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CLAIM OF PRIORITY 
     This application is a U.S. National Stage of International Application No. PCT/US2012/046857, filed Jul. 16, 2012. 
     TECHNICAL FIELD 
     The present disclosure relates to systems, assemblies, and methods for conveying perforating and/or logging tools (hereinafter referred to as a “tool string”) in a wellbore where adverse conditions may be present to challenge downward movement of the tool string in the wellbore. 
     BACKGROUND 
     In oil and gas exploration it is important to obtain diagnostic evaluation logs of geological formations penetrated by a wellbore drilled for the purpose of extracting oil and gas products from a subterranean reservoir. Diagnostic evaluation well logs are generated by data obtained by diagnostic tools (referred to in the industry as logging tools) that are lowered into the wellbore and passed across geologic formations that may contain hydrocarbon substances. Examples of well logs and logging tools are known in the art. Examples of such diagnostic well logs include Neutron logs, Gamma Ray logs, Resistivity logs and Acoustic logs. Logging tools frequently are used for log data acquisition in a wellbore by logging in an upward (up hole) direction, from a bottom portion of the wellbore to an upper portion of the wellbore. The logging tools, therefore, need first be conveyed to the bottom portion of the wellbore. In many instances, wellbores can be highly deviated, or can include a substantially horizontal section. Such wellbores make downward movement of the logging tools in the wellbore difficult, as gravitational force becomes insufficient to convey the logging tools downhole. 
     SUMMARY 
     The present disclosure relates to systems, assemblies, and methods for conveying perforating and/or logging tools (hereinafter referred to as a “tool string”) in a wellbore where adverse conditions may be present to challenge downward movement of the tool string in the wellbore. The disclosed systems, assemblies, and methods can reduce risk of damage to the tool string and increase speed and reliability of moving the tool string into and out of wellbores. For example, certain wells can be drilled in a deviated manner or with a substantially horizontal section. In some conditions, the wells may be drilled through geologic formations that are subject to swelling or caving, or may have fluid pressures that make passage of the tool string unsuitable for common conveyance techniques. The present disclosure overcomes these difficulties and provides several technical advances. 
     The present disclosure relates generally to a pump down tool string that is connected to the lower end of an electric wireline or slickline cable that is spooled off a truck located at the surface. As used herein the terms “cable” and “line” and “wireline” are used interchangeably and unless described with more specificity may include an electric wireline cable or a slickline cable. The subject method and system is used in some implementations in a cased wellbore or in other implementations is applicable in a partially cased wellbore. The tool string is especially adapted for use in highly deviated wellbores wherein it is a known practice to pump fluid from the surface behind a tool string to assist the tool in moving down the deviated wellbore. 
     General background of pump down tool technology is known in the art and is disclosed in pending application PCT/US/2010/44999. The automated pump-down system described in the afore referenced PCT patent application depends on sensor data to provide line tension and line speed. Typically, these readings would come from sensors and calculations done at the surface as prior art pump down operations do not include a tool string that has the capability to transmit this information from the tool string. Using surface data to describe events happening in the wellbore is not optimum due to the delay in the response of the sensors at the surface as well as the inaccuracies caused by the effect of wellbore conditions on the readings. Changes in tension at the cable head of the tool string and real tool string speed would not be instantaneously measured due to dampening effects of stretching of the wireline cable and different wellbore fluids. Accuracy of those measurements would also be affected by cable stretch, wellbore fluids, and well geometry. 
     If the pump pressure of the fluid behind the tool string is too great it may result in excessive downhole tension on the cable head that will result in breaking the cable or pulling the cable out from the cable head. It is desirable to control the pump pressure or line speed of the cable to keep the tension in the cable within safe parameters. 
     In some implementations, the pump down tool string of the present disclosure includes a device that measures the tension in the cable at the cable head and transmits that data as an analog signal to the surface via an electric wireline cable or other transmission means, and uses that data to control pumps and/or line speed. 
     Additionally, in some implementations the pump down tool string of the present disclosure may include a device that calculates the speed of the downhole tool string at the cable head and transmits that data as an analog or digital signal. (Examples of such devices include an accelerometer and/or a casing collar locator.) 
     In a first aspect, a system for pump down operations in a wellbore includes a tool string disposed in a wellbore, said tool string including a cable head having an upper end coupled to an electric wireline cable, a downhole tool coupled to the cable head, the downhole tool selected from the group consisting of perforating tools and downhole logging tools, and a downhole tension sensor located in the cable head, are alternatively located elsewhere in the tool string, said sensor adapted to obtain downhole wireline tension data and transmit the downhole wireline tension data via the electric wireline cable, a fluid pump with a fluid output operatively connected to the wellbore above the tool string, and a controller adapted to selectively adjust a pump fluid output rate of the fluid pump during pump down operations based on the downhole wireline tension data received from the downhole tension sensor. 
