Patent Publication Number: US-10316645-B2

Title: Autonomous untethered well object

Description:
BACKGROUND 
     For purposes of preparing a well for the production of oil or gas, at least one perforating gun may be deployed into the well via a conveyance mechanism, such as a wireline or a coiled tubing string. The shaped charges of the perforating gun(s) are fired when the gun(s) are appropriately positioned to perforate a casing of the well and form perforating tunnels into the surrounding formation. Additional operations may be performed in the well to increase the well&#39;s permeability, such as well stimulation operations and operations that involve hydraulic fracturing. The above-described perforating and stimulation operations may be performed in multiple stages of the well. 
     The above-described operations may be performed by actuating one or more downhole tools. A given downhole tool may be actuated using a wide variety of techniques, such dropping a ball into the well sized for a seat of the tool; running another tool into the well on a conveyance mechanism to mechanically shift or inductively communicate with the tool to be actuated; pressurizing a control line; and so forth. 
     SUMMARY 
     The summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     In an example implementation, a technique includes deploying an untethered object though a passageway of a string in a well; and acquiring a plurality of measurements that represent an environment of the string as the object is being communicated through the passageway. The technique includes cross-correlating the plurality of measurements and using results of the cross-correlating to identify at least one downhole feature. 
     In another example implementation, an apparatus that is usable with a well includes string and an untethered object that is adapted to be deployed in a passageway of the string, such that the untethered object travels in the passageway. The untethered object includes a magnetic field generator; antennae that are spatially separated to provide a plurality of signals generated in response to a magnetic field generated by the magnetic field generator; an expandable element; and a controller. The controller of the untethered object cross-correlates the signals; uses the cross-correlation of the signals to identify at least one downhole feature of the string; and selectively radially expands the element based at least in part on the at least one identified downhole feature. 
     In another example implementation, a technique includes deploying an untethered object though a passageway of a string in a well; sensing a property of an environment of the string as the object is being communicated through the passageway; and selectively autonomously radially expanding the untethered object in response to the sensing. Radially expanding the untethered object includes creating fluid communication between two chambers of the object at different pressures to cause translational movement of a piston of the object; and expanding a collar of the object in response to the translation of the piston. 
     In another example implementation, an apparatus that is usable with a well includes a string and an untethered object that is adapted to be deployed in the passageway such that the object travels in the passageway. The untethered object includes a first chamber at a relatively lower pressure; a second chamber at a relatively high pressure; a fluid control device between the first and second chambers; a piston; an expandable collar that is coupled to the piston; and a controller to operate the fluid control device to establish communication between the first and second chambers to selectively radially expand the untethered object. 
     In another example implementation, an apparatus that is usable with a well includes a string and an untethered object that is adapted to be deployed in a passageway of the string such that the object travels in the passageway. The untethered object includes a first chamber at a relatively lower pressure; a second chamber at a relatively high pressure; a fluid control device between the first and second chambers; a piston; an expandable collar that is coupled to the piston; and a controller to operate the fluid control device to establish communication between the first and second chambers to selectively radially expand the untethered object. 
     In another example implementation, a technique that is usable with a well includes deploying an untethered object though a passageway of a string in a well. The string comprising at least one dedicated location identification marker. The technique includes detecting a feature of the string as the object is being communicated through the passageway. The detecting includes actuating at least one mechanically-actuated switch of the object in response to engagement of the object with the at least one dedicated identification marker to register a count; and selectively autonomously operating the untethered object in response to the count. 
     In yet another example implementation, a technique includes deploying an untethered object though a passageway of a tubular member; and acquiring a plurality of measurements that represent an environment of the tubular member as the object is being communicated through the passageway. The technique includes cross-correlating the plurality of measurements and using results of the cross-correlating to identify at least one feature of the tubular member. 
     Advantages and other features will become apparent from the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a multiple stage well according to an example implementation. 
         FIG. 2  is a schematic diagram of a dart of  FIG. 1  in a radially contracted state according to an example implementation. 
         FIG. 3  is a schematic diagram of the dart of  FIG. 1  in a radially expanded state according to an example implementation. 
         FIGS. 4, 6B and 14  are flow diagrams depicting techniques to autonomously operate an untethered object in a well to perform an operation in the well according to example implementations. 
         FIG. 5  is a schematic diagram of a dart illustrating a magnetic field sensor of the dart of  FIG. 1  according to an example implementation. 
         FIG. 6A  is a schematic diagram illustrating a differential pressure sensor of the dart of  FIG. 1  according to an example implementation. 
         FIG. 7  is a flow diagram depicting a technique to autonomously operate a dart in a well to perform an operation in the well according to an example implementation. 
         FIGS. 8A and 8B  are cross-sectional views illustrating use of the dart to operate a valve according to an example implementation. 
         FIGS. 9A, 9B, 9C and 9D  are cross-sectional views illustrating use of a dart to operate a valve assembly according to an example implementation. 
         FIG. 10A  is a perspective view of a dart according to an example implementation. 
         FIG. 10B  is a cross-sectional view of the dart of  FIG. 10A  according to an example implementation. 
         FIG. 11  is a perspective view of a deployment mechanism of the dart according to a further example implementation. 
         FIG. 12  is a schematic diagram of a dart illustrating an electromagnetic coupling sensor of the dart according to an example implementation. 
         FIG. 13  is an illustration of a signal generated by the sensor of  FIG. 12  according to an example implementation. 
         FIG. 15  is a schematic diagram illustrating a balanced coil sensor of a dart according to an example implementation. 
         FIGS. 16A and 16B  are illustrations of the balanced coil sensor in proximity to different downhole features according to example implementations. 
         FIG. 17A  is an illustration of a difference of signals provided by the balanced coil sensor according to an example implementation. 
         FIG. 17B  is an illustration of signals provided by the balanced coil sensor according to an example implementation. 
         FIGS. 18A and 18B  illustrate signals provided by a balanced coil sensor according to an example implementation. 
         FIG. 19  is an illustration of a process to determine a time shift between sensed signals using cross-correlation according to an example implementation. 
         FIG. 20  is a cross-sectional view of an example section of a tubing string. 
         FIG. 21  illustrates signals provided by coils of a balanced coil sensor when passing through the tubing string section of  FIG. 20  according to an example implementation. 
         FIG. 22  is an illustration depicting a process to measure distances between features of a tubing string according to an example implementation. 
         FIG. 23  is a flow diagram depicting a technique to use cross-correlation of sensor signals to identify a downhole feature according to an example implementation. 
         FIG. 24A  is a flow diagram depicting a technique used by an untethered object to determine its speed according to an example implementation. 
         FIG. 24B  is a flow diagram depicting a technique used by an untethered object to identify downhole equipment according to an example implementation. 
         FIG. 25A  is a schematic view illustrating a dart landing in a sleeve of a valve assembly according to an example implementation. 
         FIG. 25B  is a cross-sectional view illustrating the shifting of the sleeve by the dart of  FIG. 25A  according to an example implementation. 
         FIGS. 26A and 26B  are schematic diagrams illustrating the use of mechanically-actuated switches of a dart to count downhole identification markers according to an example implementation. 
         FIG. 27  is an electrical schematic diagram illustrating the use of mechanically-actuated switches to count downhole features according to an example implementation. 
         FIG. 28  is a flow diagram depicting a technique to use mechanically-actuated switches of an untethered object to regulate activation of the object according to an example implementation. 
