Patent Publication Number: US-11661804-B1

Title: Coiled tubing injector with reactive chain tension

Description:
TECHNICAL FIELD 
     The present disclosure pertains generally, but not by way of limitation, to coiled tubing injectors for driving continuous tubing into well bores. More particularly, but not by way of limitation, the present disclosure pertains to systems and methods for tensioning drive chains within coiled tubing injectors. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Coiled tubing refers generally to continuous tubing coiled on a reel or spool. Coiled tubing is significantly faster, relative to conventional interconnected rigid piping, to trip into wellbores or trip out of wellbores. Additionally, coiled tubing can function as either temporary or permanent piping in production wells. As such, coiled tubing is commonly used in both drilling and workover operations at well sites. Coiled tubing injectors are devices for inserting coiled tubing into, and withdrawing coiled tubing from, wellbores. When in use, coiled tubing injectors are typically situated directly above a wellbore; and can position the coiled tubing within the wellbore by precisely controlling the rate at which the coiled tubing moves through die coiled tubing injector. For example, many coiled tubing injectors include a pair of counter-rotating drive chains each carrying grippers which continuously engage the coiled tubing within a “grip-zone” (e.g., an area in which the grippers are in contact with the coiled tubing) defined between the drive chains. 
     The drive chains are often rotated by one or more hydraulic motors fluidly powered by an external power unit, such as a hydraulic power pack. While tripping the coiled tubing into a wellbore, the coiled tubing is generally in a “pipe-heavy” state, in that the downward force of gravity acting on the coiled tubing is greater than any upward force acting on the coiled tubing. When the coiled tubing is pipe-heavy, the coiled tubing injector can function to control the rate at which the coiled tubing passes through the grip zone by resisting the downward force of gravity acting on the coiled tubing. As can be appreciated, a portion of each drive chain within the grip zone will be placed under constant tension when the grippers begin supporting the weight of the coiled tubing. While tripping the coiled tubing into a wellbore, the coiled tubing may switch from a pipe-heavy state to a pipe-light state, in that the downward force of gravity acting on the coiled tubing is less than an upward force acting on the coiled tubing caused by pressure within a wellbore. For example, the coiled tubing can switch from a pipe-heavy state to a pipe-light state, such as due to an obstruction located within the wellbore, internal pressure within the wellbore, or other factors. 
     When the coiled tubing becomes pipe-light, such as in response to an obstruction or internal pressure within the wellbore, the coiled tubing injector must function to push the coiled tubing into the wellbore by applying a downward, or “snubbing”, force sufficient to overcome the pressure within the wellbore. However, such an upward force will act on the coiled tubing injector in a direction opposite to the downward movement of the grippers carried by the drive chains. As can be appreciated, this can cause a portion of each drive chain passing through the grip zone to lose tension and rapidly become slack or “bunch up”, such as in an area near the drive sprockets responsible for supporting and turning the drive chains. Chain bunching can cause serious damage to the coiled tubing, the drive chains, the driven sprockets, the grippers, or other components of the coiled tubing injector. Such damage can be costly to repair, and the downtime associated with repair or replacement of the coiled tubing injector can significantly lengthen a well drilling or workover operation. 
     To help avoid the above issues, coiled tubing injectors often include a chain tension system operable to adjustably tension the drive chains. For example, a user can increase tension within the drive chains from a preset level to a predetermined level above the preset level if the coiled tubing switches from a pipe-heavy state to a pipe-light state, such as to prevent the drive chains from becoming vulnerable to chain bunching. Subsequently, if the tubing switches back to a pipe-heavy state, the user can proportionally reduce the tension within the drive chains. However, existing chain tension systems require a user to closely monitor a tubing load on the coiled tubing injector, such as to proactively predict necessary tension within the drive chains. The tubing load can be a positive load (e.g., the downward force applied to the injector by the coiled tubing) or a negative load (e.g., the upward force applied to the injector by the coiled tubing); and can change frequently during continuous operation of the coiled tubing injector. For example, if the coiled tubing encounters an obstruction within a wellbore, the tubing load can rapidly and unexpectedly switch from a positive load to a negative load. 
     As such, existing chain tension systems are susceptible to user error. For example, a user can fail to notice that a negative tubing load has arisen, such as indicating the coiled tubing has switched from a pipe-heavy state to pipe-light state; and will therefore also fail to proportionally increase the tension in the drive chains. As a result, the drive chains are likely to begin bunching. Similarly, a user can fail to notice that a positive tubing load has arisen, such as indicating that the coiled tubing has switched back from a pipe-light state to a pipe-heavy state; and will therefore also fail to proportionally decrease the tension in the drive chains. As a result, the drive chains, the drive sprockets, chain guides, and many other components of the coiled tubing injector are likely to prematurely degrade due to excessive and unnecessary tension present within the drive chains. For example, when the drive chains are tensioned for pipe-light operating conditions, outer-facing portions of the drive sprockets, and outer facing portions of idler sprockets helping to guide or support the drive chains, can be under a significant tension force. 
     SUMMARY 
     In a non-limiting example, a coiled tubing injector can include: two or more drive chains each forming a continuous loop and carrying a plurality of grippers for engaging coiled tubing within a grip zone defined between the two or more drive chains: a drive system including: a first drive sprocket engaged with a first drive chain of the two or more drive chains; a second drive sprocket engaged with a second drive chain of the two or more drive chains; at least one hydraulic motor for turning the first drive sprocket and the second drive sprocket, the at least one hydraulic motor connected to a drive line and a return line forming a drive circuit for fluidly powering the at least one hydraulic motor; and a tension system including: at least one hydraulic cylinder for tensioning the two or more drive chains; and a reactive chain tension circuit for automatically tensioning the two or more drive chains by maintaining a pressure differential between a fluid pressure within the drive line and a fluid pressure within the at least one hydraulic cylinder. 
