG. 2 , a control tool  207  manages data exchanges through the standard digital bus  206 . The control tool  207  in addition communicates with a surface system  211  through a surface bus  208 . 
     In the systems from prior art wherein a dedicated wire is required between an injector tool and a detector tool, it may be relatively delicate to provide an additional tool between the injector tool and the detector tool. For this reason, the injector tool is usually disposed at a relatively low distance from the detector tool. Because no dedicated wire is required for synchronization, the system according to the invention allows to dispose the injector tool at a greater distance from the detector tool than the systems from prior art. The measurements of the flow velocity may thus be more accurate. 
     The standard digital bus  206  allows to insure the synchronization. Preferably, an ejection counter (not represented) is activated at the ejecting of the tracer. The activating of the ejection counter allows to activate at least the detector tool  203 . The ejection counter is incremented at an acquisition frequency that is independent from a communication frequency of the communication between the plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ). The acquisition frequency may have a relatively high value, e.g. 15 kHz. The communication frequency at which the plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ) communicates may be smaller than the acquisition frequency and may vary with time. Typically, the communication frequency may be smaller than 5 Hz. The ejection counter hence allows to evaluate a time duration between the ejecting and the detecting of the tracer with a relatively high precision. A value of the injection counter may be read to evaluate the time duration between the ejecting and the detecting. 
     Alternatively, the synchronization may be performed by broadcasting a single ejection command to both the first injector tool  201  and the detector tool  203 . 
     The standard digital bus  206  may also be used to allow a communication with a second injector tool  205  located opposite the detector tool  203  when seen from the first injector  201 , so as to allow to detect a possible reverse flow in the borehole  202 . 
     The plurality of tools may also comprise a third injector tool  204  distinct from the first injector tool  201 , the third injector tool  204  being located on the same side of but more distant from the detector tool  203 . The third injector tool  204  may eject the same type of tracer as the first injector tool  201 . Because of a greater distance between the third injector tool  204  and the detector tool  203 , and because the detector tool  203  detects tracer ejected at different locations, a better accuracy of the measuring of the flow velocity is thus provided. 
     The third injector tool  204  may also eject a second tracer different from the tracer ejected by the first injector tool  201 . For example, the second tracer may be an oil tracer, intended to coalesce with an oil phase, whereas the tracer ejected by the first injector tool  201  may be a water tracer intended to coalesce with a water or brine phase. Hence the flow velocities of both the oil phase and the water phase are measured using a single detector tool. 
     In a further example embodiment (not represented in  FIG. 2 ), the well is horizontal; the first injector tool and the third injector tool may comprise an ejection port. For example, the ejection port of the first injection tool may be oriented downward so as to inject a water or brine tracer, and the ejection port of the third ejection tool may be oriented upward so as to eject an oil tracer. 
     In the system according to the invention, there is no dedicated wire through which an analog signal activates the detector tool as in prior art, and hence no wire related constraints for placing the injector tools. 
     The system according to the invention allows a multiple injector tools—single detector tool configuration, unlike the systems from prior art. The detector tools is not required to manage several analog signals from several dedicated wires coming from several ejector tools as is the case with the systems from prior art. By providing a standard digital bus  206  between each of the plurality of tools ( 201 ,  203 ,  204 ,  205 ,  207 ), the invention allows more than a single injector tool for a single detector. 
     Furthermore, the system according to the invention provides compatibility between the injector tools and any other downhole tool that uses the standard digital bus to communicate. In particular, in a case of a horizontal section, where a first injector tool having an ejection port oriented to lay with a determined layer is used, the first injector tool may be able to communicate with an orientating tool. The orientating tool may for example comprise a relative bearing measuring tool. The orientating tool may also comprise absolute angle measuring means, thus providing a reliable measurement of an orientation of the ejection port. Furthermore, the orientating tool may comprise a probe that is able to evaluate a nature of the determined layer (oil, water . . . ). Hence such an orientating tool provides various measurements that may be crucial for monitoring correctly a flow of liquid of the determined layer. The standard digital bus allows the first injector tool and any other tool to communicate with the orientating tool, and to supply both tools with power. 
     The tools of the plurality of tools, e.g. the first injector tool, may comprise a standard connector. The standard connector of the first injector tool allows a first group of electrical wires corresponding to the standard digital bus along the first injector tool to be removably connected to a second group of electrical wires. The second group of electrical wires corresponds to the standard digital bus along a distinct tool of the plurality of tools, i.e. a tool that is able to communicate using the standard digital bus. The standard connector hence allows to replace one of the tools of the plurality of tools by another, e.g. the first injector tool and the detector tool may be interchanged so as to detect a possible reverse flow. In the systems from prior art, the first injector tools comprises a dedicated connector, and the replacing of one tool by an other is thus more complex. 