     Various implementations can include some, all, or none of the following features. The system can also include a wireline speed sensor in communication with the controller, wherein the controller is adapted to selectively adjust the pump fluid output rate during pump down operations based on wireline speed data received from the wireline speed sensor. The wireline speed sensor can be located at the surface and measures the speed of the wireline as the wireline is spooled into the wellbore. The tool speed sensor can be disposed proximal to the cable head and the speed sensor can calculate the speed of the device at the cable head and can transmit that data to a system that communicates with one or more controllers. The tool speed sensor can include a casing collar locator disposed in the tool string and one or more controllers which can calculate the speed at which the casing collar locator is passing between known casing collars spaced apart at previously known distances between the known casing collars. The controller can compare the calculated speed as the casing collar locator passes additional known casing collars and can determine if the speed of the tool string is increasing or decreasing. 
     In a second aspect, a system for pump down operations in a wellbore includes a tool string disposed in a wellbore, said tool string including a cable head having an upper end coupled to an electric wireline cable, and a downhole tool coupled to the cable head, the downhole tool selected from the group consisting of perforating tools and downhole logging tools, a fluid pump with a fluid output operatively connected to the wellbore above the tool string, and a downhole tool speed sensor in communication with a system that is connected to the controller, wherein the controller is adapted to selectively adjust a pump rate during pump down operations based on wireline speed data received from the downhole tool speed sensor. 
     Various implementations can include some, all, or none of the following features. The downhole tool speed sensor can be an accelerometer disposed proximal to the cable head and wherein the tool speed sensor calculates the speed of the device at the cable head and transmits that data to a system that is in communication with one or more controllers. The downhole tool speed sensor can include a casing collar locator disposed in the tool string and one or more controllers which can calculate the speed at which the casing collar locator is passing between known casing collars spaced apart at previously known distances between the known casing collars. The controller can compare the calculated tool speed as the casing collar locator passes additional known casing collars and can determine if the speed of the tool string is increasing or decreasing. The controller can adjust either the speed at which the wireline is spooled off at the surface or the pump output based on the downhole tool speed. The system can also include a downhole tension sensor incorporated in the cable head, or alternatively located elsewhere in the tool string, said sensor adapted to obtain downhole wireline tension data and transmit the downhole wireline tension data to the surface. The controller can be adapted to adjust the pump rate based on the downhole wireline speed data unless the downhole wireline tension reaches a predetermined tension threshold, after which the controller can automatically reduce a surface wireline speed of a wireline unit and the pump rate. The controller can be adapted to selectively adjust the pump rate during pump down operations based on downhole wireline tension data received from the downhole tension sensor. The controller can selectively adjust the wireline speed during pump down operations based on wireline tension data received from the downhole tension sensor. The system can also include a pump rate sensor in communication with the controller, wherein the controller can selectively adjust the wireline speed during pump down operations based on pump rate data received from the pump rate sensor. The controller can automate at least one control function selected from the group consisting of: a pump fluid output rate for the pump unit based on at least one of a monitored wireline speed and a monitored wireline tension, and a wireline speed based on at least a monitored pump rate for the pump. The controller can include a wireline controller typically located at the surface and a pump controller that is part of the pump. If the wireline controller notifies the pump controller that a monitored tool speed is less than a predetermined threshold, the pump controller can increase a pump rate of the pump unit in response to said notification. If the wireline controller notifies the pump controller that a monitored wireline tension is more than a predetermined threshold, the pump controller can decrease a pump rate of the pump unit in response to said notification. If the pump controller notifies the wireline controller that a monitored pump rate is less than a predetermined threshold, the wireline controller can decrease a wireline speed in response to said notification. 
     In a third aspect, a method for pumping a tool string connected to an electric wireline into a wellbore includes inserting a logging tool string into a proximal upper end of the wellbore, said logging tool string including a cable head attached to a cable, a downhole tension sensor located in the cable head, or alternatively, proximal to the cable head, said sensor adapted to obtain downhole wireline tension data and transmit the downhole wireline tension data via the electric wireline cable, and a downhole wireline speed sensor, pumping a fluid into the upper proximal end of the wellbore above the tool string to assist, via fluid pressure on the tool string, movement of the tool string down the wellbore, spooling out the cable at the surface as the fluid is pumped behind the tool string and the tool string is moving down the wellbore, receiving by one or more controllers downhole wireline tension data from the downhole tensions sensor via the electric wireline cable, and receiving by the one or more controllers data from the casing collar locator via the electric wireline cable. 