     In another non-limiting example, a coiled tubing injector can include: two or more drive chains each forming a continuous loop and carrying a plurality of grippers for engaging coiled tubing within a grip zone defined between the two or more drive chains; a drive system including: a first drive sprocket engaged with a first drive chain of the two or more drive chains; a second drive sprocket engaged with a second drive chain of the two or more drive chains; a first hydraulic motor for turning the first drive sprocket; a second hydraulic motor for turning the second drive sprocket, wherein the first hydraulic motor and the second hydraulic motor are connected in parallel to a drive line and a return line forming a drive circuit for fluidly powering the first hydraulic motor and the second hydraulic motor; and a tension system including: one or more first hydraulic cylinders for tensioning the first drive chain, a second hydraulic cylinder for tensioning the second drive chain; and a reactive chain tension circuit for automatically tensioning the first drive chain and the second drive chain by maintaining a pressure differential between a fluid pressure within the drive line and a fluid pressure within the one or more first hydraulic cylinders and the one or more second hydraulic cylinders, wherein the reactive chain tension circuit is connected to a first port of the first hydraulic motor via a pilot line to provide the reactive chain tension circuit with a pressure signal indicative of a tubing load on the coiled tubing injector. 
     In an additional example, a method of using a coiled tubing injector including two or more drive chains carrying a plurality of grippers for engaging coiled tubing within a grip zone defined between a first drive chain and a second drive chain, and a drive system for turning the two or more drive chains and including at least one hydraulic motor connected to a drive line for fluidly powering the at least one hydraulic motor, can include activating the drive system to begin moving the coiled tubing through the grip zone toward a wellbore; and tensioning the two or more drive chains by a reactive chain tension circuit based on a fluid pressure within the at least one hydraulic motor, by maintaining a pressure differential between the fluid pressure within the at least one hydraulic motor and a fluid pressure within at least one hydraulic cylinder for tensioning the first drive chain and the second drive chain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a perspective view of a coiled tubing unit in place on a well pad, according to one or more examples. 
         FIG.  2    illustrates an isometric view of the coiled tubing injector of  FIG.  1   , according to one or examples. 
         FIG.  3    illustrates a side view of the coiled tubing injector of  FIGS.  1 - 2   , according to one or more examples. 
         FIG.  4    illustrates a hydraulic circuit of the coiled tubing injector of  FIGS.  1 - 3    including an example reactive chain tension circuit, according to one or more examples. 
         FIG.  5    illustrates an example graph illustrating fluid pressure with the reactive chain tension circuit of  FIG.  4    as a function of tubing load on the coiled tubing injector of  FIGS.  1 - 3   , according to one or more examples. 
         FIG.  6    illustrates a hydraulic circuit of a coiled tubing injector of the coiled tubing injector of  FIGS.  1 - 3    including an example reactive chain tension circuit, according to one or more examples. 
         FIG.  7    illustrates a method of using a coiled tubing injector, according to one or more examples. 
     
    
    
     In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
     DETAILED DESCRIPTION 
     The coiled tubing injector of the present disclosure can help to address the above issues, among others, such as by providing a tension system capable of automatically tensioning the drive chains of a coiled tubing injector in proportion to a positive tubing load or a negative tubing load acting on the coiled tubing injector. For example, the tension system can include a reactive chain tension circuit configured to maintain a pressure differential between a fluid pressure within one or more hydraulic motors for turning the drive chains and a fluid pressure within one or more hydraulic cylinders for tensioning the drive chains. In such an example, the reactive tension circuit can receive a pressure signal from the one or more hydraulic motors to adjust the fluid pressure within the one or more hydraulic cylinders, such as in response to an increasing negative load on the one or more hydraulic motors and fall in proportion to a decreasing negative load on the one or more hydraulic motors, such as to ensure the drive chains are under constant tension sufficient to prevent chain bunching in pipe-light operation conditions. 
     In view of the above, the reactive chain tension circuit can make operating coiled tubing injectors easier for a user while preventing coiled tubing injectors from becoming susceptible to drive chain bunching. For example, rather than closely monitoring the tubing load and operate a chain tension system, the user can be free to devote attention to other aspects of drilling or workovers operations at a well site. Additionally, the reactive chain tension circuit can improve the reliability and overall service life of coiled tubing injectors. For example, as the reactive chain tension circuit can automatically decrease tension within the drive chains as the tubing load decreases, the reactive chain tension circuit can prevent any unnecessary wear of the drive chains, drive sprockets, chain guides, or other components of coiled tubing injectors. Further, the reactive chain tension circuit can help to reduce the overall cost of various operations at well sites. For example, by preventing a user from tensioning the drive chains above a level necessary to prevent chain bunching, the reactive chain tension circuit can reduce the likelihood of operational downtime caused by repair or replacement of a coiled tubing injector. 
       FIG.  1    illustrates a perspective view of a coiled tubing system  100  in place on a well pad  102 , according to one or more embodiments. As shown in  FIG.  1   , the coiled tubing system  100  can include a tubing spool  104  containing a high linear footage of coiled tubing  106 . The coiled tubing system  100  can include a coiled tubing injector  108 . The coiled tubing injector  108  can be operable to advance the coiled tubing  106  into, or withdraw the coiled tubing  106  from, a wellbore of the well pad  102 . 
     In one example, such as shown in  FIG.  1   , the coiled tubing injector  108  can be suspended above a well bore by a crane  110 , such as to help enable the coiled tubing injector  108  to pull the coiled tubing  106  from the tubing spool  104  and lower the coiled tubing  106  into a wellbore. In some examples, the coiled tubing  106  can be connected to various attachments, such as a drilling assembly  112 . In such an example, the coiled tubing  106  can be connected to a drilling fluid source on the well pad  102  to fluidly power the drilling assembly  112  during a well bore drilling operation. The coiled tubing system  100  can include a hydraulic power unit, such as, but not limited to, a power pack  113 . The power pack  113  can be connected to the coiled tubing injector  108  to fluidly power the coiled tubing injector  108 . 
       FIG.  2    illustrates an isometric view of the coiled tubing injector  108  of  FIG.  1   , according to one or more embodiments.  FIG.  3    illustrates a side view of the coiled tubing injector  108  of  FIGS.  1 - 2   , according to one or more embodiments.  FIGS.  2 - 3    are discussed below concurrently. The coiled tubing injector  108  is an example coiled tubing injector generally representative of various coiled tubing injectors. For example, the coiled tubing injector  108  can include any of the components or features described in U.S. Pat. Nos. 8,544,536, 8,701,754, or U.S. Pat. No. 10,024,123, each of which is hereby incorporated by reference in its entirety. 