     The standard digital bus may for example be a Production Service Platform bus (PSP). The PSP bus comprises two power wires dedicated to power transportation, and two signal wires dedicated to signal transportation. The standard digital bus may also be any digital bus that allows a communicating between devices. 
     The plurality of tools comprises at least the first injector tool and the detector tool. The plurality of tools of the present invention may comprise other devices such as logging devices that also communicate using the standard digital bus. The system according to the invention may also comprise some extra tools that communicate using a dedicated wire, or using a second bus distinct from the standard digital bus. 
     Injector Tool 
       FIG. 3  illustrates an example of an injector tool for ejecting a tracer in a tool system for monitoring a flow of liquid within a borehole according to the invention. The injector tool  303  comprises measuring means  310  to measure an ejected quantity of the ejected tracer. 
     The measuring means  310  may, as represented in  FIG. 3 , be located within the injector tool  303 . As the tracer  302  stored into a reservoir  304  is typically expelled by a displacement of a piston  307 , the measuring means  310  may measure a displacement of the piston  307  relative to a body  311  of the injector tool  303 . An electro-valve  305  controls an opening  312  through which the tracer may be ejected from the injector tool  303 . During an ejection operation, actuating means, e.g. a spring (not represented on  FIG. 3 ) move the piston  307  when the electro-valve  305  opens the opening  312  and the tracer is ejected. The measuring means  310  allow to measure the ejected quantity of tracer. 
     For example, the piston  307  may comprise three magnetic rings  301  that are mounted thereon so as to have a same movement as the piston  307 . When the piston  307  slides along the body  311  of the injector tool  303 , Hall Effect switches  309  mounted on the body  311  detect a displacement of the magnetic rings  301 . Such measuring means that involve the Hall Effect may be designed to avoid any contact between the tracer and themselves. Hence it is possible to use corrosive tracers. 
       FIG. 4  illustrates a preferred embodiment of measuring means according to the present invention. In this embodiment, the Hall Effect switches  401  are organized in four independent arrays. Each array of switches may be longitudinally disposed with a determined azimuthal angle, even if the Hall Effect switches  401  are represented on a same axis on  FIG. 4 . Each array is tied to a corresponding wire ( 403   a ,  403   b ,  403   c ,  403   d ). A signal  405  on a determined wire  403   b  is generated by a passage of a magnetic ring  402  close to one of the Hall effect switches  401   b  belonging to the array corresponding to the determined wire  403   b.    
     The three magnetic rings  402  located on an extension  404  of a piston are separated from each other by a distance that is 33% higher than a space between two Hall Effect switches, so as to provide a measurement that is three times more accurate than with a single magnetic ring. 
     The signal  405  may be processed so as to generate a pulse  406  at its rising edge. By observing the pulses corresponding to the four electrical wires, it is possible to measure a relative displacement of the piston relative to the Hall Effect switches. 
     In another embodiment of the present invention, an absolute displacement of a piston relative to a body may be measured. 
     Such a measurement allows to control the ejected quantity of tracer.  FIG. 5  illustrates an example of an algorithm to be executed by an electronic card downhole. The electronic card may be part of a tool system comprising the injector tool and a detector tool, the detector tool allowing to detect the ejected tracer. A desired number N corresponding to a value of a desired quantity of tracer is received in box  501  from a surface system. The ejecting of the tracer then starts at box  502 . In a preferred embodiment, the starting is performed by opening an electro-valve. 
     The opening of the electro-valve allows a displacing of the piston actuated for example by a spring. The measuring means provide a measurement of the ejected quantity of tracer, which is proportional to a displacement of the piston. During the ejecting of the tracer, a measured number T 2  corresponding to the displacement of the piston, and hence, to the measured ejected quantity of tracer is regularly read at box  503 . The measured number T 2  is compared in box  505  to the desired number N. If the measured number T 2  is smaller than the desired number N, i.e. the measured ejected quantity of tracer is smaller than the value of the desired quantity, then a new value of the measured number T 2  is read (in box  503 ) from the measuring means, thus repeating an ejection cycle. 
     When the measured number T 2  is substantially equal to the desired number N, after a number n of ejection cycles, the ejected quantity of tracer is substantially equal to the desired quantity. The ejection may hence be interrupted at box  506 : the electro-valve is closed. 
     This example algorithm enables to eject a desired quantity of tracer, wherein the value of the desired quantity is received from the surface system at each ejection operation. The value of the desired quantity of tracer may thus be changed, which allows to increase the quantity of ejected tracer for a proper detection of the tracer at the detector tool, or on the contrary, to reduce the quantity and save some tracer. 
     Furthermore, with an injector tool according to the invention, it is insured that the desired quantity of tracer has really been ejected. The systems from prior art that comprise only a sensor to detect an end of stroke of the piston fail to provide any information about a position of the piston before the ejection, and hence it is only supposed that the piston was at a proper position. 