     
    
    
     DETAILED DESCRIPTION 
     In general, systems and techniques are disclosed herein for purposes of deploying an untethered object into a well and using an autonomous operation of the object to perform a downhole operation. In this context, an “untethered object” refers to an object that travels at least some distance in a well passageway without being attached to a conveyance mechanism (a slickline, wireline, coiled tubing string, and so forth). As specific examples, the untethered object may be a dart, a ball or a bar. However, the untethered object may take on different forms, in accordance with further implementations. In accordance with some implementations, the untethered object may be pumped into the well (i.e., pushed into the well with fluid), although pumping may not be employed to move the object in the well, in accordance with further implementations. 
     In general, the untethered object may be used to perform a downhole operation that may or may not involve actuation of a downhole tool As just a few examples, the downhole operation may be a stimulation operation (a fracturing operation or an acidizing operation as examples); an operation performed by a downhole tool (the operation of a downhole valve, the operation of a single shot tool, or the operation of a perforating gun, as examples); the formation of a downhole obstruction; or the diversion of fluid (the diversion of fracturing fluid into a surrounding formation, for example). Moreover, in accordance with example implementations, a single untethered object may be used to perform multiple downhole operations in multiple zones, or stages, of the well, as further disclosed herein. 
     In accordance with example implementations, the untethered object is deployed in a passageway (a tubing string passageway, for example) of the well, autonomously senses its position as it travels in the passageway, and upon reaching a given targeted downhole position, autonomously operates to initiate a downhole operation. The untethered object is initially radially contracted when the object is deployed into the passageway. The object monitors its position as the object travels in the passageway, and upon determining that it has reached a predetermined location in the well, the object radially expands. The increased cross-section of the object due to its radial expansion may be used to effect any of a number of downhole operations, such as shifting a valve, forming a fluid obstruction, actuating a tool, and so forth. Moreover, because the object remains radially contracted before reaching the predetermined location, the object may pass through downhole restrictions (valve seats, for example) that may otherwise “catch” the object, thereby allowing the object to be used in, for example, multiple stage applications in which the object is used in conjunction with seats of the same size so that the object selects which seat catches the object. 
     In general, the untethered object is constructed to sense its downhole position as it travels in the well and autonomously respond based on this sensing. As disclosed herein, the untethered object may sense its position based on features of the string, markers, formation characteristics, and so forth, depending on the particular implementation. As a more specific example, for purposes of sensing its downhole location, the untethered object may be constructed to, during its travel, sense specific points in the well, called “markers” herein. Moreover, as disclosed herein, the untethered object may be constructed to detect the markers by sensing a property of the environment surrounding the object (a physical property of the string or formation, as examples). The markers may be dedicated tags or materials installed in the well for location sensing by the object or may be formed from features (sleeve valves, casing valves, casing collars, and so forth) of the well, which are primarily associated with downhole functions, other than location sensing. Moreover, as disclosed herein, in accordance with example implementations, the untethered object may be constructed to sense its location in other and/or different ways that do not involve sensing a physical property of its environment, such as, for example, sensing a pressure for purposes of identifying valves or other downhole features that the object traverses during its travel. 
     Referring to  FIG. 1 , as a more specific example, in accordance with some implementations, a multiple stage well  90  includes a wellbore  120 , which traverses one or more formations (hydrocarbon bearing formations, for example). As a more specific example, the wellbore  120  may be lined, or supported, by a tubing string  130 , as depicted in  FIG. 1 . The tubing string  130  may be cemented to the wellbore  120  (such wellbores typically are referred to as “cased hole” wellbores); or the tubing string  130  may be secured to the formation by packers (such wellbores typically are referred to as “open hole” wellbores). In general, the wellbore  120  extends through one or multiple zones, or stages  170  (four stages  170 - 1 ,  170 - 2 ,  170 - 3  and  170 - 4 , being depicted as examples in  FIG. 1 ) of the well  90 . 
     It is noted that although  FIG. 1  depicts a laterally extending wellbore  120 , the systems and techniques that are disclosed herein may likewise be applied to vertical wellbores. In accordance with example implementations, the well  90  may contain multiple wellbores, which contain tubing strings that are similar to the illustrated tubing string  130 . Moreover, depending on the particular implementation, the well  90  may be an injection well or a production well. Thus, many variations are contemplated, which are within the scope of the appended claims. 
     In general, the downhole operations may be multiple stage operations that may be sequentially performed in the stages  170  in a particular direction (in a direction from the toe end of the wellbore  120  to the heel end of the wellbore  120 , for example) or may be performed in no particular direction or sequence, depending on the implementation. 
     Although not depicted in  FIG. 1 , fluid communication with the surrounding reservoir may be enhanced in one or more of the stages  170  through, for example, abrasive jetting operations, perforating operations, and so forth. 
     In accordance with example implementations, the well  90  of  FIG. 1  includes downhole tools  152  (tools  152 - 1 ,  152 - 2 ,  152 - 3  and  152 - 4 , being depicted in  FIG. 1  as examples) that are located in the respective stages  170 . The tool  152  may be any of a variety of downhole tools, such as a valve (a circulation valve, a casing valve, a sleeve valve, and so forth), a seat assembly, a check valve, a plug assembly, and so forth, depending on the particular implementation. Moreover, the tool  152  may be different tools (a mixture of casing valves, plug assemblies, check valves, and so forth, for example). 
     A given tool  152  may be selectively actuated by deploying an untethered object through the central passageway of the tubing string  130 . In general, the untethered object has a radially contracted state to permit the object to pass relatively freely through the central passageway of the tubing string  130  (and thus, through tools of the string  130 ), and the object has a radially expanded state, which causes the object to land in, or, be “caught” by, a selected one of the tools  152  or otherwise secured at a selected downhole location, in general, for purposes of performing a given downhole operation. For example, a given downhole tool  152  may catch the untethered object for purposes of forming a downhole obstruction to divert fluid (divert fluid in a fracturing or other stimulation operation, for example); pressurize a given stage  170 ; shift a sleeve of the tool  152 ; actuate the tool  152 ; install a check valve (part of the object) in the tool  152 ; and so forth, depending on the particular implementation. 
     For the specific example of  FIG. 1 , the untethered object is a dart  100 , which, as depicted in  FIG. 1 , may be deployed (as an example) from the Earth surface E into the tubing string  130  and propagate along the central passageway of the string  130  until the dart  100  senses proximity of the targeted tool  152  (as further disclosed herein), radially expands and engages the tool  152 . It is noted that the dart  100  may be deployed from a location other than the Earth surface E, in accordance with further implementations. For example, the dart  100  may be released by a downhole tool. As another example, the dart  100  may be run downhole on a conveyance mechanism and then released downhole to travel further downhole untethered. 
     In accordance with an example implementation, the tools  152  may be sleeve valves that may be initially closed when run into the well  90  but subsequently shifted open when engaged by the dart  100  for purposes for performing fracturing operations from the heel to the toe of the wellbore  120  (for the example stages  170 - 1 ,  170 - 2 ,  170 - 3  and  170 - 4  depicted in  FIG. 1 ). In this manner, for this example, before being deployed into the wellbore  120 , the dart  100  is configured, or programmed, to sequentially target the tools  152  of the stages  170 - 1 ,  170 - 2 ,  170 - 3  and  170 - 4  in the order in which the dart  100  encounters the tools  152 . 