     As shown in  FIG.  2   , the coiled tubing injector  108  can include two or more drive chains  114 , such as a first drive chain  116  ( FIG.  2   ) and a second drive chain  118  ( FIG.  2   ). The first drive chain  116  and the second drive chain  118  can each form a continuous loop. The coiled tubing injector  108  can include a drive system  120  ( FIG.  2   ) for turning or otherwise rotating the first drive chain  116  and the second drive chain  118 . For example, the drive system  120  can include a first drive sprocket  122  ( FIG.  2   ), a first idler sprocket  124  (shown in shadow in  FIG.  3   ), a second drive sprocket  126  ( FIG.  2   ), and a second idler sprocket  128  (shown in shadow in  FIG.  3   ). The first drive chain  116  can concurrently engage a first drive sprocket  122  and a first idler sprocket  124 ; and the second drive chain  118  can concurrently engage a second drive sprocket  126  and a second idler sprocket  128 . 
     The first drive sprocket  122  can be connected to a first driveshaft  230  (represented in  FIG.  4   ), the second drive sprocket  126  can be connected to a second driveshaft (represented in  FIG.  4   ), the first idler sprocket  124  can be connected to a first idler shaft  134  ( FIG.  2   ), and the second idler sprocket can be connected to a second idler shaft  136  ( FIG.  2   ). In  FIG.  2   , a top surface  125  of the coiled tubing injector  108  is partially cut away to reveal the first drive sprocket  122  and the second drive sprocket  126 . In some examples, the first driveshaft  230 , the second driveshaft  232 , the first idler shaft  134 , and the second idler shaft  136  can extend parallel to, and laterally offset from, one another to enable the first drive sprocket  122 , the first idler sprocket  124 , the second drive sprocket  126 , and the second idler sprocket  128  to be laterally opposed within a common plane. 
     The drive system  120  can include at least one hydraulic motor. For example, as shown in  FIG.  2   , the drive system  120  can include a first hydraulic motor  140  and a second hydraulic motor  142 . The first hydraulic motor  140  and the second hydraulic motor  142  can be fixed displacement motors. The first hydraulic motor  140  and the second hydraulic motor  142  can turn or otherwise rotate the first drive sprocket  122  and the second drive sprocket  126 , respectively. For example, the first driveshaft  230  and the second driveshaft  232  can be operably coupled to the first hydraulic motor  140  and the second hydraulic motor  142 , respectively, such as through a first gearbox  144  ( FIG.  2   ) and a second gearbox  146  ( FIG.  2   ), respectively. The first hydraulic motor  140  and the second hydraulic motor  142  can be configured to rotate in opposite directions to thereby cause the first drive chain  116  and the second drive chain  118  to counter-rotate relative to each other. The coiled tubing injector  108  can include a first brake  148  ( FIG.  2   ) and a second brake  150  ( FIG.  2   ). The first brake  148  can be operably coupled to the first driveshaft  230  and the second brake  150  can be coupled to the second driveshaft  232 . The first drive chain  116  and the second drive chain  118  can each carry a plurality of grippers  152  ( FIG.  2   ) shaped to conform to and at least partially encircle the coiled tubing  106  ( FIG.  3   ) therebetween. 
     The plurality of grippers  152  can define a grip zone  154  ( FIG.  3   ). The grip zone  154  can be an area between the first drive chain  116  and the second drive chain  118  in which the plurality of grippers  152  are in contact with and at least partially encircle the coiled tubing  106 . Although not visible, the coiled tubing injector  108  can include at least two skates, one for each of the first drive chain  116  and the second drive chain  118 , and a plurality of hydraulic cylinders for pushing the at least two skates toward each other. The at least two skates move the plurality of grippers  152  within the grip zone  154  toward each other under hydraulic pressure provided by the hydraulic cylinders to thereby cause the plurality of grippers  152  to forcibly engage the coiled tubing  106 . Examples of such skates and hydraulic cylinders are shown in U.S. Pat. Nos. 5,309,990 and 5,918,671, which are hereby incorporated by reference in their entirety. 
     The coiled tubing injector  108  can include a tension system  156  ( FIG.  2   ). The tension system  156  can include the first idler sprocket  124 , the second idler sprocket  128 , the first idler shaft  134 , and the second idler shaft  136 . For example, the first idler shaft  134  and the second idler shaft  136  can be moveably connected to a frame  161  ( FIG.  2   ) configured to support various components of the coiled tubing injector  108 , such as including, but not limited to, the first driveshaft  230 , the second driveshaft  232 , the first idler shaft  134 , and the second idler shaft  136 . The tension system  156  can include at least one hydraulic cylinder, such as a one or more first hydraulic cylinders  158  and the one or more second hydraulic cylinders  160 . 
     The one or more first hydraulic cylinders  158  and the one or more second hydraulic cylinders  160  can be arranged within the frame  161  with respect to the first idler shaft  134  and the second idler shaft  136 , respectively, such as to enable the one or more first hydraulic cylinders  158  to adjust the distance between the first driveshaft  230  and the first idler shaft  134 , and the distance between the second driveshaft  232  and the second idler shaft  136 , by translating the first idler shaft  134  and the second idler shaft  136  with respect to the frame  161 . The one or more first hydraulic cylinders  158  and the one or more second hydraulic cylinders  160  are thereby operable to place the first drive chain  116  and the second drive chain  118  under constant tension during rotation of the first drive sprocket  122 , the first idler sprocket  124 , the second drive sprocket  126 , and the second idler sprocket  128 . The tension system  156  can include a reactive chain tension circuit  262  ( FIG.  4   ). 
     The reactive chain tension circuit  262  can be configured to automatically adjust the fluid pressure within the one or more first hydraulic cylinders  158  and the one or more second hydraulic cylinders  160 , and thereby automatically tension the first drive chain  116  and the second drive chain  118 . For example, the reactive chain tension circuit  262  can be in fluid communication with the first hydraulic motor  140  and/or the second hydraulic motor  142  to receive a pressure signal indicative of a positive or a negative tubing load on the coiled tubing injector  108 . That is, the first hydraulic motor  140  and the second hydraulic motor  142  can be the primary motive element driving the coiled tubing  106 , and the amount of power used (in the form of hydraulic pressure) to advance or withdraw the coiled tubing  106  into, or out of, a wellbore is related to the upward or downward force on the coiled tubing  106 . The reactive chain tension circuit  262  can then use the pressure signal to increase or decrease the fluid pressure within the one or more first hydraulic cylinders  158  and the one or more second hydraulic cylinders  160  to maintain a pressure differential between the fluid pressure within the first hydraulic motor  140  and the second hydraulic motor  142  and the fluid pressure within the one or more first hydraulic cylinders  158  and the one or more second hydraulic cylinders  160 . 
