Patent Publication Number: US-10323505-B2

Title: Radioactive tag detection for downhole positioning

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage entry of PCT/US2016/013084 filed Jan. 12, 2016, said application is expressly incorporated herein in its entirety. 
     TECHNICAL FIELD 
     The present technology pertains to positioning of downhole tools, and more particularly to positioning of downhole tools via radioactive tag detection. 
     BACKGROUND 
     After an oil or gas well has been drilled, well operators often carry out various tasks to prepare the well for production of hydrocarbons. These tasks, known as completion operations, typically include inserting and cementing a casing or liner within the wellbore to prevent the walls of the wellbore from caving in. A downhole tool, such as a perforating gun, can then be conveyed downhole via a wireline or a wellbore tubular and positioned adjacent to a formation of interest. Once in position, one or more packers can be set, and explosive charges within the perforating gun can be fired to create holes, or perforations, within the casing, the cement, and the formation. In this manner, fluid communication between the wellbore and the formation can be established. 
     However, accurately positioning the downhole tool at the intended downhole location relies heavily on the ability to correctly determine the downhole position of the tool in relation to the formation of interest. Current solutions measure the length of the wellbore tubular as it conveyed downhole to determine when the downhole tool has reached the known depth of the formation. Unfortunately, these solutions are subject to improper or inaccurate measurements of the length of the wellbore tubular due to inconsistent lengths of tubulars, tubular stretch and compression, well deviations, and the like, resulting in erroneous placement of the downhole tool. Other solutions conduct additional logging runs to collect well logs which can be used to correlate the position of the downhole tool with the depth of the well. However, these logging runs often necessitate the removal of the wellbore tubular to deploy a wireline logging tool within the wellbore. Further, the additional logging runs required by these solutions are very expensive and time consuming, especially in offshore applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features of the disclosure can be obtained, a more particular description of the principles briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only exemplary embodiments of the disclosure and are not therefore to be considered to be limiting of its scope, the principles herein are described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  illustrates a schematic diagram of an example system for positioning of downhole tools; 
         FIG. 2  illustrates a graphical representation of a gamma ray log for a radiation detector; 
         FIG. 3  illustrates a schematic diagram of another example system for positioning of downhole tools; 
         FIG. 4  illustrates graphical representations of gamma ray logs for two radiation detectors; 
         FIG. 5  illustrates an exemplary method embodiment for positioning a downhole tool using a single radiation detector; 
         FIG. 6  illustrates an exemplary method embodiment for positioning a downhole tool using two radiation detectors; and 
         FIGS. 7A and 7B  illustrate schematic diagrams of example computing systems for use with example system embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the disclosure are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the disclosure. 
     Additional features and advantages of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or can be learned by practice of the herein disclosed principles. The features and advantages of the disclosure can be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the disclosure will become more fully apparent from the following description and appended claims, or can be learned by the practice of the principles set forth herein. 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features. The description is not to be considered as limiting the scope of the embodiments described herein. 
     The phrase “wellbore tubular” is defined as one or more types of connected tubulars, and can include, but is not limited to, drill string, tool string, completion string, production string, tubing, production tubing, jointed tubing, coiled tubing, casings, liners, drill pipe, combinations thereof, or the like. The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. Further, the use of directional terms such as above, below, upper, lower, upward, downward, uphole, downhole and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward or uphole direction being toward the surface of the well, the downward or downhole direction being toward the bottom of the well. 
     The approaches set forth herein describe systems and methods quickly determining the position of a wellbore tubular and its components in relation to a radioactive tag disposed in within a wellbore. The system includes one or more radiation detectors disposed on a wellbore tubular and configured to detect a radioactive tag within a wellbore. By detecting the radioactive tag, the wellbore tubular can be correlated to the depth of the tag, and the downhole position of the wellbore tubular components in relation to a formation of interest can be known. This critical position information can be communicated to a surface of the wellbore (e.g., through telemetry), stored for later verification, and/or used to automatically activate downhole tools once they are at the proper location within the wellbore. 
     Disclosed are systems and methods for positioning of downhole tools via radioactive tag detection. The method comprises positioning a radiation detector at a first position within a wellbore, logging radiation data while the radiation detector is moved from the first position to a position adjacent to or past a radioactive marker disposed within the wellbore, determining, based on the radiation data, a time at which the radiation detector is adjacent to the radioactive marker, and calculating, based on the time, a distance between the first position of the radiation detector and the radioactive marker. 
     The present disclosure is described in relation to an offshore well operation  100  depicted schematically in  FIG. 1 . A semi-submersible platform  102  is centered over a submerged formation  104  located below sea floor  106 . A subsea conduit  108  extends from a surface  110  of platform  102  to a wellhead installation  112 , including blowout preventers  114 . Platform  102  has a hoisting apparatus  116  and a derrick  118  for raising and lowering wellbore tubulars, such as wellbore tubular  120 . 