     Continuing the example, the dart  100  is released into the central passageway of the tubing string  130  from the Earth surface E, travels downhole in the tubing string  130 , and when the dart  100  senses proximity of the tool  152  of the stage  170 - 1  along the dart&#39;s path, the dart  100  radially expands to engage a dart catching seat of the tool  152 . Using the resulting fluid barrier, or obstruction, that is created by the dart  100  landing in the tool  152 , fluid pressure may be applied uphole of the dart  100  (by pumping fluid into the tubing string  130 , for example) for purposes of creating a force to shift the sleeve of the tool  152  (a sleeve valve, for this example) to open radial fracture ports of the tool  152  with the surrounding formation in the stage  170 - 1 . 
     The dart  100  is constructed to subsequently radially contract to release itself from the tool  152  (as further disclosed herein) of the stage  170 - 1 , travel further downhole through the tubing string  130 , radially expand in response to sensing proximity of the tool  152  of the stage  170 - 2 , and land in the tool of the stage  170 - 2  to create another fluid obstruction. Using this fluid obstruction, the portion of the tubing string  130  uphole of the dart  100  may be pressurized for purposes of fracturing the stage  170 - 1  and shifting the sleeve valve of the stage  170 - 2  open. Thus, the above-described process repeats in the heel-to-toe fracturing, in accordance with an example implementation, as the fracturing proceeds downhole until the stage  170 - 4  is fractured. It is noted that although  FIG. 1  depicts four stages  170 - 1 ,  170 - 2 ,  170 - 3  and  170 - 4 , the heel-to-toe fracturing may be performed in fewer or more than four stages, in accordance with further implementations. 
     Although examples are disclosed herein in which the dart  100  is constructed to radially expand at the appropriate time so that a tool  152  of the string  130  catches the dart  100 , in accordance with other implementations disclosed herein, the dart  100  may be constructed to secure itself to an arbitrary position of the string  130 , which is not part of a tool  152 . Thus, many variations are contemplated, which are within the scope of the appended claims. 
     For the example that is depicted in  FIG. 1 , the dart  100  is deployed in the tubing string  130  from the Earth surface E for purposes of engaging one of the tool  152  (i.e., for purposes of engaging a “targeted tool  152 ”). The dart  100  autonomously senses its downhole position, remains radially contracted to pass through tool(s)  152  (if any) uphole of the targeted tool  152 , and radially expands before reaching the targeted tool  152 . In accordance with some implementations, the dart  100  senses its downhole position by sensing the presence of markers  160  which may be distributed along the tubing string  130 . 
     For the specific example of  FIG. 1 , each stage  170  contains a marker  160 , and each marker  160  is embedded in a different tool  152 . The marker  160  may be a specific material, a specific downhole feature, a specific physical property, a radio frequency (RF) identification (RFID), tag, and so forth, depending on the particular implementation. 
     It is noted that each stage  170  may contain multiple markers  160 ; a given stage  170  may not contain any markers  160 ; the markers  160  may be deployed along the tubing string  130  at positions that do not coincide with given tools  152 ; the markers  160  may not be evenly/regularly distributed as depicted in  FIG. 1 ; and so forth, depending on the particular implementation. Moreover, although  FIG. 1  depicts the markers  160  as being deployed in the tools  152 , the markers  160  may be deployed at defined distances with respect to the tools  152 , depending on the particular implementation. For example, the markers  160  may be deployed between or at intermediate positions between respective tools  152 , in accordance with further implementations. Thus, many variations are contemplated, which are within the scope of the appended claims. 
     In accordance with an example implementation, a given marker  160  may be a magnetic material-based marker, which may be formed, for example, by a ferromagnetic material that is embedded in or attached to the tubing string  130 , embedded in or attached to a given tool housing, and so forth. By sensing the markers  160 , the dart  100  may determine its downhole position and selectively radially expand accordingly. As further disclosed herein, in accordance with an example implementation, the dart  100  may maintain a count of detected markers. In this manner, the dart  100  may sense and log when the dart  100  passes a marker  160  such that the dart  100  may determine its downhole position based on the marker count. 
     Thus, the dart  100  may increment (as an example) a marker counter (an electronics-based counter, for example) as the dart  100  traverses the markers  160  in its travel through the tubing string  130 ; and when the dart  100  determines that a given number of markers  160  have been detected (via a threshold count that is programmed into the dart  100 , for example), the dart  100  radially expands. 
     For example, the dart  100  may be launched into the well  90  for purposes of being caught in the tool  152 - 3 . Therefore, given the example arrangement of  FIG. 1 , the dart  100  may be programmed at the Earth surface E to count two markers  160  (i.e., the markers  160  of the tools  152 - 1  and  152 - 2 ) before radially expanding. The dart  100  passes through the tools  152 - 1  and  152 - 2  in its radially contracted state; increments its marker counter twice due to the detection of the markers  152 - 1  and  152 - 2 ; and in response to its marker counter indicating a “2,” the dart  100  radially expands so that the dart  100  has a cross-sectional size that causes the dart  100  to be “caught” by the tool  152 - 3 . 
     Referring to  FIG. 2 , in accordance with an example implementation, the dart  100  includes a body  204  having a section  200 , which is initially radially contracted to a cross-sectional diameter D 1  when the dart  100  is first deployed in the well  90 . The dart  100  autonomously senses its downhole location and autonomously expands the section  200  to a radially larger cross-sectional diameter D 2  (as depicted in  FIG. 3 ) for purposes of causing the next encountered tool  152  to catch the dart  100 . 
     As depicted in  FIG. 2 , in accordance with an example implementation, the dart  100  include a controller  224  (a microcontroller, microprocessor, field programmable gate array (FPGA), or central processing unit (CPU), as examples), which receives feedback as to the dart&#39;s position and generates the appropriate signal(s) to control the radial expansion of the dart  100 . As depicted in  FIG. 2 , the controller  224  may maintain a count  225  of the detected markers, which may be stored in a memory (a volatile or a non-volatile memory, depending on the implementation) of the dart  100 . 
     In this manner, in accordance with an example implementation, the sensor  230  provides one or more signals that indicate a physical property of the dart&#39;s environment (a magnetic permeability of the tubing string  130 , a radioactivity emission of the surrounding formation, and so forth); the controller  224  use the signal(s) to determine a location of the dart  100 ; and the controller  224  correspondingly activates an actuator  220  to expand a deployment mechanism  210  of the dart  100  at the appropriate time to expand the cross-sectional dimension of the section  200  from the D 1  diameter to the D 2  diameter. As depicted in  FIG. 2 , among its other components, the dart  100  may have a stored energy source, such as a battery  240 , and the dart  100  may have an interface (a wireless interface, for example), which is not shown in  FIG. 2 , for purposes of programming the dart  100  with a threshold marker count before the dart  100  is deployed in the well  90 . 
     The dart  100  may, in accordance with example implementations, count specific markers, while ignoring other markers. In this manner, another dart may be subsequently launched into the tubing string  130  to count the previously-ignored markers (or count all of the markers, including the ignored markers, as another example) in a subsequent operation, such as a remedial action operation, a fracturing operation, and so forth. In this manner, using such an approach, specific portions of the well  90  may be selectively treated at different times. In accordance with some example implementations, the tubing string  130  may have more tools  152  (see  FIG. 1 ), such as sleeve valves (as an example), than are needed for current downhole operations, for purposes of allowing future refracturing or remedial operations to be performed. 