     Such a pressure differential can include a first pressure threshold and a second pressure threshold. In one example, the first pressure threshold can be a minimum pressure, based on a tubing load on the coiled tubing injector  108 , selected to prevent bunching of the first drive chain  116  and the second drive chain  118 ; and the second pressure threshold can be a maximum pressure, based on a tubing load on the coiled tubing injector  108 , before which the first drive chain  116  and the second drive chain  118  experience excessive tension. In view of the above, the tension system  256  can automatically tension the first drive chain  116  and the second drive chain  118  based on a tubing load on the coiled tubing injector  108 . 
       FIG.  4    illustrates a hydraulic circuit of the coiled tubing injector of  FIGS.  1 - 3    including an example reactive chain tension circuit  262 , according to one or more embodiments. In  FIG.  4   , the power pack  113 , the first drive sprocket  122 , the second drive sprocket  126 , the first hydraulic motor  140 , the second hydraulic motor  142 , the first brake  148 , the second brake  150 , the one or more first hydraulic cylinders  158 , and the one or more second hydraulic cylinders  160  shown in  FIG.  2    are discussed with regard to reference numbers  213 ,  222 ,  226 ,  248 ,  250 ,  258 , and  260 , respectively. The hydraulic circuit  200  is one example of a simplified hydraulic circuit that can be used with the coiled tubing injector  108  shown in  FIGS.  1 - 3    above. The hydraulic circuit  200  can include a drive circuit  201 , such as formed by a drive line  266  and a return line  268 . The drive line  266  and the return line  268  can be configured for connection to the power pack  213 , such as to supply the hydraulic circuit  200  with pressurized hydraulic fluid at a preset or charge pressure. Fluid pressure within the drive line  266  can change in response to a negative tubing load on the coiled tubing injector  108 , and fluid pressure within the return line  268  can change in response to a positive tubing load on the coiled tubing injector  108 . 
     The first hydraulic motor  240  can be connected to the drive line  266  through branch  266 A, such as via port  267 A of a first counterbalance valve  267 . The second hydraulic motor  242  can be connected to the drive line  266  through branch  266 B, such as via port  269 A of a second counterbalance valve  269 . The first hydraulic motor  240  can be connected to the return line  268  through branch  268 A, such as via port  267 B of the first counterbalance valve  267 . The second hydraulic motor  242  can be connected to the return line  268  through branch  268 B, such as via port  269 B of the second counterbalance valve  269 . The first counterbalance valve  267  and the second counterbalance valve  269  can be configured to regulate the flow of hydraulic fluid through the first hydraulic motor  240  and the second hydraulic motor  242 , respectively. For example, the first counterbalance valve  267  and the second counterbalance valve  269  can be operable to start, stop, or otherwise control the speed of the first hydraulic motor  240  and the second hydraulic motor  242 , respectively. 
     The first hydraulic motor  240  can include a shaft  241  connected to the first brake  248  and the first gearbox  244 , which can in turn rotate the first drive sprocket  222  via an output shaft. The second hydraulic motor  242  can include a shaft  243  connected to the second brake  250  and the second gearbox  246 , which can in turn rotate the second drive sprocket  226 . The tension system  156  ( FIG.  1   ) can include a cylinder drive line  270  and an external fluid source  285 . The cylinder drive line  270  can be connected directly to the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260 . In one example, such as shown in  FIG.  4   , the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  each include two hydraulic cylinders. The cylinder drive line  270  can be connected to the external fluid source  285 . The external fluid source  285  can supply pressurized hydraulic fluid to cylinder drive line  270 . Fluid pressure within the cylinder drive line  270  can dictate the amount of tension within the first drive chain  116  ( FIG.  2   ) and the second drive chain  118  ( FIG.  2   ). Pressurized hydraulic fluid within the cylinder drive line  270  can be discharged to a drain line  271  through a brake manifold assembly  272  for controlling the first brake  248  and the second brake  250 , such as through lines  273 ,  274 ,  275 , and  276 . 
     A user can manually adjust the fluid pressure within the cylinder drive line  270 , such as via one or more user inputs to the external fluid source  285 . For example, a user can increase or decrease the fluid pressure within the cylinder drive line  270  to a limit (e.g., a first pressure threshold or a second pressure threshold) implemented by the reactive chain tension circuit  262 . The reactive chain tension circuit  262  can include a pilot line  277 . The pilot line  277  can be in fluid communication with the drive line  266 . For example, the pilot line  277  can be connected to a first port  240 A of the first hydraulic motor  240 . The first port  240 A can be a port that is in fluid communication with the drive line  266 , at least in that fluid pressure at the first port  240 A can be equal to the fluid pressure within the drive line  266 . In other examples, the pilot line  277  can be connected directly to the drive line  266 , or to a first port  242 A of the second hydraulic motor  242 . 
     The reactive chain tension circuit  262  can include a first valve  279  and a second valve  280 . The first valve  279  and the second valve  280  can be connected to the pilot line  277  in parallel, such as through branch  277 A and branch  277 B, respectively. The pilot line  277  can thereby provide a pressure signal indicative of a tubing load on the coiled tubing injector  108  to the reactive chain tension circuit  262 . The first valve  279  and the second valve  280  can be, for example, but not limited to, pilot-operated pressure control valves. The reactive chain tension circuit  262  can include an intermediary line  281  and a tension line  282 . The first valve  279  can be connected to the second valve  280  through the intermediary line  281 . The tension line  282  can be connected to the cylinder drive line  270 . The intermediary line  281  can be connected to the tension line  282 , and thereby the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260 , through the tension line  282 . 
     The reactive chain tension circuit  262  can include an auxiliary fluid source  283  and an auxiliary line  284 . The auxiliary fluid source  283  can be connected to the first valve  279  through the auxiliary line  284 . In some examples, the auxiliary line  284  can include an adjustable orifice  286 . The adjustable orifice  286  can be operable to control a fluid flow rate between the auxiliary fluid source  283  and the first valve  279 . In one example, the first valve  279  can be configured to maintain the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  above a first pressure threshold. The first pressure threshold can be a relative fluid pressure set at a fixed interval above, or below, the pressure signal within the pilot line  277 , such as selected to meet the unique requirements of an individual coiled tubing injector. In one example, the first pressure threshold can be a pressure about 100 pounds per square inch greater than the pressure signal (e.g., the fluid pressure within the pilot line  277 ) provided by the branch  277 A to the first valve  279 . 