     Continuing with  FIG. 1 , a wellbore  122  extends through the various earth strata including formation  104 . At least a portion of a casing  124  can be cemented within wellbore  122  by cement  126 . Note that, in this specification, the terms “liner” and “casing” are used interchangeably to describe one or more layers of tubular materials which are used to form protective linings in wellbores. Liners and casings may be made from any material such as metals, plastics, composites, or the like, may be expanded or unexpanded as part of an installation procedure, and may be segmented or continuous. Additionally, it is not necessary for a liner or casing to be cemented in a wellbore. Any type of liner or casing may be used in keeping with the principles of the present invention. 
     Wellbore tubular  120  can be raised or lowered within wellbore  122  to conduct various operations on one or more formations of interest, such as formation  104 . Moreover, wellbore tubular  120  can include various wellbore components to support such operations. For example, wellbore tubular  120  can include one or more packers  128 ,  130  to provide zonal isolation for the production of hydrocarbons in certain formation of interest within wellbore  122 . When set, packers  128 ,  130  can isolate zones of the annulus between wellbore tubular  120  and wellbore  122 . In this manner, formation fluids, such as those from formation  104 , may enter the annulus between wellbore tubular  120  and wellbore  122  in between packers  128 ,  130 . 
     Wellbore tubular  120  can also include one or more downhole tools, such as a perforating gun  132 . Wellbore tubular  120  can be moved within wellbore  122  to position perforating gun  132  adjacent to a formation of interest, such as formation  104 . Once in position, a string of shaped charges within perforating gun  132  can be fired to create holes, or perforations, within casing  124 , cement  126 , and/or formation  104 . In this manner, fluid communication between formation  104  and wellbore  122  can be established. 
     As noted above, accurately positioning a downhole tool at the intended downhole location relies heavily on the ability to correctly determine the downhole position of the tool in relation to the formation of interest. Measuring the length of the wellbore tubular as it conveyed downhole often results in an inaccurate determination of the depth of the downhole tool. Further, additional logging runs to correlate the position of the downhole tool with the depth of the well are very expensive and time consuming and often necessitate the removal of the wellbore tubular to deploy a wireline logging tool within the wellbore. Accordingly, the systems and techniques disclosed herein allow for the positioning of a downhole tool using radioactive tag detection. 
     For example,  FIG. 1  illustrates a radiation detector  134  coupled with wellbore tubular  120 . Radiation detector  134  can be a gamma ray detector configured to measure the radioactivity within wellbore  122  and its surrounding formations and devices. For instance, radiation detector  134  can include a scintillator, such as thallium-doped sodium-iodide, coupled with a photomultiplier and one or more processors and/or storage devices. When gamma rays emitted by radioactive materials within wellbore  122  and its surrounding formations/devices enter radiation detector  134 , their energy can be absorbed by the scintillator and re-emitted in the form of light. The light can be detected by the photomultiplier, which can convert the light energy into an electric pulse. By measuring the number of electric pulses per unit time, referred to herein as “counts”, radiation detector  134  can determine the intensity of the radiation at a given depth within wellbore  122 . 
     Radiation detector  134  can be coupled with a downhole tools unit  136  disposed on wellbore tubular  120 . Downhole tools unit  136  can be located at a downhole location within wellbore  122  and can include one or more processors and storage devices to process and/or store data received from radiation detector  134 , as well as to send instructions to radiation detector  134 . Further, downhole tools unit  136  can include a downhole telemetry tool configured to communicate with a surface telemetry tool included within a surface tools unit  138  located at a surface location of wellbore  122  (e.g., surface  110 ). Communication between tools units  136 ,  138  can include any technique known in the art, such as by acoustic telemetry, optical telemetry, electromagnetic telemetry, pulse telemetry, electrical lines, and the like. In this manner, tools units  136 ,  138  can enable bidirectional communication between surface  110  of platform  102  and downhole devices located within wellbore  122  (e.g., packers  128 ,  130 , perforation gun  132 , radiation detector  134 , downhole tools unit  136 , etc.). Moreover, tools units  136 ,  138  can enable radiation detector  134  to send measured radiation data to surface  110  of platform  102  in real-time for processing, storage, and/or analysis by one or more processors and storage devices included within surface tools unit  138  and/or disposed at a remote location. 
     In operation, wellbore tubular  120  can be raised or lowered to dispose a downhole tool, such as perforating gun  132 , at an initial position within wellbore  122 . The initial position of perforating gun  132  can be broken into two components: a known distance  142  between perforating gun  132  and radiation detector  134 , and a measured distance  144  between radiation detector  134  and surface  110 . Known distance  142  can be determined, for example, by measuring the fixed length between perforating gun  132  and radiation detector  134  prior to lowering wellbore tubular  120  below surface  110 . Measured distance  144  can be determined, for example, by measuring the length of wellbore tubular  120  between radiation detector  134  and surface  110  as wellbore tubular  120  is deployed downhole. However, as previously discussed, measuring the length of wellbore tubular  120  as it is deployed below surface  110  is not a reliable indicator of the actual downhole position of wellbore tubular  120  and its associated components due to factors such as inconsistent lengths of tubulars, tubular stretch and compression, well deviations, and the like. 