     In accordance with example implementations, the sensor  230  senses a magnetic field. In this manner, the tubing string  130  may contain embedded magnets, and sensor  230  may be an active or passive magnetic field sensor that provides one or more signals, which the controller  224  interprets to detect the magnets. However, in accordance with further implementations, the sensor  230  may sense an electromagnetic coupling path for purposes of allowing the dart  100  to electromagnetic coupling changes due to changing geometrical features of the string  130  (thicker metallic sections due to tools versus thinner metallic sections for regions of the string  130  where tools are not located, for example) that are not attributable to magnets. In other example implementations, the sensor  230  may be a gamma ray sensor that senses a radioactivity. Moreover, the sensed radioactivity may be the radioactivity of the surrounding formation. In this manner, a gamma ray log may be used to program a corresponding location radioactivity-based map into a memory of the dart  100 . 
     Regardless of the particular sensor  230  or sensors  230  used by the dart  100  to sense its downhole position, in general, the dart  100  may perform a technique  400  that is depicted in  FIG. 4 . Referring to  FIG. 4 , in accordance with example implementations, the technique  400  includes deploying (block  404 ) an untethered object, such as a dart, through a passageway of a string and autonomously sensing (block  408 ) a property of an environment of the string as the object travels in the passageway of the string. The technique  400  includes autonomously controlling the object to perform a downhole function, which may include, for example, selectively radially expanding (block  412 ) the untethered object in response to the sensing. 
     Referring to  FIG. 5  in conjunction with  FIG. 2 , in accordance with an example implementation, the sensor  230  of the dart  100  may include a coil  504  for purposes of sensing a magnetic field. In this manner, the coil  504  may be formed from an electrical conductor that has multiple windings about a central opening. When the dart passes in proximity to a ferromagnetic material  520 , such as a magnetic marker  160  that contains the material  520 , magnetic flux lines  510  of the material  520  pass through the coil  504 . Thus, the magnetic field that is sensed by the coil  504  changes in strength due to the motion of the dart  100  (i.e., the influence of the material  520  on the sensed magnetic field changes as the dart  100  approaches the material  520 , coincides in location with the material  520  and then moves past the material  520 ). The changing magnetic field, in turn, induces a current in the coil  504 . The controller  224  (see  FIG. 2 ) may therefore monitor the voltage across the coil  504  and/or the current in the coil  504  for purposes of detecting a given marker  160 . The coil  504  may or may not be pre-energized with a current (i.e., the coil  504  may passively or actively sense the magnetic field), depending on the particular implementation. 
     It is noted that  FIGS. 2 and 5  depict a simplified view of the sensor  230  and controller  224 , as the skilled artisan would appreciate that numerous other components may be used, such as an analog-to-digital converter (ADC) to convert an analog signal from the coil  504  into a corresponding digital value, an analog amplifier, and so forth, depending on the particular implementation. 
     In accordance with example implementations, the dart  100  may sense a pressure to detect features of the tubing string  130  for purposes of determining the location/downhole position of the dart  100 . For example, referring to  FIG. 6A , in accordance with example implementations, the dart  100  includes a differential pressure sensor  620  that senses a pressure in a passageway  610  that is in communication with a region  660  uphole from the dart  100  and a passageway  614  that is in communication with a region  670  downhole of the dart  100 . Due to this arrangement, the partial fluid seal/obstruction that is introduced by the dart  100  in its radially contracted state creates a pressure difference between the upstream and downstream ends of the dart  100  when the dart  100  passes through a valve. 
     For example, as shown in  FIG. 6A , a given valve may contain radial ports  604 . Therefore, for this example, the differential pressure sensor  620  may sense a pressure difference as the dart  100  travels due to a lower pressure below the dart  100  as compared to above the dart  100  due to a difference in pressure between the hydrostatic fluid above the dart  100  and the reduced pressure (due to the ports  604 ) below the dart  100 . As depicted in  FIG. 6A , the differential pressure sensor  620  may contain terminals  624  that, for example, electrically indicate the sensed differential pressure (provide a voltage representing the sensed pressure, for example), which may be communicated to the controller  224  (see  FIG. 2 ). For these example implementations, valves of the tubing string  130  are effectively used as markers for purposes of allowing the dart  100  to sense its position along the tubing string  130 . 
     Therefore, in accordance with example implementations, a technique  680  that is depicted in  FIG. 6B  may be used to autonomously operate the dart  100 . Pursuant to the technique  680 , an untethered object is deployed (block  682 ) in a passageway of the string; and the object is used (block  684 ) to sense pressure as the object travels in a passageway of the string. The technique  680  includes selectively autonomously operating (block  686 ) the untethered object in response to the sensing to perform a downhole operation. 
     In accordance with some implementations, the dart  100  may sense multiple indicators of its position as the dart  100  travels in the string. For example, in accordance with example implementations, the dart  100  may sense both a physical property and another downhole position indicator, such as a pressure (or another property), for purposes of determining its downhole position. Moreover, in accordance with some implementations, the markers  160  (see  FIG. 1 ) may have alternating polarities, which may be another position indicator that the dart  100  uses to assess/corroborate its downhole position. In this regard, magnetic-based markers  160 , in accordance with an example implementation, may be distributed and oriented in a fashion such that the polarities of adjacent magnets alternate. Thus, for example, one marker  160  may have its north pole uphole from its south pole, whereas the next marker  160  may have its south pole uphole from its north pole; and the next the marker  160 - 3  may have its north pole uphole from its south pole; and so forth. The dart  100  may use the knowledge of the alternating polarities as feedback to verify/assess its downhole position. 
     Thus, referring to  FIG. 7 , in accordance with an example implementation, a technique  700  for autonomously operating an untethered object in a well, such as the dart  100 , includes determining (decision block  704 ) whether a marker has been detected. If so, the dart  100  updates a detected marker count and updates its position, pursuant to block  708 . The dart  100  further determines (block  712 ) its position based on a sensed marker polarity pattern, and the dart  100  may determine (block  716 ) its position based on one or more other measures (a sensed pressure, for example). If the dart  100  determines (decision block  720 ) that the marker count is inconsistent with the other determined position(s), then the dart  100  adjusts (block  724 ) the count/position. Next, the dart  100  determines (decision block  728 ) whether the dart  100  should radially expand the dart based on determined position. If not, control returns to decision block  704  for purposes of detecting the next marker. 
     If the dart  100  determines (decision block  728 ) that its position triggers its radially expansion, then the dart  100  activates (block  732 ) its actuator for purposes of causing the dart  100  to radially expand to at least temporarily secure the dart  100  to a given location in the tubing string  130 . At this location, the dart  100  may or may not be used to perform a downhole function, depending on the particular implementation. 
     In accordance with example implementations, the dart  100  may contain a self-release mechanism. In this regard, in accordance with example implementations, the technique  700  includes the dart  100  determining (decision block  736 ) whether it is time to release the dart  100 , and if so, the dart  100  activates (block  740 ) its self-release mechanism. In this manner, in accordance with example implementations, activation of the self-release mechanism causes the dart&#39;s deployment mechanism  210  (see  FIGS. 2 and 3 ) to radially contract to allow the dart  100  to travel further into the tubing string  130 . Subsequently, after activating the self-release mechanism, the dart  100  may determine (decision block  744 ) whether the dart  100  is to expand again or whether the dart has reached its final position. In this manner, a single dart  100  may be used to perform multiple downhole operations in potentially multiple stages, in accordance with example implementations. If the dart  100  is to expand again (decision block  744 ), then control returns to decision block  704 . 