     The first valve  279  can be controlled by the pressure signal provided by the branch  277 A of the pilot line  277 . For example, when the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  is less than the first pressure threshold, the first valve  279  can open to enable fluid from the auxiliary fluid source  283  to flow through the first valve  279  until the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  is equal to the first pressure threshold. Once the fluid pressure within the tension line  282  is equal to the first pressure threshold, the first valve  279  can close to prevent fluid from the auxiliary fluid source  283  from flowing through the first valve  279 , such as until the pressure signal within the pilot line  277  indicates the pressure within the tension line  282  is below the first pressure threshold. In view of the above the first valve  279  can selectively connect the auxiliary fluid source  283  to the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  based on the pressure signal from the pilot line  277 . 
     The reactive chain tension circuit  262  can include a discharge line  287 . The discharge line  287  can be connected to the drain line  271  via the brake manifold assembly  272 , such as through lines  274 ,  275 , and  276 . The second valve  280  can be configured to maintain the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  below a second pressure threshold. The second pressure threshold can be a relative fluid pressure set at a fixed interval above, or below, the pressure signal within the pilot line  277 , such as selected to meet the unique requirements an individual coiled tubing injector. In one non-limiting example, the second pressure threshold can be a pressure about 400 pounds per square inch greater than the pressure signal (e.g., the fluid pressure within the pilot line  277 ) provided by the branch  277 B to the second valve  280 . The second valve  280  can be controlled by the pressure signal provided by the branch  277 B of the pilot line  277 . 
     For example, when the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  is equal to the second pressure threshold, the second valve  280  can open to enable fluid to flow through the second valve  280  into the discharge line  287  until the fluid pressure within the tension line  282  is below the second pressure threshold. Once the fluid pressure within the tension line  282  is below the second pressure threshold, the second valve  280  can close to prevent fluid from flowing through the second valve  280  until the pressure signal within the pilot line  277  is below the second pressure threshold. In view of the above, the second valve  280  can selectively connect the tension line  282  to the discharge line  287  based on the pressure signal from the pilot line  277 . 
     In some examples, the intermediary line  281  can include a directional control valve  288 . The directional control valve  288  can be, but is not limited to, a check valve. The directional control valve  288  can be configured to prevent fluid flow from the tension line  282  to the first valve  279 , such as to help reduce pressure leakage associated with the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260 . For example, the directional control valve  288  can be located on the intermediary line  281  downstream of the first valve  279 , such as between the first valve  279  and a point at which the intermediary line  281  is connected to the tension line  282 . In some examples, the reactive chain tension circuit  262  can include a two-way valve  289 . The two-way valve  289  can be, for example, but is not limited, to a manually operable ball valve. The two-way valve  289  can be opened by a user to operably connect, or disconnect, the reactive chain tension circuit  262  to the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  by enabling or preventing fluid flow through the tension line  282 . 
       FIG.  5    is an example graph  300  illustrating hydraulic fluid pressure with the reactive chain tension circuit  262  of  FIG.  4    as a function of tubing load of the coiled tubing injector of  FIGS.  1 - 3   . Section  302  of the graph  300  can represent a period where the coiled tubing injector  108  is experiencing a positive tubing load, in pounds. A positive tubing load can indicate the coiled tubing  106  is in a pipe-heavy state; and the first hydraulic motor  240  ( FIG.  4   ) and the second hydraulic motor  242  ( FIG.  4   ) are working to resist a downward force of gravity acting on the coiled tubing  106 . Section  304  of the graph  300  can represent a period where the coiled tubing injector  108  is experiencing a negative tubing load, in pounds. A negative tubing load can indicate that the coiled tubing is in a pipe-light state; and the first hydraulic motor  240  and the second hydraulic motor  242  are working to overcome an upward force acting on the coiled tubing  106 , such as from within a wellbore. 
     A line  306  can represent an example minimum pressure within the one or more first hydraulic cylinders  258  ( FIG.  4   ) and the one or more second hydraulic cylinders  260  ( FIG.  4   ) relative to a tubing load, such as necessary or recommended to prevent chain bunching. Traditionally, this minimum pressure is maintained by a user, such as by manually increasing or decreasing the fluid pressure within the one or more first hydraulic cylinders  258  ( FIG.  4   ) and the one or more second hydraulic cylinders  260  using the external fluid source  285  ( FIG.  4   ) A line  308  can represent action, in the form of a pressure curve, of the first valve  279  ( FIG.  4   ) in response to the pressure signal provided by the pilot line  277  ( FIG.  4   ). The first pressure threshold is shown by the line  308  as 100 pounds per square inch above the pressure signal provided by the pilot line  277 . A line  310  can represent action, in the form of a pressure curve, of the second valve  280  ( FIG.  4   ) in response to the pressure signal provided by the pilot line  277 . The second pressure threshold is shown by the line  310  as 400 pounds per square inch above the pressure signal provided by the pilot line  277 . 
     The first valve  279  can maintain the fluid pressure within the reactive chain tension circuit  262 , and thereby the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260 , above the line  306  (e.g., the minimum pressure) and the minimum pressure threshold when the coiled tubing injector  108  is experiencing a positive load. For example, the fluid pressure within the pilot line  277  will not increase above a charge pressure (illustrated in  FIG.  5    as 500 pounds per square inch) provided by the power pack  213  ( FIG.  4   ) of the drive circuit  201 , as only the fluid pressure within the return line  268  will increase when the coiled tubing injector  108  is experiencing a positive tubing load. As such, in section  302 , the first valve  279  will generally be in a closed state to continuously maintain the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  above the line  306 . 
     The first valve  279  can maintain the fluid pressure within the reactive chain tension circuit  262 , and thereby the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260 , above the line  306  (e.g., the minimum pressure) and the minimum pressure threshold when the coiled tubing injector  108  is experiencing a negative load. For example, as the fluid pressure within the pilot line  277  will increase in response to an increasing negative tubing load on the coiled tubing injector  108 , the first valve  279  will open to enable fluid from the auxiliary fluid source  283  ( FIG.  4   ) to flow through the first valve  279  until the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  is equal to the first pressure threshold. As such, in section  304 , the first valve  279  will generally alternate between an open state and a closed state to continuously maintain the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  above the pressure signal provided by the pilot line  277 . 
     The second valve  280  can maintain the fluid pressure within the reactive chain tension circuit  262 , and thereby the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260 , below a second pressure threshold. For example, when the fluid pressure within the pilot line  277  decreases in response to a decreasing negative tubing load on the coiled tubing injector  108 , the second valve  280  can open to enable fluid from the reactive chain tension circuit  262  to flow through the second valve  280  until the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  is less than the second pressure threshold. 