     Accordingly, a radioactive tag  140  can be placed within casing  124 , cement  126 , and/or the formation surrounding wellbore  122 . Radioactive tag  140  can be a radioactive pip tag, although those skilled in the art will appreciate that any radioactive marker capable of providing a detectable radioactive signature can be used. A distance between radioactive tag  140  and a formation of interest can be known and verified, for example, through one or more previously conducted well logging runs. For instance,  FIG. 1  illustrates a known distance  146  between radioactive tag  140  and formation  104 . Thus, by correlating the depth of wellbore tubular  120  with the depth of radioactive tag  140 , an accurate relationship between formation  104  and the downhole position of wellbore tubular  120  can be established. 
     Such a relationship can be established, for example, by correlating the depth of radiation detector  134  with the depth of radioactive tag  140 . To do so, radiation detector  134  can be disposed at an initial position within wellbore  122  having an unknown distance above or below radioactive tag  140 . For example,  FIG. 1  depicts radiation detector  134  located at an unknown distance  148  below radioactive tag  140  when disposed at an initial position at measured distance  144  below surface  110 . 
     From the initial position, radiation detector  134  can be activated to collect logs, such as gamma ray logs, of the radiation intensity within wellbore  122  and its surrounding formations beginning at an initial time, T 0 . In some cases, radiation detector  134  can receive an activation signal from downhole tools unit  136 , surface tools unit  138 , and/or stored instructions which enables radiation detector  134  to begin measuring the radiation intensity. From here, the measured data can be processed and/or stored within radiation detector  134 , and/or sent from radiation detector  134  to downhole tools unit  136  and/or surface tools unit  138  for processing, storage, and/or analysis. In other cases, radiation detector  134  can continuously measure the radiation intensity within wellbore  122  and its surrounding formations, and an indication of the initial time T 0  can be made on the logs of the measured data. 
     Once radiation detector  134  is activated, wellbore tubular  120  can be raised or lowered to move radiation detector  134  from its initial position to a second position that is adjacent to or past radioactive tag  140 . The second position can be a predetermined distance from the initial position (e.g., wellbore tubular  120  can be raised/lowered a predetermined distance, raised/lowered for a predetermined time, etc.), or the second position can be dynamically determined based on real-time analysis of the radiation intensity logs as wellbore tubular  120  is moved within wellbore  122 . 
     As radiation detector  134  is moved to the second position adjacent to or past radioactive tag  140 , the gamma ray counts measured by radiation detector  134  can be recorded as a function of time. For example,  FIG. 2  illustrates an exemplary gamma ray log  200  produced from the measurements taken by radiation detector  134  as wellbore tubular  120  is moved from its initial position to a second position where radiation detector  134  is past radioactive tag  140 . Such a log can be produced, for example, by one or more processors, storage units, and/or software modules within tools units  136 ,  138  and/or radiation detector  134 . As illustrated, gamma ray log  200  begins at an initial time T 0  ( 202 ) which can correspond to the time at which radiation detector  134  is at its initial position (e.g., an initial position at measured distance  144  below surface  110 ). The counts measured by radiation detector  134  can change over time as wellbore tubular  120  is moved within wellbore  122 , with a relative maximum in the measured counts occurring at a time T tag  ( 204 ). By comparing the measured counts to a threshold value or otherwise analyzing gamma ray log  200 , it can be determined that time T tag  ( 204 ) corresponds to the time at which radiation detector  134  is adjacent to radioactive tag  140 . 
     Using the information from gamma ray log  200 , unknown distance  148  between radioactive tag  140  and the initial position of radiation detector  134  can be calculated. As a non-limiting example, unknown distance  148  can be calculated using equation (1) below, where D is the unknown distance (e.g., unknown distance  148 ), T tag  is the time at which radiation detector  134  is adjacent to radioactive tag  140  (e.g., T tag    204 ), T 0  is the time at which radiation detector  134  is at its initial position (e.g., T 0    202 ), and V is the velocity of radiation detector  134  between times T 0  and T tag .
 
 D =( T   tag   −T   0 )* V   (1)
 
     In some cases, the velocity V can be a predetermined constant velocity. For example, wellbore tubular  120  can be raised or lowered at a constant velocity between times T 0  and T tag , and velocity V can be assumed to be equivalent to the constant velocity of wellbore tubular  120 . In other cases, the velocity V between times T 0  and T tag  can be measured or calculated. Moreover, in some cases, one or more accelerometers can be included within radiation detector  134 , within downhole tools unit  136 , and/or elsewhere on wellbore tubular  120  and can measure the acceleration of wellbore tubular  120  between times T 0  and T tag . The measured acceleration data, as well as other known data, can then be used to calculate, qualify, and/or modify the velocity V between times T 0  and T tag . 