     Various aspects can include some, all, or none of the following features. The method can also include determining if the downhole tool string speed is increasing or decreasing. The method can also include monitoring, by a controller, a downhole wireline speed, monitoring, by the controller, a downhole wireline tension, and automatically controlling, by the controller, a pump rate for pumping the tool into the wellbore based on at least one of the monitored downhole tool speed and monitored downhole wireline tension. The method can also include receiving downhole sensor data and determining the tool speed and the wireline tension from the sensor data. The method can also include increasing the pump rate in response to a reduction in the monitored tool speed. The method can also include changing the pump rate in accordance with a difference between the monitored tool speed and a predetermined threshold. The method can also include changing the wireline speed at the surface in response to a monitored pump rate. The method can also include monitoring by a controller a pump rate for pumping the tool into the wellbore, and automatically controlling, by the controller, a tool speed for the tool being pumped into the wellbore based on at least the monitored pump rate. 
     In the drawings and description that follow, like parts are typically marked throughout the specification and drawings with the same reference numerals. The drawing figures are not necessarily to scale. Certain features of the disclosure may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present disclosure is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the disclosure to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an inclusive fashion, and thus should be interpreted to mean “including, but not limited to.” Unless otherwise specified, any use of any form of the terms “connect,” “engage,”, “couple,” “attach,”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. Reference to up or down will be made for purposes of description with “up,” “upper,” “upwardly” or “upstream” meaning toward the surface of the well and with “down,” “lower,” “downwardly” or “downstream” meaning toward the terminal end of the well, regardless of the wellbore orientation. In addition, in the discussion and claims that follow, it may be sometimes stated that certain components or elements are in fluid communication. By this it is meant that the components are constructed and interrelated such that a fluid could be communicated between them, as via a passageway, tube, or conduit. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
     Disclosed herein are systems and methods for automated monitoring and control of pump down operations. More specifically, the pump rate of a pump unit (or units), the line speed for a logging/perforating (L/P) unit, and the line tension for the L/P unit may be automatically monitored and controlled to enable efficient pump down operations. In at least some embodiments, pump down operations may be based on a predetermined line speed, a predetermined line tension and/or a predetermined pump rate. However, if any of these parameters change during pump down operations, the other parameters will be adjusted automatically. The techniques disclosed herein improve safety of pump down operations by eliminating the possibility of pumping the tools off the end of the wireline cable or other catastrophes. 
     As a specific example, if the monitored line tension surpasses a desired threshold, the line speed will be automatically reduced to maintain the desired line tension and the pump rate will be reduced in accordance with the amount of change in the line speed. Thereafter, if the monitored line tension drops below the predetermined threshold, the line speed will be automatically increased (up to a desired line speed) and the pump rate will be increased in accordance with the line speed. Similarly, changes in the monitored pump rate during pump down operations may result in automated changes to the line tension and/or line speed of the L/P unit. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side schematic that illustrates operations of a logging tool conveying system. 
         FIG. 2  illustrates a conceptual block diagram of a logging tool conveying system. 
         FIG. 3  illustrates a block diagram of a control system for pump down operations. 
         FIGS. 4A and 4B  illustrate other control systems which may be distributed between the wireline unit, pumping unit, the wireline tool, and a separate controller. 
         FIG. 5  illustrates a block diagram of a pump down control application. 
         FIG. 6  illustrates a method  600  in accordance with an embodiment of the disclosure. 
         FIG. 7A  is a side view of a logging tool string assembly applicable to operations illustrated in  FIG. 1 . 
         FIG. 7B  is a side view of a perforation tool assembly applicable to the operations illustrated in  FIG. 1 . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIGS. 1 to 6  illustrate operations of a pump down tool string  200  (including implementations of the tool string  200   a  and  200   b  of  FIGS. 7A and 7B ). The system  100  includes surface equipment above the ground surface  105  and a wellbore  150  and its related equipment and instruments below the ground surface  105 . In general, surface equipment provides power, material, and structural support for the operation of the pump down tool string  200 . In the embodiment illustrated in the side schematic of  FIG. 1 , the surface equipment includes a drilling rig  102  and associated equipment, and a data logging and control truck  115 . The rig  102  may include equipment such as a rig pump  122  disposed proximal to the rig  102 . The rig  102  can include equipment used when a well is being logged or later perforated such as a tool lubrication assembly  104  and a pack off pump  120 . In some implementations a blowout preventer  103  will be attached to a casing head  106  that is attached to an upper end of a well casing  112 . The rig pump  122  provides pressurized drilling fluid to the rig and some of its associated equipment. A wireline and control truck  115  monitors the data logging operation and receives and stores logging data from the logging tools and/or controls and directs perforation operations. Below the rig  102  is the wellbore  150  extending from the surface  105  into the earth  110  and passing through a plurality of subterranean geologic formations  107 . The wellbore  150  penetrates through the formations  107  and in some implementations forms a deviated path, which may include a substantially horizontal section as illustrated in  FIG. 1 . The wellbore  150  may be reinforced with one or more casing strings  112  and  114 . 