     Additionally, during section  302  or section  304 , a user can operate the external fluid source  285  to manually increase the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  up to the second pressure threshold. Upon the occurrence of such an event, the second valve  280  can open to discharge pressure through the discharge line  287  ( FIG.  4   ) until the pressure signal provided to the second valve  280  by the pilot line  277  falls below the second pressure threshold. Additionally, during section  302  or section  304 , a user can operate the external fluid source  285  to manually decrease the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  to first second pressure threshold. Upon the occurrence of such an event, the first valve  279  can open to enable fluid from the auxiliary fluid source  283  ( FIG.  4   ) to flow through the first valve  279  until the fluid pressure within the one or more first hydraulic cylinders  258  and the one or more second hydraulic cylinders  260  is again equal to the first pressure threshold. 
       FIG.  6    illustrates a hydraulic circuit  400  of the coiled tubing injector  108  of  FIGS.  1 - 3    including an example reactive chain tension circuit  462 , according to one or more examples. In  FIG.  6   , the first hydraulic motor  240 , the first port  240 A, the second hydraulic motor  242 , the port  242 A, the one or more first hydraulic cylinders  258 , the one or more second hydraulic cylinders  260 , the drive line  266 , the drain line  271 , the second valve  280 , the intermediary line  481 , the tension line  482 , the discharge line  487 , the directional control valve  488 , and the two-way valve  489  are discussed with regard to reference numbers  440 ,  440 A,  442 ,  442 A,  458 ,  460 ,  466 ,  471 ,  480 ,  481 ,  482 ,  487 ,  488 , and  489 , respectively. The hydraulic circuit  400  is one example of a simplified hydraulic circuit that can be used with the coiled tubing injector  108  shown in  FIGS.  1 - 3    above. The hydraulic circuit  400  can be similar to the hydraulic circuit  200  discussed with regard to  FIG.  4    above, except in that the reactive chain tension circuit  462  can be realized using a different combination of components. The reactive chain tension circuit  462  can include a pilot line  490 , a flow divider  491 , the second valve  480 , the intermediary line  481 , the tension line  482 , and the discharge line  487 . 
     The pilot line  490  can be in fluid communication with the drive line  466 . For example, the pilot line  490  can be connected to a first port  440 A of the first hydraulic motor  440 . The first port  440 A can be a port that is in fluid communication with the drive line  466 , at least in that fluid pressure at the first port  440 A can be equal to the fluid pressure within the drive line  466 . In other examples, the pilot line  490  can be connected directly to the drive line  466 , or to a first port  442 A of the second hydraulic motor  442 . The flow divider  491  and the second valve  480  can be connected to the pilot line  490  in parallel, such as through branch  490 A and branch  490 B, respectively. The pilot line  490  can thereby provide a pressure signal indicative of a tubing load on the coiled tubing injector  108  to the reactive chain tension circuit  462 . The second valve  480  can be connected to the flow divider  491  through the intermediary line  481 . 
     In some examples, the branch  490 A of the pilot line  490  can include an adjustable orifice  493 . The adjustable orifice  493  can be operable to control a fluid flow rate between the first hydraulic motor  440  and the flow divider  491 . For example, the adjustable orifice  493  can be located on branch  490 A of the pilot line  490 . In some examples, the pilot line  490  can include a pressure reducing valve  494 . The pressure reducing valve  494  can be configured to limit a maximum fluid pressure at the flow divider  491  and the second valve  480 . The flow divider  491  can include a first port  495 , a second port  496 , and a third port  497 . In one example, the flow divider  491  can be configured to maintain the fluid pressure within the one or more first hydraulic cylinders  458  and the one or more second hydraulic cylinders  460  above a first pressure threshold. For example, the flow divider  491  can be configured as a pressure intensifier. In such an example, the first port  495  can be connected to the branch  490 A of the pilot line  490 , the second port  496  can be connected to the drain line  471  through a flow line  498 , and the third port  497  can be connected to the second valve  480  via the intermediary line  481 . 
     In operation, fluid pressure at the third port  497 , and the thereby the tension line  482 , the one or more first hydraulic cylinders  458 , and the one or more second hydraulic cylinders  460 , will be proportionally greater than the fluid pressure at the first port  495  and the pilot line  490  by the amount of fluid pressure that the second port  496  and the flow line  498  direct to the drain line  471 . For example, if the flow divider  491  is has a two-to-one ratio between the third port  497  and the second port  496 , the second port  496  can divert one-third of the fluid pressure at the first port  495 , and therefore the fluid pressure within the one or more first hydraulic cylinders  458  and the one or more second hydraulic cylinders  460  will be one-third greater than the fluid pressure within the pilot line  490 . In this way, the flow divider  491  can maintain the fluid pressure within the one or more first hydraulic cylinders  458  and the one or more second hydraulic cylinders  460  above a first pressure threshold, such as when the first pressure threshold is a pressure greater than the fluid pressure within the first hydraulic motor  240  and the drive line  466 . 
     In other examples, the flow divider  491  can be configured as a pressure reducer. In such an example, such as shown in  FIG.  4   , the first port  495  can be connected to the second valve  480  via the intermediary line  481 , the second port  496  can be connected to the flow line  498 , and the third port  497  can be connected the branch  490 A of the pilot line  490 . In operation, fluid pressure at the first port  495 , and the thereby the tension line  482 , the one or more first hydraulic cylinders  458 , and the one or more second hydraulic cylinders  460 , will be proportionally less than the amount of fluid pressure at the third port  497  and the pilot line  490  by the amount of the fluid pressure that the second port  496  and the flow line  498  bring into the flow divider  491  from the drain line  471 . 
     For example, if the flow divider  491  has a two-to-one ratio between the third port  497  and the second port  496 , and the second port  496  contributes zero, or otherwise a nominal, fluid pressure to the second port  496 , the fluid pressure within the one or more first hydraulic cylinders  458  and the one or more second hydraulic cylinders  460  will be one-third less than the fluid pressure within the pilot line  490 . In this way, the flow divider  491  can maintain the fluid pressure within the one or more first hydraulic cylinders  458  and the one or more second hydraulic cylinders  460  above a first pressure threshold, such as when the first pressure threshold is a pressure less than the fluid pressure within the first hydraulic motor  440  and the drive line  466 . 