     As a more specific example,  FIGS. 8A and 8B  depict engagement of the dart  100  with a valve assembly  810  of the tubing string  130 . As an example, the valve assembly  810  may be a casing valve assembly, which is run into the well  90  closed and which may be opened by the dart  100  for purposes of opening fluid communication between the central passageway of the string  130  and the surrounding formation. For example, communication with the surrounding formation may be established/opened through the valve assembly  810  for purposes of performing a fracturing operation. 
     In general, the valve assembly  810  includes radial ports  812  that are formed in a housing of the valve assembly  810 , which is constructed to be part of the tubing string  130  and generally circumscribe a longitudinal axis  800  of the assembly  810 . The valve assembly  810  includes a radial pocket  822  to receive a corresponding sleeve  814  that may be moved along the longitudinal axis  800  for purposes of opening and closing fluid communication through the radial ports  812 . In this manner, as depicted in  FIG. 8A , in its closed state, the sleeve  814  blocks fluid communication between the central passageway of the valve assembly  810  and the radial ports  812 . In this regard, the sleeve  814  closes off communication due to seals  816  and  818  (o-ring seals, for example) that are disposed between the sleeve  814  and the surrounding housing of the valve assembly  810 . 
     As depicted in  FIG. 8A , in general, the sleeve  814  has an inner diameter D 2 , which generally matches the expanded D 2  diameter of the dart  100 . Thus, referring to  FIG. 8B , when the dart  100  is in proximity to the sleeve  814 , the dart  100  radially expands the section  200  to close to or at the diameter D 2  to cause a shoulder  200 -A of the dart  100  to engage a shoulder  819  of the sleeve  814  so that the dart  100  becomes lodged, or caught in the sleeve  814 , as depicted in  FIG. 8B . Therefore, upon application of fluid pressure to the dart  100 , the dart  100  translates along the longitudinal axis  800  to shift open the sleeve  814  to expose the radial ports  812  for purposes of transitioning the valve assembly  810  to the open state and allowing fluid communication through the radial ports  812 . 
     In general, the valve assembly  810  depicted in  FIGS. 8A and 8B  is constructed to catch the dart  100  (assuming that the dart  100  expands before reaching the valve assembly  810 ) and subsequently retain the dart  100  until (and if) the dart  100  engages a self-release mechanism. 
     In accordance with some implementations, the valve assembly may contain a self-release mechanism, which is constructed to release the dart  100  after the dart  100  actuates the valve assembly. As an example,  FIGS. 9A and 9B  depict a valve assembly  900  that also includes radial ports  910  and a sleeve  914  for purposes of selectively opening and closing communication through the radial ports  910 . In general, the sleeve  914  resides inside a radially recessed pocket  912  of the housing of the valve assembly  900 , and seals  916  and  918  provide fluid isolation between the sleeve  914  and the housing when the valve assembly  900  is in its closed state. Referring to  FIG. 9A , when the valve assembly  910  is in its closed state, a collet  930  of the assembly  910  is attached to and disposed inside a corresponding recessed pocket  940  of the sleeve  914  for purposes of catching the dart  100  (assuming that the dart  100  is in its expanded D 2  diameter state). Thus, as depicted in  FIG. 9A , when entering the valve assembly  900 , the section  200  of the dart  100 , when radially expanded, is sized to be captured inside the inner diameter of the collet  930  via the shoulder  200 -A seating against a stop shoulder  913  of the pocket  912 . 
     The securement of the section  200  of the dart  100  to the collet  930 , in turn, shifts the sleeve  914  to open the valve assembly  900 . Moreover, further translation of the dart  100  along the longitudinal axis  902  moves the collet  930  outside of the recessed pocket  940  of the sleeve  914  and into a corresponding recessed region  950  further downhole of the recessed region  912  where a stop shoulder  951  engages the collet  930 . This state is depicted in  FIG. 9B , which shows the collet  930  as being radially expanded inside the recess region  940 . For this radially expanded state of the collet  930 , the dart  100  is released, and allowed to travel further downhole. 
     Thus, in accordance with some implementations, for purposes of actuating, or operating, multiple valve assemblies, the tubing string  130  may contain a succession, or “stack,” of one or more of the valve assemblies  900  (as depicted in  FIGS. 9A and 9B ) that have self-release mechanisms, with the very last valve assembly being a valve assembly, such as the valve assembly  800 , which is constructed to retain the dart  100 . 
       FIGS. 9C and 9D  illustrate a dart  101  according to a further example implementation. For this example, the dart  101  is used to shift a valve assembly  960 , with  FIG. 9C  illustrating the radially contracted dart  101  entering the valve assembly  960  and  FIG. 9D  illustrating the shifting of the valve assembly  960 . 
     More specifically, referring to  FIG. 9C , in accordance with example implementations, the dart  101  has a C-ring  1070 , which the dart  101  radially expands for purposes of engaging an inner sleeve  962  of sleeve valve  960 . In this regard,  FIG. 9C  depicts the dart  101  in proximity to a restricted profile, or seat  964 , of the inner sleeve  962 .  FIG. 9D  depicts engagement of the C-ring  1070  with the seat  964 . In this engaged position, fluid pressure may be applied uphole of the dart  101  for purposes of shifting the inner sleeve  962  downhole to open radial flow ports (not shown) of the valve assembly  960 . 
     Referring to  FIG. 10A , in general, the dart  101  has a tubular housing  1001  and an annular seal element  1092 , which generally surrounds the housing  1001 . As described further below, in accordance with example implementations, the dart  101  is constructed to retract an internal piston to cause the closure  1071  of the C-ring  1070  to impinge upon a spear  1075  that is fixed to the housing  1001  for purposes of radially expanding the ring  1070 . 
     Referring to  FIG. 10B , in accordance with example implementations, the dart  101  includes a deployment mechanism that is formed from an atmospheric pressure chamber  1050  and a chamber  1060  that is initially isolated from the atmospheric pressure chamber  1050  and initially exerts a hydrostatic pressure against the piston  1075 . More specifically, in accordance with an example implementation, the piston  1075  controls the alignment of radial ports  1052  of the housing  1001  and radial ports  1041  of a mandrel  1074  that is connected to the piston  1075 . In the dart&#39;s radially contracted state, the piston  1075  is in a position to isolate the ports  1052  from the ports  1041 . In this manner, in accordance with example implementations, a pressure chamber  1060  (a hydrostatic pressure chamber, for example) acts against the piston  1075  in a direction to keep the C-ring  1070  unexpanded. 
     In accordance with example implementations, to expand the C-ring  1070 , the dart  101  reduces the pressure in the chamber  1060  to cause the piston  1075  to shift in the opposite direction. In this manner, the dart  101  radially expands the C-ring  1070  by opening fluid communication between the chamber  1060  and the atmospheric chamber  1050 . This causes the piston  1075  to move into space  1060  and pull the C-ring  1070  into the spear  1075  may be radially expanded in response to fluid at hydrostatic pressure being communicated through the radial ports  1052 . 