     The tool string  200  may be attached with a cable/wireline  111  via a cable head  211 . The conveying process is conducted by pumping a fluid from the rig pump  122  into the upper proximal end of the casing string  112  (or  114 ) above the tool string  200  to assist, via fluid pressure on the tool string  200 , movement of the tool string  200  down the wellbore  150 . The pump pressure of the fluid above the tool string  200  is monitored, for example, by the truck  115 , because the fluid pressure can change during the conveying process and exhibit patterns indicating events such as sticking of the tool string in the wellbore. As the tool string  200  is pumped (propelled) downwards by the fluid pressure that is pushing behind the tool string  200 , the cable  111  is spooled out at the surface by the truck  115 . A cable tension sensing device  117  is located at the surface and provides cable tension data to control track  115 . A speed sensor device  119  located at the surface provides surface cable speed data to control track  115 . 
     In some implementations the tool string will have sufficient weight that gravity will convey the tool string down the wellbore without the assistance of pump fluid pressure. 
       FIG. 7A  is a side view of an exemplary logging tool string  200   a  and  200   b  applicable to the operations of a general tool string  200  illustrated in  FIGS. 1 to 6 . In some implementations the tool string  200  may be implemented and tool string  200   a  as illustrated in  FIG. 7A  and include various data logging instruments used for data acquisition; for example, a casing collar locator  220 , a telemetry gamma ray tool  231 , a density neutron logging tool  241 , a borehole sonic array logging tool  243 , a compensated true resistivity tool array  251 , among others as are well known in the art. 
     The tool string is securely connected with the cable  111  by cable head tool  211 . The tool string may include a downhole tension sensing device  213  and a downhole speed sensing device such as an accelerometer  215 . As the tool string  200  is propelled down the bore of the drill string by the fluid pressure, the rate at which the cable  111  is spooled out maintains movement control of the tool string  200  at a desired speed. 
     In other possible configurations, the tool string  200  may include other data logging instruments besides those discussed in  FIG. 7A , or may include a subset of the presented instruments. 
     Referring to  FIG. 7B , in other implementations the tool string  200  may be implemented as tool string  200   b  as illustrated and include the casing collar locator  220 , a firing head and perforating gun  250 , as are well known in the art. In some implementations the tool string  200  includes a tension load cell  213  and/or triaxial accelerometer speed sensing device  215 . 
     Referring to  FIG. 7A , wherein an exemplary tool string  200   a  is illustrated inside a casing string  114 . Casing collars  116  are couplings that connect two joints of pipe together. The coupling adds mass to the casing string  114  at the connections and the change in mass can be measured. In most cased wellbores, there will be an existing record of the location of the casing collars relative to the actual known depth of most casing collars in the wellbore trajectory. This is typically done by running a log with a gamma ray detector and a casing collar locator. The actual known depth of the casing collars is entered into a processor. 
     As used herein with regard to speed calculations and speed adjustments and corrections factors, the term “actual known depth” is the depth as determined from the casing collar locator log. The depth may also be referred to as the “expected depth.” The measured depth is the depth as calculated based on the measured amount of cable/line spooled out and measured at the surface. 
     In some methods of operations of the tool string  200 ,  200   a ,  200   b , before entering a section of the wellbore that is highly deviated from vertical, a casing collar at a known depth will be recorded and the current depth will be adjusted or the delta will be noted. The line will be spooled into the well, the casing collar locator data, as well as the downhole line tension data will be transmitted uphole to a surface processor that is part of the system. Downhole tension data is used in speed correction algorithms that use line tension. As the tool passes a casing collar, the depth of the collar will be noted as well as the time. The average line or tool speed over the interval between collars will be calculated and compared to the average line or tool speed measured at the surface and the average calculated downhole speed. The recorded depth of the casing collar will be compared to the expected actual depth. The expected actual depth of the casing collar is based on previously recorded measurements used to determine the actual depth of the casing collar. This could be a Gamma Ray/CCL log or some other method of correlating the casing collar depth to the reference depth for the well. 