     The flow divider  491  can help the reactive chain tension circuit  462  meet the unique requirements of different coiled tubing injectors. For example, the flow divider  491  can be selected based on various components of a coiled tubing injector, such as to help ensure that a pressure curve defined by fluid flow through the flow divider  491  can closely correspond to a minimum pressure within the one or more first hydraulic cylinders  458  and the one or more second hydraulic cylinders  460  relative to a tubing load on the coiled tubing injector. In one such example, the flow divider  491  can be selected to control fluid flow within the reactive chain tension circuit  462  such that the line  308  ( FIG.  5   ) and the line  310  ( FIG.  5   ) shown in section  304  ( FIG.  5   ) of the graph  300  ( FIG.  5   ) extend parallel to, and laterally offset from, the line  306  (e.g., the minimum pressure). 
       FIG.  7    illustrates an example method  500  of using a coiled tubing injector. Any of the above examples of the coiled tubing injector  108 , and the reactive chain tension circuits  262  and  462 , described with regard to  FIGS.  1 - 5    above can be used in the method  500  of using a coiled tubing injector. The discussed steps or operations can be performed in parallel or in a different sequence without materially impacting other operations. The method  500  as discussed includes operations that can be performed by multiple different actors, devices, and/or systems. It is understood that subsets of the operations discussed in the method  500  can be attributable to a single actor device, or system, and could be considered a separate standalone process or method. 
     The method  500  can optionally include operation  502 . The operation  502  can include connecting the reactive chain tension circuit to the at least one hydraulic motor. For example, a user can open a two-way valve to operably connect the reactive chain tension circuit to the at least one hydraulic cylinder to thereby enable fluid flow through the reactive chain tension circuit to the at least one hydraulic cylinder. The method operation can optionally include operation  504 . The operation  504  can include adjusting an orifice between the reactive chain tension circuit and the drive line. For example, a user can engage an adjustable orifice to set or otherwise adjust a fluid flow rate between the at least one hydraulic motor and the reactive chain tension circuit. 
     The method  500  can include operation  506 . The operation  506  can include activating the drive system to begin moving the coiled tubing through the grip zone toward a wellbore. For example, a user can engage with, such as via one or more user inputs to, a hydraulic power pack or power unit to charge the drive line and a return connected to the at least one hydraulic motor to enable the at least one hydraulic motor to begin turning the two or more drive chains carrying the plurality of grippers. 
     The method can include operation  508 . The operation  508  can include tensioning the two or more drive chains by a reactive chain tension circuit based on a fluid pressure within the at least one hydraulic motor, by maintaining a pressure differential between the fluid pressure within the at least one hydraulic motor and a fluid pressure within at least one hydraulic cylinder for tensioning the at least two drive chains. For example, when the drive system begins moving the coiled tubing through the grip zone, the reactive chain tension circuit can receive a pressure signal from the at least one hydraulic motor indicative of a tubing load on the coiled tubing injector to enable the reactive tension circuit to automatically adjust the tension within the first drive chain and the second drive chain based on the tubing load, such as by maintaining the fluid pressure within the at least one hydraulic cylinder at a set or otherwise fixed interval above or below the pressure signal. 
     In some examples, the operation  508  can include actuating a first valve to maintain a fluid pressure within the hydraulic reactive tension circuit above a first pressure threshold. For example, when the fluid pressure within the reactive chain tension circuit, and thereby the at least one hydraulic cylinder, is less than the first pressure threshold, the first valve can open to enable fluid, such as from an external fluid source, to flow through the first valve until the fluid pressure within the at least one hydraulic cylinder is equal to the first pressure threshold. In some examples, the operation  508  can include actuating a second valve to maintain the fluid pressure within the hydraulic reactive tension circuit below a second pressure threshold. For example, when the fluid pressure within the reactive chain tension circuit, and thereby the at least one hydraulic cylinder, is equal to the second pressure threshold, the second valve can open to enable fluid to flow through the second valve to a drain line until the fluid pressure within the at least one hydraulic cylinder is below the second pressure threshold. 
     In some examples, the operation  508  can include directing hydraulic fluid through a flow divider to maintain a fluid pressure within the hydraulic reactive tension circuit above a first pressure threshold. For example, the flow divider can be configured as a pressure intensifier, and fluid within the pilot line at the pressure within the drive line and the least one hydraulic motor can be passed through the flow divider to increase the fluid pressure by an amount equal to or greater than the first pressure threshold. 
     The foregoing systems and devices, etc. are merely illustrative of the components, interconnections, communications, functions, etc. that can be employed in carrying out examples in accordance with this disclosure. Different types and combinations of sensor or other portable electronics devices, computers including clients and servers, implants, and other systems and devices can be employed in examples according to this disclosure. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventor also contemplates examples in which only those elements shown or described are provided. 
     Moreover, the present inventor also contemplates examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of“at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. 
     This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     EXAMPLES 
     The following, non-limiting examples, detail certain aspects of the present subject matter to solve the challenges and provide the benefits discussed herein, among others. 
     Example 1 is a coiled tubing injector comprising: two or more drive chains each forming a continuous loop and carrying a plurality of grippers for engaging coiled tubing within a grip zone defined between the two or more drive chains; a drive system including: a first drive sprocket engaged with a first drive chain of the two or more drive chains; a second drive sprocket engaged with a second drive chain of the two or more drive chains; at least one hydraulic motor for turning the first drive sprocket and the second drive sprocket, the at least one hydraulic motor connected to a drive line and a return line forming a drive circuit for fluidly powering the at least one hydraulic motor; and a tension system including: at least one hydraulic cylinder for tensioning the two or more drive chains, and a reactive chain tension circuit for automatically tensioning the two or more drive chains by maintaining a pressure differential between a fluid pressure within the drive line and a fluid pressure within the at least one hydraulic cylinder. 
     In Example 2, the subject matter of Example 1 includes, where the reactive chain tension circuit includes a first valve configured to maintain the fluid pressure within the at least one hydraulic cylinder above a first pressure threshold; and a second valve configured to maintain the fluid pressure within the at least one hydraulic cylinder below a second pressure threshold. 
     In Example 3, the subject matter of Example 2 includes, wherein the reactive chain tension circuit includes a pilot line connecting the first valve and the second valve in parallel to the drive line to provide the first valve and the second valve with a pressure signal indicative of a tubing load on the coiled tubing injector. 
     In Example 4, the subject matter of Example 3 includes, wherein the at least one hydraulic motor includes a first port in fluid communication with the drive line and the pilot line of the reactive chain tension circuit. 