     Once unknown distance  148  between radioactive tag  140  and the initial position of radiation detector  134  is calculated, the depth of radiation detector  134  can be correlated with the depth of radioactive tag  140 . In this manner, an accurate relationship between formation  104  and the downhole position of wellbore tubular  120  and its associated components can be established. For example, the calculated distance  148  can be used to offset measured distance  144  so that measured distance  144  corresponds to the depth of the radioactive tag  140 . From here, one or more downhole tools and/or other components disposed on wellbore tubular  120  can be positioned downhole using the known relationship between radioactive tag  140  and formation  104  (e.g., known distance  146 ) and one or more known distances between radiation detector  134  and the other components of wellbore tubular  120  (e.g., known distance  142 ). For example, once correlated, wellbore tubular  120  can be raised or lowered to position perforating gun  132  adjacent to formation  104  using known distances  142  and  146 . Further, wellbore tubular  120  can be raised or lowered to position packers  128 ,  130  above and/or below formation  104  using known distance  146  and known distances between radiation detector  134  and packers  128 ,  130 . Once in position, packers  128 ,  130  can be set (automatically or manually), and perforating gun  132  can be fired (automatically or manually) to perforate casing  124 , cement  126 , and/or formation  104 . 
       FIG. 3  illustrates a schematic diagram of an offshore well operation  300 . Well operation  300  is substantially similar to well operation  100  and therefore, to avoid repetition, only the differences between the two will be described. As illustrated, well operation  300  includes a first radiation detector  302  and a second radiation detector  304 . Radiation detectors  302 ,  304  are separated by a known distance and can each be a gamma ray detector configured to measure the radioactivity within wellbore  122  and its surrounding formations and devices. Each of radiation detectors  302 ,  304  can include one or more processors and storage devices to process and store measured data, execute instructions, and the like. Further, each of radiation detectors  302 ,  304  can be coupled with a downhole tools unit  136  configured for bidirectional communication with a surface tools unit  138 . In this manner, each of radiation detectors  302 ,  304  can receive instructions from a surface  110  and can send measured data to surface  110  in real-time. 
     As previously discussed, wellbore tubular  120  can be raised or lowered to dispose a downhole tool, such as perforating gun  132 , at an initial position within wellbore  122 . The initial position of perforating gun  132  can be broken into two components, including a known distance  306  between perforating gun  132  and radiation detector  302 , and a measured distance  308  between radiation detector  302  and surface  110 . Further, a distance  310  between radioactive tag  140  and a formation  104  can be known and verified, for example, through one or more previously conducted well logging runs. 
     To establish an accurate relationship between formation  104  and the downhole position of wellbore tubular  120  and its associated components, one or more of radiation detectors  302 ,  304  can be correlated with radioactive tag  140 . To do so, radiation detectors  302 ,  304  can be disposed at respective initial positions within wellbore  122  having unknown distances above or below radioactive tag  140 . For example,  FIG. 3  depicts radiation detector  302  located at an unknown distance  312  below radioactive tag  140  when disposed at an initial position at measured distance  308  below surface  110 . 
     From the initial positions, radiation detectors  302 ,  304  can be activated to collect logs, such as gamma ray logs, of the radiation intensity within wellbore  122  and its surrounding formations beginning at an initial time, T 0 . Once radiation detectors  302 ,  304  are activated, wellbore tubular  120  can be raised or lowered to move radiation detectors  302 ,  304  from their initial positions to secondary positions. In cases where the initial positions of radiation detectors  302 ,  304  are both above radioactive tag  140 , radiation detector  304  can be moved to a secondary position below radioactive tag  140 , and radiation detector  302  can be moved to a secondary position adjacent to or below radioactive tag  140 . In cases where the initial positions of radiation detectors  302 ,  304  are both below radioactive tag  140 , radiation detector  302  can be moved to a secondary position above radioactive tag  140 , and radiation detector  304  can be moved to a secondary position adjacent to or above radioactive tag  140 . 
     As radiation detectors  302 ,  304  are moved to their respective secondary positions, the gamma ray counts measured by each of radiation detectors  302 ,  304  can be recorded as a function of time. For example,  FIG. 4  illustrates exemplary gamma ray logs  400 ,  402  produced from the measurements taken by radiation detectors  302 ,  304 , respectively, as wellbore tubular  120  is moved from its initial position to a second position. Gamma ray logs  400 ,  402  can begin at an initial time T 0  ( 404 ) which can correspond to the time at which radiation detectors  302 ,  304  are at their initial positions. The counts measured by radiation detectors  302 ,  304  can change over time as wellbore tubular  120  is moved within wellbore  122 , with a relative maximum in the measured counts occurring at a time T tag1  ( 406 ) for radiation detector  302  and at time T tag2  ( 408 ) for radiation detector  304 . By comparing the measured counts to a threshold value or otherwise analyzing gamma ray logs  400 ,  402 , it can be determined that time T tag1  ( 406 ) corresponds to the time at which radiation detector  302  is adjacent to radioactive tag  140 , and time T tag2  ( 408 ) corresponds to the time at which radiation detector  304  is adjacent to radioactive tag  140 . 