     For purposes of controlling fluid communication between chambers  1050  and  1060 , the dart  101  includes a flow control device, such as a rupture disc  1020 . The controller  224  selectively actuates the actuator  220  of the dart  101  for purposes of rupturing the rupture disc  1020  to establish communication with the atmospheric  1050  chamber for purposes of causing the mandrel  1080  to translate to expand the C-ring  1070 . 
     As an example, in accordance with some implementations, the actuator  220  may include a linear actuator  1020 , which, when activated by the controller  224 , controls a linearly operable member to puncture the rupture disc  1020  for purposes of establishing communication with the atmospheric chamber  1050 . In further implementations, the actuator  220  may include an exploding foil initiator (EFI) to activate a pyrotechnic material for purposes of puncturing the rupture disc  1020  (either directly or by forcing a projectile through the disc  1020  using the pressure generated by expanding gases, for example). The rupture disc  1020  may be an electric rupture disc. Moreover, communication path between the chambers may have an aperture, flutes, channels or other features to regulate fluid to flow from the hydrostatic chamber to the atmospheric chamber. Thus, many implementations are contemplated, which are within the scope of the appended claims. 
     Among its other features, as depicted in  FIG. 10B , in accordance with example implementations, the dart  101  may include an electronic board  1032  that contains the circuitry for the controller  224  and a battery  1022  to provide power to the board  1032 . The dart may further include windings  1076  that may form coils, and are used for purposes of sensing downhole features (valves, collars and so forth), as further described herein. In this manner, the windings  1076  may form one or more receiver coils (or antennae) of a balanced coil sensor or electromagnetic sensor, in general, in accordance with example implementations. More specifically, as further described herein, the controller  224  may process signals received from the receiver coils to identify downhole features, identify identification markers and determine a speed of the dart  101 , among other functions. The dart  101  may further include a check valve  1034  that has a dissolvable ball  1036  for purposes of establishing downhole flow through the dart  101  after a predetermined time elapses to allow the dart  101  to be initially used to establish a fluid barrier to shift a valve assembly and divert fluid (such as in a fracturing operation). Additionally, as depicted in  FIGS. 10A and 10B , in accordance with example implementations, the dart  101  may have a nose end  1072  with a receptacle  1073  to receive a tail end  1030  of another dart  101 . Thus, multiple darts  101  may be stacked end-to-end, depending on the particular application in which the darts  101  are used. 
     Although the dart  101  is depicted as having a C-ring  1070  as its expandable deployment element, in general, the dart may have any of a number of different deployment elements, depending on the particular implementations. As other examples, the deployment element may be a collet sleeve, an inflatable bladder, an elastomer packer-type element that is compressed in response to the hydrostatic pressure, and so forth. Thus, many implementations are contemplated, which are within the scope of the appended claims. 
     In accordance with some example implementations, dart may have a self-release mechanism. For example, in accordance with example implementations, the dart may have a self-release mechanism that is formed from a reservoir and a metering valve, where the metering valve serves as a timer. In this manner, in response to the dart radially expanding, a fluid begins flowing into a pressure relief chamber. For example, the metering valve may be constructed to communicate a metered fluid flow between hydrostatic and atmospheric pressure chambers for purposes of resetting the deployment element of the dart to a radially contracted state to allow the dart to travel further into the well. As another example, in accordance with some implementations, one or more components of the dart, such as the deployment mechanism may be constructed of a dissolvable material, and the dart may release a solvent from a chamber at the time of its radial expansion to dissolve the mechanism. 
     As yet another example,  FIG. 11  depicts a portion of a dart  1100  in accordance with another example implementation. For this implementation, a deployment mechanism  1102  of the dart  1100  includes slips  1120 , or hardened “teeth,” which are designed to be radially expanded for purposes of gripping the wall of the tubing string  130 , without using a special seat or profile of the tubing string  130  to catch the dart  1100 . In this manner, the deployment mechanism  1102  may contains sleeves, or cones, to slide toward each other along the longitudinal axis of the dart to force the slips  1120  radially outwardly to engage the tubing string  130  and stop the dart&#39;s travel. Thus, many variations are contemplated, which are within the scope of the appended claims. 
     Other variations are contemplated, which are within the scope of the appended claims. For example,  FIG. 12  depicts a dart  1200  according to a further example implementation. In general, the dart  1200  includes an electromagnetic coupling sensor that is formed from two antennae, or receiver coils  1214  and  1216 , and a transmitter coil  1210  that resides between the receiver coils  1215  and  1216 . As shown in  FIG. 12 , the receiver coils  1214  and  1216  have respective magnetic moments  1215  and  1217 , respectively, which are opposite in direction. It is noted that the moments  1215  and  1217  that are depicted in  FIG. 12  may be reversed, in accordance with further implementations. As also shown in  FIG. 12 , the transmitter  1210  has an associated magnetic moment  1211 , which is pointed upwardly in  FIG. 12 , but may be pointed downwardly, in accordance with further implementations. 
     In general, the electromagnetic coupling sensor of the dart  1200  senses geometric changes in a tubing string  1204  in which the dart  1200  travels. More specifically, in accordance with some implementations, the controller (not shown in  FIG. 12 ) of the dart  1200  algebraically adds, or combines, the signals from the two receiver coils  1214  and  1216 , such that when both receiver coils  1214  and  1216  have the same effective electromagnetic coupling the signals are the same, thereby resulting in a net zero voltage signal. However, when the electromagnetic coupling sensor passes by a geometrically varying feature of the tubing string  1204  (a geometric discontinuity or a geometric dimension change, such as a wall thickness change, for example), the signals provided by the two receiver coils  1214  and  1216  differ. This difference, in turn, produces a non-zero voltage signal, thereby indicating to the controller that a geometric feature change of the tubing string  1204  has been detected. 
     Such geometric variations may be used, in accordance with example implementations, for purposes of detecting certain geometric features of the tubing string  1204 , such as, for example, sleeves or sleeve valves of the tubing string  1204 . Thus, by detecting and possibly counting sleeves (or other tools or features), the dart  1200  may determine its downhole position and actuate its deployment mechanism accordingly. 
     Referring to  FIG. 13  in conjunction with  FIG. 12 , as a more specific example, an example signal is depicted in  FIG. 13  illustrating a signature  1302  of the combined signal (called the “VDIFF” signal in  FIG. 13 ) when the electromagnetic coupling sensor passes in proximity to an illustrated geometric feature  1220 , such as an annular notch for this example. 
     Thus, referring to  FIG. 14 , in accordance with example implementations, a technique  1400  includes deploying (block  1402 ) an untethered object and using (block  1404 ) the object to sense an electromagnetic coupling as the object travels in a passageway of the string. The technique  1400  includes selectively autonomously operating the untethered object in response to the sensing to perform a downhole operation, pursuant to block  1406 . 
     Thus, in general, implementations are disclosed herein for purposes of deploying an untethered object through a passageway of the string in a well and sensing a position indicator as the object is being communicated through the passageway. The untethered object selectively autonomously operates in response to the sensing. As disclosed above, the property may be a physical property such as a magnetic marker, an electromagnetic coupling, a geometric discontinuity, a pressure or a radioactive source. In further implementations, the physical property may be a chemical property or may be an acoustic wave. Moreover, in accordance with some implementations, the physical property may be a conductivity. In yet further implementations, a given position indicator may be formed from an intentionally-placed marker, a response marker, a radioactive source, magnet, microelectromechanical system (MEMS), a pressure, and so forth. The untethered object has the appropriate sensor(s) to detect the position indicator(s), as can be appreciated by the skilled artisan in view of the disclosure contained herein. 