     In Example 5, the subject matter of Example 4 includes, wherein the tension system includes an auxiliary fluid source; and wherein the first valve is configured to maintain the fluid pressure within the at least one hydraulic cylinder above the first pressure threshold by selectively connecting the auxiliary fluid source to the at least one hydraulic cylinder based on the pressure signal. 
     In Example 6, the subject matter of Example 5 includes, wherein the first valve is connected to the auxiliary fluid source with an auxiliary line including an adjustable orifice operable to control a fluid flow rate between the auxiliary fluid source and the reactive chain tension circuit. 
     In Example 7, the subject matter of Examples 5-6 includes, wherein the first valve and the second valve are connected to the reactive chain tension circuit by a tension line, the tension line including a two-way valve operable to disconnect the reactive chain tension circuit from the tension system. 
     In Example 8, the subject matter of Example 7 includes, wherein the first valve and the second valve are connected to each other via an intermediary line extending therebetween, the intermediary line including a directional control valve positioned to prevent fluid flow from the tension line to the first valve to reduce leakage associated with the at least one hydraulic cylinder, and wherein the tension line is connected to the intermediary line between the directional control valve and the second valve. 
     In Example 9, the subject matter of Example 8 includes, wherein the second valve is configured to maintain the fluid pressure within the at least one hydraulic cylinder below the second pressure threshold by selectively connecting the tension line to a discharge line. 
     In Example 10, the subject matter of Example 9 includes, an external fluid source connected to the at least one hydraulic cylinder, wherein the external fluid source is manually operable to increase the fluid pressure within the at least one hydraulic cylinder to the second pressure threshold or decrease the fluid pressure within the at least one hydraulic cylinder to the first pressure threshold. 
     Example 11 is a coiled tubing injector comprising two or more drive chains each forming a continuous loop and carrying a plurality of grippers for engaging coiled tubing within a grip zone defined between the two or more drive chains; a drive system including: a first drive sprocket engaged with a first drive chain of the two or more drive chains; a second drive sprocket engaged with a second drive chain of the two or more drive chains; a first hydraulic motor for turning the first drive sprocket; a second hydraulic motor for turning the second drive sprocket, wherein the first hydraulic motor and the second hydraulic motor are connected in parallel to a drive line and a return line forming a drive circuit for fluidly powering the first hydraulic motor and the second hydraulic motor; and a tension system including: one or more first hydraulic cylinders for tensioning the first drive chain; one or more second hydraulic cylinders for tensioning the second drive chain; and a reactive chain tension circuit for automatically tensioning the first drive chain and the second drive chain by maintaining a pressure differential between a fluid pressure within the drive line and a fluid pressure within the first hydraulic cylinder and the second hydraulic cylinder, wherein the reactive chain tension circuit is connected to a first port of the first hydraulic motor via a pilot line to provide the reactive chain tension circuit with a pressure signal indicative of a tubing load on the coiled tubing injector. 
     In Example 12, the subject matter of Example 11 includes, wherein the reactive chain tension circuit includes: a flow divider configured to maintain the fluid pressure within the first hydraulic cylinder and the second hydraulic cylinder above a first pressure threshold; and a pilot operated pressure control valve configured to maintain the fluid pressure within the first hydraulic cylinder and the second hydraulic cylinder below a second pressure threshold, wherein the pilot line connects the flow divider and the pilot operated pressure control valve to the drive line in parallel. 
     In Example 13, the subject matter of Example 12 includes, wherein the pilot line includes a pressure reducing valve configured to limit a maximum fluid pressure at the flow divider and the pilot operated pressure control valve. 
     In Example 14, the subject matter of Example 13 includes, wherein the pilot line includes an adjustable orifice operable to control a fluid flow rate between the first hydraulic motor and the pressure reducing valve; and wherein the reactive chain tension circuit includes a two-way valve operable to disconnect the reactive chain tension circuit from the drive line. 
     Example 15 is a method of using a coiled tubing injector including two or more drive chains carrying a plurality of grippers for engaging coiled tubing within a grip zone defined between a first drive chain and a second drive chain, and a drive system for turning the two or more drive chains and including at least one hydraulic motor connected to a drive line for fluidly powering the at least one hydraulic motor, the method comprising: activating the drive system to begin moving the coiled tubing through the grip zone toward a wellbore; and tensioning the two or more drive chains by a reactive chain tension circuit based on a fluid pressure within the at least one hydraulic motor, by maintaining a pressure differential between the fluid pressure within the at least one hydraulic motor and a fluid pressure within at least one hydraulic cylinder for tensioning the first drive chain and the second drive chain. 
     In Example 16, the subject matter of Example 15 includes, wherein tensioning the two or more drive chains by the reactive chain tension circuit includes actuating a first valve to maintain a fluid pressure within the reactive chain tension circuit above a first pressure threshold; and actuating a second valve to maintain the fluid pressure within the reactive chain tension circuit below a second pressure threshold. 
     In Example 17, the subject matter of Examples 15-16 includes, wherein tensioning the two or more drive chains by the reactive chain tension circuit includes: directing hydraulic fluid through a flow divider to maintain a fluid pressure within the reactive chain tension circuit above a first pressure threshold, and actuating a pilot operated pressure control valve to maintain the fluid pressure within the reactive chain tension circuit below a second threshold pressure. 
     In Example 18, the subject matter of Example 17 includes, wherein directing hydraulic fluid through a flow divider to increase a fluid pressure within the reactive chain tension circuit to a first threshold pressure includes actuating a pressure reducing valve to limit a fluid pressure at the flow divider and the pilot operated pressure control valve. 
     In Example 19, the subject matter of Examples 15-18 includes, wherein tensioning the two or more drive chains by the reactive chain tension circuit includes first adjusting an orifice between the reactive chain tension circuit and the drive line. 
     In Example 20, the subject matter of Examples 15-19 includes, wherein tensioning the two or more drive chains by the reactive chain tension circuit first includes first connecting the reactive chain tension circuit to the at least one hydraulic motor. 
     Example 21 is at least one machine-readable medium including instructions that, when executed by processing circuitry, cause the processing circuitry to perform operations to implement of any of Examples 1-20. 
     Example 22 is an apparatus comprising means to implement of any of Examples 1-20. 
     Example 23 is a system to implement of any of Examples 1-20. 
     Example 24 is a method to implement of any of Examples 1-20.