     Using the information from gamma ray logs  400 ,  402 , a velocity of wellbore tubular  120  as it is moved from its initial position to its second position can be calculated. In particular, the velocity of wellbore tubular  120  between times T tag1  ( 406 ) and T tag2  ( 408 ) can be calculated. As a non-limiting example, the velocity can be calculated using equation (2) below, where V calculated  is the velocity of wellbore tubular  120  between times T tag1  and T tag2 , D rd  is the known distance between radiation detectors  302 ,  304 , T tag1  is the time at which radiation detector  302  is adjacent to radioactive tag  140  (e.g., T tag1    406 ), and T tag2  is the time at which radiation detector  304  is adjacent to radioactive tag  140  (e.g., T tag2    408 ). 
     
       
         
           
             
               
                 
                   
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     Using the calculated velocity V calculated , the unknown distance  312  between radioactive tag  140  and the initial position of radiation detector  302  can be calculated. As a non-limiting example, unknown distance  312  can be calculated using equation (3) below, where D is the unknown distance (e.g., unknown distance  312 ), T tag1  is the time at which radiation detector  302  is adjacent to radioactive tag  140  (e.g., T tag1    406 ), T tag2  is the time at which radiation detector  304  is adjacent to radioactive tag  140  (e.g., T tag2    408 ), T 0  is the time at which radiation detectors  302 ,  304  are at their initial positions (e.g., T 0    404 ), and V calculated  is the velocity of wellbore tubular  120  between times T tag1  and T tag2 .
 
 D =( T   tag2   −T   0 )* V   calculated =( T   tag1   −T   0 )* V   calculated   −D   rd   (3)
 
     In some cases, one or more accelerometers can be included within radiation detectors  302 ,  304 , within downhole tools unit  136 , and/or elsewhere on wellbore tubular  120  and can measure the acceleration of wellbore tubular  120  as it is moved within wellbore  122 . The measured acceleration data, as well as other known data, can then be used to qualify and/or modify the velocity V calculated  and can enhance the calculation by extrapolating velocities that are not constant. 
     Once unknown distance  312  between radioactive tag  140  and the initial position of radiation detector  302  is calculated, the depth of radiation detector  302  and/or radiation detector  304  can be correlated with the depth of radioactive tag  140 . In this manner, an accurate relationship between formation  104  and the downhole position of wellbore tubular  120  and its associated components can be established. For example, the calculated distance  312  can be used to offset measured distance  308  so that measured distance  308  corresponds to the depth of the radioactive tag  140 . From here, one or more downhole tools and/or other components disposed on wellbore tubular  120  can be positioned downhole using the known relationship between radioactive tag  140  and formation  104  (e.g., known distance  310 ) and one or more known distances between radiation detectors  302 ,  304  and the other components of wellbore tubular  120  (e.g., known distance  306 ). For example, once correlated, wellbore tubular  120  can be raised or lowered to position perforating gun  132  adjacent to formation  104  using known distances  306  and  310 . Further, wellbore tubular  120  can be raised or lowered to position packers  128 ,  130  above and/or below formation  104  using known distance  310  and known distances between radiation detectors  302 ,  304  and packers  128 ,  130 . Once in position, packers  128 ,  130  can be set (automatically or manually), and perforating gun  132  can be fired (automatically or manually) to perforate casing  124 , cement  126 , and/or formation  104 . 
     Although  FIG. 1  depicts an offshore operation, it should be understood by those skilled in the art that the present disclosure is equally well suited for use in onshore operations. Further, even though  FIG. 1  depicts a specific wellbore configuration, it should be understood by those skilled in the art that the present disclosure is equally well suited for use in wellbores having other orientations including vertical wellbores, slanted wellbores, multilateral wellbores and the like. 
     Moreover, it should be noted that the configurations and distances described in relation to the figures are for purposes of explanation and are not intended to limit the scope of the disclosure. For example, the downhole tools unit (e.g., downhole tools unit  136 ) can be separate from radiation detector (e.g., radiation detector  134 ) as illustrated, or can be included within the radiation detector to for a single unit. Further, the initial position of radiation detector(s) can be above the radioactive tag and is not limited to being below the radioactive tag as illustrated. In addition, well operations  100 ,  300  are not limited to perforation operations as described, but can also include other operations such as inspection, evaluation, analysis, collection, stimulation, perforation, and the like. To support such operations, wellbore tubular can include any number of components, and can include different components than those depicted in the figures. Moreover, the processes described herein can be executed manually (e.g., by user interaction), automatically (e.g., by one or more processors and storage devices controlling the well operations), or a combination of both. 
     Having disclosed some basic system components and concepts, the disclosure now turns to the example method embodiments shown in  FIGS. 5-6 . The steps outlined herein can be implemented in any combination thereof, including combinations that exclude, add, or modify certain steps. 