     The injector tool according to the invention also allows to measure a duration of the ejecting. The electrical card may comprise a frame counter FRCT, as illustrated in the algorithm of  FIG. 5 . When the ejecting starts at box  502 , the frame counter FRCT is initialized. Then, at each ejection cycle, the frame counter FRCT is incremented in box  504 . After n ejection cycles, when the ejecting is interrupted in box  506 , the frame counter has been incremented n times. The value of the frame counter FRCT is a function of the duration of the ejecting; it is transmitted at box  507  to the surface system. Knowing the duration of the injecting allows to model a behavior of the ejected tracer within the borehole, and hence to predict a shape of a measured signal at the detector tool. A time at which the ejected tracer is detected may thus be measured more accurately. 
     The measuring means may be any mean to detect a displacement of a piston relative to a body, and, more generally, any means that provide an evaluating of an ejected quantity of the tracer. 
     Injector Tool Configuration 
       FIG. 6  illustrates an example of a portion of an injector tool intended to eject a tracer for monitoring a flow of liquid  609  in a well according to the invention. The injector tool  601  comprises a first group of hydraulic parts ( 603 ;  610 ;  616 ) intended to be in contact with the tracer, and a second group of electrical elements ( 604 ;  605 ;  617 ). The electrical elements ( 604 ;  605 ;  617 ) may be any element able to conduct electricity for power or signal transportation, e.g. an electronic card (not represented), Hall Effect sensors  617 , etc. The hydraulic parts ( 603 ;  610 ;  616 ) may be any piece that may be in contact with the tracer, e.g. a reservoir  603 , a piston  616 , etc. The hydraulic parts ( 603 ;  610 ;  616 ) of the first group may be accessed and replaced during a maintenance operation. The electrical elements ( 604 ;  605 ;  617 ) of the second group remain protected during the maintenance operation. 
     The hydraulic parts ( 603 ;  610 ;  616 ) of the first group may be directly accessed without cutting electrical wires  605  as in prior art. Furthermore, in a case of a rig site maintenance operation, the electrical elements ( 604 ;  605 ;  617 ) of the second group remain isolated from contamination by an external fluid in the well even during the maintenance operations, so that they do not need to be replaced. There is hence no need to touch any electrical element ( 604 ;  605 ;  617 ) of the second group, either for accessing one of the hydraulic parts ( 603 ;  610 ;  616 ), or for replacing them after a deterioration due to the maintenance operation. 
     Typically, the injector tool according to the invention comprises an electro-valve  606  that, when opened, allows to expel the tracer. The electro-valve  606  discloses a solenoid seat  610  through which the tracer is expelled, the solenoid seat  610  belonging to the first group. The electro-valve also discloses an electrical portion, e.g. a solenoid coil  604  that is relied to a command wire  619 . The solenoid coil  604  is used to control a movement of a plunger  611  and belongs to the second group. The tracer stored in the reservoir  603  passes through a tube (not represented) and is kept in the solenoid seat  610 . The movement of the plunger  611  allows the tracer to be expelled. 
     In the injector tool according to the invention, the electro-valve  606  is oriented so that the solenoid seat  610  is at a peripheral position compared to the solenoid coil  604 , i.e. the solenoid seat  610  may be accessed without removing the solenoid coil  604 . 
     Furthermore, the electro-valve  606  may disclose a high pressure barrier  608  that isolates the solenoid coil  604  from the solenoid seat  610 . The solenoid coil  604  remains protected when the solenoid seat  610  is replaced. 
     In the example embodiment illustrated in  FIG. 6 , it is necessary to remove a seat locker  602  to access the solenoid seat  610 . 
     Similarly, the electrical wires  605  of the injector tool belong to the second group and remain protected during a replacing of the reservoir  603 . The electrical wires are located within a chamber  614 . The Hall Effect sensors  617  allowing a measurement of a quantity of ejected tracer are also located within the chamber  614 . A connector  620  allows to connect the electrical wires with another electronic system, e.g. the electronic card (not represented) of the injector tool  601 , or electrical wires of a second tool (not represented). The connector  620  may comprise a first portion  612  that is mounted on an outside tube  621 , and a second portion  613  that is mounted on an inside tube, e.g. the chamber  614 . 
     During the maintenance operation, the first portion  612  of the connector  620  is removed. The second portion  613  of the connector continues to protect the electrical wires  605  and the Hall Effect sensors  617  from the liquid  609  within the well. A spring  615 , the piston  616  and the reservoir  603  may hence slide along the chamber  614  so as to be replaced. 
     By separating the electrical elements and the hydraulic parts, and by protecting the electrical elements, the invention allows to remove and replace the hydraulic parts much more easily than in prior art. Hence corrosive tracers may be used. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.