     Other implementations are contemplated and are within the scope of the appended claims. For example, in accordance with further example implementations, the dart may have a container that contains a chemical (a tracer, for example) that is carried into the fractures with the fracturing fluid. In this manner, when the dart is deployed into the well, the chemical is confined to the container. The dart may contain a rupture disc (as an example), or other such device, which is sensitive to the tubing string pressure such that the disc ruptures at fracturing pressures to allow the chemical to leave the container and be transported into the fractures. The use of the chemical in this manner allows the recovery of information during flowback regarding fracture efficiency, fracture locations, and so forth. 
     As another example of a further implementation, the dart may contain a telemetry interface that allows wireless communication with the dart. For example, a tube wave (an acoustic wave, for example) may be used to communicate with the dart from the Earth surface (as an example) for purposes of acquiring information (information about the dart&#39;s status, information acquired by the dart, and so forth) from the dart. The wireless communication may also be used, for example, to initiate an action of the dart, such as, for example, instructing the dart to radially expand, radially contract, acquire information, transmit information to the surface, and so forth. 
     In accordance with example implementations, the dart may contain a balanced coil sensor  1500  that is depicted in  FIG. 15 . The balanced coil sensor  1500  includes a magnetic field generator, or center coil  1504 , which is energized, or driven, by the dart to produce a magnetic field (represented by flux lines  1510 ). In this manner the dart contains a driver that applies a voltage to terminals  1504 -A and  1504 -B of the coil  1504  to produce the magnetic field. This magnetic field, in turn, is influenced by the environment of the dart (the string  130  and its features, for example), and the magnetic field is sensed by receiver antennae, or receiver coils  1506  and  1508 , of the balanced coil sensor  1500  to produce respective signals. In this manner, the receiver coils  1506  and  1508  may be disposed at equal distances (spaced apart at equal distances from the coil  1504  along the longitudinal axis of the dart, for example) such that the coil  1506  provides a signal across its terminals  1506 -A and  1506 -B, and the coil  1508  provides a signal across its terminals  1508 -A and  1508 -B. In accordance with example implementations, the coil  1504 ,  1506  and/or  1508  may be formed from the windings  1076  (see  FIG. 10B ), although, the coil  1504 ,  1506  and/or  1508  may be formed from windings of the dart that are disposed at other locations, in accordance with further, example implementations. The signals that are provided by the receiver coils  1506  and  1508  may differ at any point in time, depending on whether the influence of the surrounding tubing string  130 . In this manner, if the balanced coil sensor  103  is within a uniform section of the tubing string  130  (such as in a straight pipe portion), then the signals are the same. However, the signals differ at a given time when the geometry of the string  130  through which the balanced coil sensor  1500  passes changes, as the magnetic field through each receiver coil  1506  is different. 
     In this manner, referring to  FIG. 16A , for the case in which the sensor is disposed inside a generally uniform tubular section  1623  of the tubing string  130 , the flux lines  1501  are equally distributed; and as such, the coils  1506  and  1508  generally provide the same signals. Thus, the difference of the signals is zero, or small. This is to be contrasted to the case in which the balanced coil sensor  1500  propagates in a tubular member section, which has distributed features, such example section  1624  of  FIG. 16B . For this example, the section  1624  has a thicker wall section  1624 , which, as depicted in  FIG. 16B  causes the flux lines  1510  in the coils  1506  and  1508  to differ, thereby causing the coils  1506  and  1506  to produce different signals. 
       FIG. 17B  depicts signals  1704  and  1708  that are generated by two receiver coils of a balanced coil sensor as a dart (or other untethered object carrying the sensor) propagates through the well.  FIG. 17A  depicts a difference  1710  of the signals  1704  and  1708 . As discussed below, the difference may be used for purposes of identifying specific downhole features as well as determining a speed of the dart. In this manner, at times T 1 , T 2 , T 3 , T 4 , T 5 , T 6  and T 7  in  FIG. 17A , the difference signal  1710  abruptly changes amplitude, thereby indicating a geometry change (i.e., a feature) of the tubing string  130 . As depicted in  FIG. 17B , the changes in the difference signal  1710  are associated with time shifts between the signals  1704  and  1708 , as one receiver coil of the balanced coil sensor passes by the feature of the tubing string  130 , and in a short time thereafter, the other coil of the balanced coil sensor passes by the feature. The time shift between the signals is a function of the speed of the dart. 
     More specifically,  FIGS. 18A and 18B  depict two example signals  1800  and  1804  from the two receiver coils of a balanced coil sensor, in accordance with example implementations. For this example, the coil producing the signal  1804  is located uphole from the coil that produces the signal  1800  by a distance called “Δx” herein. The dart&#39;s speed and the time difference, or time shift (called “Δt”) may be represented as follows: 
                     Δ   ⁢           ⁢   t     =         Δ   ⁢           ⁢   x     speed     .             Eq   .           ⁢   1               
As describe herein the dart&#39;s controller  224  may cross-correlate the receiver coil signals for such purposes as determining the time shift, determining a speed of the dart and identifying downhole features.
 
     In accordance with example implementations, the controller  224  (see  FIG. 2 , for example) may apply a correlation process  1900  is illustrated in  FIG. 19  for example receiver signals  1800  and  1804 . Referring to  FIG. 19 , the correlation process  1900  involves cross-correlating the signal  1800  with candidate time-shifted versions (represented by time-shifted signals  1804 - 1 ,  1804 - 2 ,  1804 - 3 ,  1804 - 4 ,  1804 - 5  and  1804 - 6 , in  FIG. 19 ) of the other signal  1804  for purposes of deriving a correlation curve  1904 . The correlation curve  1904  has a maximum correlation  1906 . The maximum correlation  1906 , in turn, corresponds to the time shift Δt between the receiver coil signals  1800  and  1804 . Moreover, using the relationship of Eq. 1 and knowledge of a distance Δx between given features of the tubing string  130 , the controller  224  may determine the speed of the dart as follows: 
     
       
         
           
             
               
                 
                   speed 
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         t 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         at 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         maximum 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         correlation 
                       
                     
                     . 
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
       FIG. 20  depicts an example downhole section  2000  of the tubing string  130 , which has various geometric features  2004 ,  2006  and  2008  (as examples) which may be detected by a balanced coil sensor of a dart or other untethered object. In this regard,  FIG. 21  depicts two corresponding signals  2102  and  2104  that may be generated by a balanced coil sensor as the object passes through the central passageway of the section  2000 . Using a determined speed of the dart is determined, the controller  224  may use the receiver coil signals to identify specific downhole features. 