       FIG. 5  illustrates an example process for positioning a downhole tool using a single radiation detector. At step  500 , a radiation detector, such as radiation detector  134 , can be positioned at a first position within a wellbore. The radiation detector can be coupled with a wellbore tubular, and positioning the radiation detector can include raising or lowering the wellbore tubular within the wellbore. The radiation detector can be activated at the first position to begin measuring the radiation intensity data within the wellbore. From here, the radiation detector can be moved from the first position to a position adjacent to or past a radioactive tag, such as radioactive tag  140 , disposed within the wellbore (step  510 ). The radiation data collected by radiation detector can be used to produce a log, such as a gamma ray log. 
     At step  520 , a time at which the radiation detector is at a position adjacent to the radioactive tag can be determined. Such a determination can be made through analysis of the log created in step  510  by one or more processors and storage devices or by a user. Next, at step  530 , a distance between the first position of the radiation detector and the radioactive tag can be calculated based on the time determined in step  520 . The calculation of the distance can also include other information, such as a constant or measured velocity of the radiation detector as it is moved within the wellbore, acceleration data, and the like. Once calculated, the distance can be used to correlate the wellbore tubular and its components with the radioactive tag. By correlating the downhole position, a position of a downhole tool, such as perforating gun  132 , can be adjusted so that it is properly located in relation to a formation of interest, such as formation  104  (step  540 ). 
       FIG. 6  illustrates an example process for positioning a downhole tool using a single radiation detector. At step  600 , a first radiation detector, such as radiation detector  302 , and a second radiation detector, such as radiation detector  304 , can be positioned at respective initial positions within a wellbore. The radiation detectors can be coupled with a wellbore tubular, and positioning the radiation detectors can include raising or lowering the wellbore tubular within the wellbore. The radiation detectors can be located at a fixed distance from one another on the wellbore tubular. The radiation detectors can be activated at the initial positions to begin measuring the radiation intensity data within the wellbore. From here, the radiation detectors can be moved from the initial positions to secondary positions past a radioactive tag, such as radioactive tag  140 , disposed within the wellbore (step  610 ). The radiation data collected by radiation detectors can be used to produce one or more logs, such as a gamma ray logs. 
     At step  620 , a first time at which the first radiation detector is at a position adjacent to the radioactive tag and a second time at which the second radiation detector is at a position adjacent to the radioactive tag can be determined. Such a determination can be made through analysis of the log(s) created in step  610  by one or more processors and storage devices or by a user. Next, at step  630 , a velocity of the radiation detectors can be calculated between the first and second times based on the fixed distance between the radiation detectors and the first and second times. Subsequently, at step  640 , a distance between the initial position of at least one of the radiation detectors and the radioactive tag can be calculated based on the times determined in step  620  and the velocity determined in step  630 . The calculation of the distance can also include other information, such as a constant or measured velocity of the radiation detector as it is moved within the wellbore, acceleration data, and the like. Once calculated, the distance can be used to correlate the wellbore tubular and its components with the radioactive tag. By correlating the downhole position, a position of a downhole tool, such as perforating gun  132 , can be adjusted so that it is properly located in relation to a formation of interest, such as formation  104  (step  640 ). 
       FIG. 7A  and  FIG. 7B  illustrate example computing systems for use with example system embodiments. The more appropriate embodiment will be apparent to those of ordinary skill in the art when practicing the present technology. Persons of ordinary skill in the art will also readily appreciate that other system embodiments are possible. 
       FIG. 7A  illustrates a conventional system bus computing system architecture  700  wherein the components of the system are in electrical communication with each other using a bus  705 . System  700  can include a processing unit (CPU or processor)  710  and a system bus  705  that couples various system components including the system memory  715 , such as read only memory (ROM)  720  and random access memory (RAM)  725 , to the processor  710 . The system  700  can include a cache of high-speed memory connected directly with, in close proximity to, or integrated as part of the processor  710 . The system  700  can copy data from the memory  715  and/or the storage device  730  to the cache  712  for quick access by the processor  710 . In this way, the cache can provide a performance boost that avoids processor  710  delays while waiting for data. These and other modules can control or be configured to control the processor  710  to perform various actions. Other system memory  715  may be available for use as well. The memory  715  can include multiple different types of memory with different performance characteristics. The processor  710  can include any general purpose processor and a hardware module or software module, such as module  1   732 , module  2   734 , and module  3   736  stored in storage device  730 , configured to control the processor  710  as well as a special-purpose processor where software instructions are incorporated into the actual processor design. The processor  710  may essentially be a completely self-contained computing system, containing multiple cores or processors, a bus, memory controller, cache, etc. A multi-core processor may be symmetric or asymmetric. 
     To enable user interaction with the computing device  700 , an input device  745  can represent any number of input mechanisms, such as a microphone for speech, a touch-sensitive screen for gesture or graphical input, keyboard, mouse, motion input, speech and so forth. An output device  742  can also be one or more of a number of output mechanisms known to those of skill in the art. In some instances, multimodal systems can enable a user to provide multiple types of input to communicate with the computing device  700 . The communications interface  740  can generally govern and manage the user input and system output. There is no restriction on operating on any particular hardware arrangement and therefore the basic features here may easily be substituted for improved hardware or firmware arrangements as they are developed. 
     Storage device  730  is a non-volatile memory and can be a hard disk or other types of computer readable media which can store data that are accessible by a computer, such as magnetic cassettes, flash memory cards, solid state memory devices, digital versatile disks, cartridges, random access memories (RAMs)  725 , read only memory (ROM)  720 , and hybrids thereof. 
     The storage device  730  can include software modules  732 ,  734 ,  736  for controlling the processor  710 . Other hardware or software modules are contemplated. The storage device  730  can be connected to the system bus  705 . In one aspect, a hardware module that performs a particular function can include the software component stored in a computer-readable medium in connection with the necessary hardware components, such as the processor  710 , bus  705 , output device  742 , and so forth, to carry out the function. 
       FIG. 7B  illustrates an example computer system  750  having a chipset architecture that can be used in executing the described method and generating and displaying a graphical user interface (GUI). Computer system  750  can be computer hardware, software, and firmware that can be used to implement the disclosed technology. System  750  can include a processor  755 , representative of any number of physically and/or logically distinct resources capable of executing software, firmware, and hardware configured to perform identified computations. Processor  755  can communicate with a chipset  760  that can control input to and output from processor  755 . Chipset  760  can output information to output device  765 , such as a display, and can read and write information to storage device  770 , which can include magnetic media, and solid state media. Chipset  760  can also read data from and write data to RAM  775 . A bridge  780  for interfacing with a variety of user interface components  785  can be provided for interfacing with chipset  760 . Such user interface components  785  can include a keyboard, a microphone, touch detection and processing circuitry, a pointing device, such as a mouse, and so on. In general, inputs to system  750  can come from any of a variety of sources, machine generated and/or human generated. 
     Chipset  760  can also interface with one or more communication interfaces  790  that can have different physical interfaces. Such communication interfaces can include interfaces for wired and wireless local area networks, for broadband wireless networks, as well as personal area networks. Some applications of the methods for generating, displaying, and using the GUI disclosed herein can include receiving ordered datasets over the physical interface or be generated by the machine itself by processor  755  analyzing data stored in storage  770  or  775 . Further, the machine can receive inputs from a user via user interface components  785  and execute appropriate functions, such as browsing functions by interpreting these inputs using processor  755 . 
     It can be appreciated that systems  700  and  750  can have more than one processor  710  or be part of a group or cluster of computing devices networked together to provide greater processing capability. 
     Methods according to the aforementioned description can be implemented using computer-executable instructions that are stored or otherwise available from computer readable media. Such instructions can comprise instructions and data which cause or otherwise configure a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The computer executable instructions may be binaries, intermediate format instructions such as assembly language, firmware, or source code. Computer-readable media that may be used to store instructions, information used, and/or information created during methods according to the aforementioned description include magnetic or optical disks, flash memory, USB devices provided with non-volatile memory, networked storage devices, and so on. 
     For clarity of explanation, in some instances the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software. 
     The computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se. 
     Devices implementing methods according to these disclosures can comprise hardware, firmware and/or software, and can take any of a variety of form factors. Such form factors can include laptops, smart phones, small form factor personal computers, personal digital assistants, rackmount devices, standalone devices, and so on. Functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device. 
     The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures. 
     Although a variety of information was used to explain aspects within the scope of the appended claims, no limitation of the claims should be implied based on particular features or arrangements, as one of ordinary skill would be able to derive a wide variety of implementations. Further and although some subject matter may have been described in language specific to structural features and/or method steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to these described features or acts. Such functionality can be distributed differently or performed in components other than those identified herein. Rather, the described features and steps are disclosed as possible components of systems and methods within the scope of the appended claims. Moreover, claim language reciting “at least one of” a set indicates that one member of the set or multiple members of the set satisfy the claim. 
     STATEMENTS OF THE DISCLOSURE INCLUDE 
     Statement 1: A method, comprising positioning a radiation detector at a first position within a wellbore, logging, via the radiation detector, radiation data while the radiation detector is moved from the first position to a second position adjacent to or past a radioactive marker disposed within the wellbore, determining, based on the radiation data, a time at which the radiation detector is adjacent to the radioactive marker as it is moved from the first to the second position, and calculating, based on the time, a distance between the first position of the radiation detector and the radioactive marker. 
     Statement 2: The method according to Statement 1, wherein calculating comprises determining a velocity of the radiation detector as it is moved within the wellbore, and multiplying the time by the velocity to calculate the distance between the first position of the radiation detector and the radioactive marker. 
     Statement 3: The method according to Statement 1 or 2, wherein the velocity is a predetermined constant velocity. 