     An example process  2200  that may be used by the controller  224  for this purpose is depicted in  FIG. 22 . In  FIG. 22  the section  200  superimposed on the signals  2102  and  2104  to depict amplitude changes in the signals  2102  and  2104  due to features  2204 ,  2204 ,  2208 ,  2210  and  2212  of the section  2000 . As can be seen from  FIG. 22 , the signals respond to a given feature at slightly different times, which is due to one receiver coil passing the feature before the other. The controller  224  may use the signals  2102  and  2104 , either singularly, or through a combination (via a difference signal, for example) to identify these features of the section  2000 . For example, the controller  224  may identify a specific feature of the tubing string (or downhole equipment, in general) by determining the time for the balanced coil sensor to pass from one feature to the next, derive a distance between these features using the already-derived speed of the dart, and then using this distance (or a set of such distances) to identify downhole equipment. For example, the controller  224  may use this technique to identify sleeve valve assemblies so that the controller  224  may count sleeve valve assemblies through which the dart passes for purposes of determine when to expand the dart. 
     Referring to  FIG. 23 , to summarize, a technique  2300  in accordance with example implementations includes acquiring (block  2302 ) measurements using sensors that are disposed at different locations on an untethered object and cross-correlating (block  2304 ) the measurements. At least one downhole feature may then be identified (block  2306 ) based at least in part on the cross-correlation. 
     As a more specific example,  FIGS. 24A and 24B  depict techniques to use a balanced coil sensor according to example implementations. Referring to  FIG. 24A , a technique  2400  for determining the speed of the object includes acquiring (block  2402 ) first and second signals that represent measurements acquired at different axial locations on the untethered object and then proceeding with an iterative process to identify the time shift between the signals. In this manner, the technique  2400  includes applying (block  2404 ) the next time shift to the second measurement and determining (block  2406 ) a cross-correlation of the first signal and the time-shifted second signal. A determination is then made (decision block  2410 ) whether to continue the iterative process. In this regard, in accordance with some implementations, the cross-correlations may be logged and tracked so that the maximum correlation, or peak, may be identified. At this point, the time shift has been identified. When the decision is made (decision block  2410 ) to longer continue the process of finding the maximum correlation, the time shift has been identified, i.e., the time shift corresponds to the maximum correlation. At this point, the speed of the untethered object may be determined based at least in part on the maximum cross-correlation, as depicted in block  2416 . 
       FIG. 24B  depicts a technique  2440  for purposes of using the determined speed of the untethered object, along with signals provided by the balanced coil sensor for purposes of identifying specific downhole features. In this regard, the technique  2440  includes using (block  2442 ) signals representing measurements acquired at different axial locations on an untethered object to identify physical features of the string. One or more distances are then determined (block  2446 ) between the features based on the timing of the measurements and the speed of the untethered object. Specific downhole equipment may then be identified (block  2450 ) based at least in part on these determined distance(s). 
     It is noted that although the balanced coil sensor is described in the examples above, a number of different sensors other than receiver coils of a balanced coil sensor may be used for the above-described cross-correlation measurement processing. Moreover, sensors other than electromagnetic sensors may be used, in accordance with example implementations, such as acoustic and nuclear sensors, to name just a few. The cross-correlation techniques may, in general, provide a real time speed measurement or may be used in an autonomous mode with a downhole tool in general to allow the tool to independently determine its location and identify specific features of equipment downhole. 
     Referring to  FIG. 25A , in accordance with example implementations, a dart  2500  includes mechanically-actuated electrical switches  2602  for purposes of counting features (restrictions, for example) which serve as identification markers in the well.  FIG. 25A  depicts the dart  2500  when landed in an inner sleeve  2532  of a valve assembly  2520 . For this example, the valve assembly  2520  includes a restriction, or seat  2540 , which is engaged by a C-ring of the dart  2500 . At its tail end, the dart  2500  includes multiple mechanically-actuated fingers  2502 , which may be, for example, circumferentially arranged about the longitudinal axis of the dart  2500 . Each finger  2502  for this example is connected at one end to the housing of the dart  2500  and has a free end at its other end for purposes of allowing the finger  2502  to be bent inwardly toward an associated switch  2602  to actuate the switch  2602  (transition the switch  2602  from an electrical open state to an electrical closed state, for example) when the finger  2502  enters a cross-sectional restriction of the tubing string  130 . Referring to  FIG. 25B , the dart  2500  may be shifted, in this example, for purposes of translating the sleeve  2532  of the valve assembly  2520 . 
       FIG. 26A  depicts the fingers  2502  when in proximity to the valve seat  2540 . As depicted in  FIG. 26A , the dart  2500  includes mechanically-actuated switches  2602  that are located in proximity to associated members  2502 . In this regard, as depicted in  FIG. 26A , in accordance with example implementations, each mechanically-actuated switch  2602  may be associated with a corresponding finger  2502 . The switch  2602  extends radially from the body of the dart  2500  so that when the finger  2502  extends inside the restriction  2540 , as depicted in  FIG. 26B , contact is made between the finger  2502  and the switch  2602  to actuate the switch  2602  (close the switch, for example). 
     Thus, as a given dart propagates through the passageway of a tubing string, switches of the dart may be momentarily engaged and released, which allows the dart  2500  to count the number of restrictions through which the dart passes. In accordance with example implementations, the dart  2500  may have a set of multiple circumferentially-arranged switches  2602  (and associated members  2502  so that a given feature is not detected by the dart  2500  until all of the switches of the set have been simultaneously actuated. Moreover, in accordance with some example implementations, the set of switches  2602  may be disposed at predetermined axial lengths along the axis of the dart  2500  so that predetermined features of downhole equipment cause the set of switches to be simultaneously engaged, thereby registering a count. 
     Thus, referring to  FIG. 27 , in accordance with some example implementations, the dart may contain circuitry  2700  for purposes of counting specific downhole features. The circuitry includes at least one set  2704  of switches  2602  (example switches  2602 - 1 ,  2602 - 2 ,  2602 - 3  . . .  2602 -N, being depicted in  FIG. 27 ), which are simultaneously actuated for purposes of forming a current path that is detected by the controller  224  for purposes of registering a count of an identified feature. In this manner, in response to detecting the closed current path, the controller  224  registers the event by incrementing a count (incrementing a count value that is stored by the controller  224 , for example); and the controller  224  may use an actuator (via signal(s) provided on output terminal(s)  2710  of the controller  224 ) of the dart to radially expand the dart in response to the count reaching a predetermined value. 
     In general, proximity switches, such as the described switches  2602 , or the like, may be implemented to count sleeve restrictions as the untethered object is going downhole. Assuming that the dart is be caught by the Nth sleeve valve assembly, after the dart reaches the N−1th sleeve, the controller  224  responds by radially expanding the dart. In accordance with example implementations, there may be multiple proximity switches tuned only to read a specific gap distance. For example, four switches may be used but it should be appreciated that any number of switches may be implemented. In the example, it may take a minimum of three switches to create a count. The fourth switch would, therefore, be a redundant switch in case one fails down hole. The distance may be dialed in to make a count once three switches were within the restriction diameter or where sensing proximity. If only two switches were sensing proximity, a count would not be registered because the other two switches would be too far away from the other walls. In other embodiments, a single proximity sensor may be configured to sense proximity to certain elements in a sleeve, valve or other downhole tool. 
     Referring to  FIG. 28 , to summarize, a technique  2800  in accordance with example implementations includes detecting one or more physical features of downhole equipment using mechanically-actuated switches of an untethered object, pursuant to block  2802 . The technique  2800  includes selectively actuating the untethered object (selectively radially expanding the object, for example) based on the detected feature(s), pursuant to block  2804 . 
     While a limited number of examples have been disclosed herein, 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.