     Statement 4: The method according to any of Statements 1-3, further comprising measuring, by an accelerometer, an acceleration of the radiation detector as it is moved within the wellbore, and modifying the velocity based on the acceleration of the radiation detector. 
     Statement 5: The method according to any of Statements 1-4, wherein the radiation detector is coupled with a wellbore tubular, and moving the radiation detector comprises raising or lowering the wellbore tubular within the wellbore. 
     Statement 6: The method according to any of Statements 1-5, further comprising positioning a downhole tool coupled with the wellbore tubular within the wellbore, and adjusting the position of the downhole tool based on the distance between the first position of the radiation detector and the radioactive tag. 
     Statement 7: The method according to any of Statements 1-6, wherein the radiation detector is a gamma ray detector and the radioactive marker is a radioactive pip tag. 
     Statement 8: The method according to any of Statements 1-7, wherein the radiation detector is coupled with a downhole telemetry unit that sends the radiation data to a surface telemetry unit in real-time. 
     Statement 9: A method, comprising positioning a first radiation detector and a second radiation detector at respective initial positions within a wellbore, wherein the first and second radiation detectors are separated by a fixed distance, logging radiation data while the radiation detectors are moved past a radioactive marker disposed within the wellbore, determining, based on the radiation data, a first time at which the first radiation detector is adjacent to the radioactive marker and a second time at which the second radiation detector is adjacent to the radioactive marker, calculating, based on the first and second times and the fixed distance, a velocity of the radiation detectors between the first time and the second time, and calculating, based on the first and second times and the velocity, a distance between the initial position of at least one of the first and second radiation detectors and the radioactive marker. 
     Statement 10: The method according to Statement 9, wherein the first and second radiation detectors are coupled with a wellbore tubular, and moving the radiation detectors comprises lowering or raising the wellbore tubular within the wellbore. 
     Statement 11: The method according to Statement 9 or 10, further comprising positioning a downhole tool coupled with the wellbore tubular within the wellbore, and adjusting the position of the downhole tool based on the distance between the initial position of at least one of the first and second radiation detectors detector and the radioactive tag. 
     Statement 12: The method according to any of Statements 9-11, wherein the radiation detectors are gamma ray detectors and the radioactive marker is a radioactive pip tag. 
     Statement 13: The method according to any of Statements 9-12, wherein the radiation detectors are coupled with a downhole telemetry unit that sends the radiation data to a surface telemetry unit in real-time. 
     Statement 14: The method according to any of Statements 9-13, further comprising measuring, by an accelerometer, an acceleration of the radiation detectors as they are moved between the first time and the second time, and modifying the velocity based on the acceleration of the radiation detectors. 
     Statement 15: A system, comprising a radioactive marker disposed within a wellbore, a first radiation detector for measuring radiation data within the wellbore, a wellbore tubular coupled with the radiation detector to position the radiation detector at an initial position within the wellbore and to move the radiation detector from the initial position to a position adjacent to or past the radioactive marker, a processor coupled with the radiation detector for receiving the radiation data, and a computer-readable storage medium having stored therein instructions which, when executed by the processor, cause the processor to perform operations comprising determining, based on the radiation data, a first time at which the radiation detector is adjacent to the radioactive marker, and calculating, based on the time, a distance between the initial position of the radiation detector and the radioactive marker. 
     Statement 16: The system according to Statement 15, wherein calculating comprises determining a velocity of the radiation detector as it is moved within the wellbore, and multiplying the time by the velocity to calculate the distance between the initial position of the radiation detector and the radioactive marker. 
     Statement 17: The system according to Statement 15 or 16, further comprising a downhole tool coupled with the wellbore tubular, wherein the wellbore tubular adjusts a position of the downhole tool based on the distance between the initial position of the radiation detector and the radioactive tag. 
     Statement 18: The system according to any of Statements 15-17, further comprising a surface telemetry unit located at a surface of the wellbore, the surface telemetry unit coupled with the processor and the computer-readable storage medium, and a downhole telemetry unit located at a downhole location within the wellbore, the downhole telemetry unit coupled with the radiation detector for sending the radiation data to the surface telemetry unit. 
     Statement 19: The system according to any of Statements 15-18, further comprising a second radiation detector coupled with the wellbore tubular, wherein the wellbore tubular positions the second radiation detector at an initial position a fixed distance from the first radiation detector and moves the second radiation detector past the radioactive tag, and wherein the computer-readable storage medium stores additional instructions which, when executed by the processor, cause the processor to perform operations comprising determining, based on the radiation data, a second time at which the second radiation detector is adjacent to the radioactive marker, calculating, based on the first and second times and the fixed distance, a velocity of the radiation detectors between the first time and the second time, and calculating, based on the first and second times and the velocity, a distance between the initial position of at least one of the first and second radiation detectors and the radioactive marker. 
     Statement 20: The system according to any of Statements 15-19, wherein the radiation detectors are gamma ray detectors and the radioactive marker is a radioactive pip tag.