       FIG. 2  illustrates a conceptual block diagram of the logging tool conveying system  210 . During the pump down operations illustrated and described in  FIGS. 1 to 7B , automated monitoring and control of various operational parameters are performed. In at least some embodiments, the pump rate of a pump unit (or units), the line speed for a logging/perforating (L/P) unit, and the line tension for the L/P unit may be sensed by a downhole tool string  200 ,  200   a ,  200   b  may be automatically monitored by a surface system  260 , and controlled by a pump controller  270  to enable efficient pump down operations. Of course, the automatic monitoring and control of parameters such as the propelling force and rate for advancing the tool string into the borehole, the line speed for a wireline unit, and the line tension for the wireline unit is useful for any wireline tool in which the tool string is conveyed into the borehole (cased or uncased) and where it is desired to coordinate control of both the pumping unit and the feed of the tool on the wireline. Such principles may be applied to any wireline logging tool, for example. Although a pumping unit is typical for use in pump down operations, other driving units are known which may be used for advancing wireline tools, such as powered tractors, and it is equally important that the driving force be balanced with wireline speed and wireline tension for such tools also. 
     As a specific example, suppose it is desired to run a tool string at a line speed of 500 feet per minute in the vertical portion  147  of wellbore  150  and run the tool at a line speed of 375 feet per minute in the horizontal portion  148  of wellbore  150 . Further, suppose the L/P control unit is always trying to hold 3000 lbs of tension on tools going in the hole. For this set of desired parameters, the L/P control unit initially sets the line tension parameter at 3000 lbs and the line speed parameter at 500 ft/min (for vertical portion  47 ) and later 375 ft/min (for horizontal portion  48 ). In response, the tech control center (TCC)/pump control unit automates the pump rate to achieve the L/P variables. Once the tool string starts down wellbore  10 , the TCC/pump sets an auto pump rate that ramps up to the L/P variables (e.g., within  30  seconds or so). If any of these parameters change during the pump down operations, the other parameters will be adjusted automatically. The techniques disclosed herein improve safety of pump down operations by eliminating the possibility of pumping the tools off the end of the wireline cable or other catastrophes. 
       FIG. 3  illustrates a block diagram of a control system  300  for pump down operations of the tool string  200  in accordance with an embodiment of the disclosure. The control system components are most usefully located at the surface, as part of the wireline unit, pumping unit or as part of a separate remote control unit. Surface control components facilitate access for maintenance and ensuring accurate control signal transmission to the wireline unit and pumping unit. It is equally possible, however, for some or all components of the control system to be installed on the downhole tool. Such an arrangement may be appropriate where it is desired to integrate the combined control functionality for the wireline unit and pumping unit into the tool itself (e.g., where the tool may be a separately provided integer from the wireline unit and is configured to interface with each of the wireline unit and the pumping unit). In such cases, the tool is ideally provided along with a remote input/output device for monitoring and/or setting control parameters for the tool/control system from the surface. As shown, the control system  300  comprises a controller  302  coupled to a wireline unit  306  and to a pump unit  308 . The controller  302  may replace one or both of the individual controllers usually provided to each of the wireline unit  306  and pump unit  308 . Where only one of the individual controllers is replaced, the controller  302  is configured to interface with the existing controller of the other unit. Alternatively, an entirely separate controller  302  may be provided that is configured to interface with the existing individual control units of both the wireline unit  306  and pumping unit  308 . Advantageously, the controller  302  may be configured to interface with the individual control units of a wide range of existing pumping units and wireline units, making the controller adaptable to different wireline and pumping equipment, including the equipment of different manufacturers and/or a variety of different wireline tools. In some applications, the interface between controller  302  and the pumping unit  308  and/or wireline unit  306  may be wireless, for example, via WiFi, Bluetooth or over a telephone or internet connection, for example. Appropriate transmitter/receiver equipment may be connected to the wireline unit  306  and pumping unit  308  to permit the controller  302  to interface with them. The controller  302  is thereby able to be configured to provide commands to the wireline unit  306  to control wireline movement during pump down operations, such as pump-and-perf operations. The controller  302  may also be configured to provide commands to the pump unit  308  to control pumping during pump down operations. This may obviate the necessity for a separate operator to control each of the wireline unit  306  and the pumping unit  308 , the pump down operation able to precede either entirely automatically under the control of controller  302 , or with input from a single operator into the controller  302 . In at least some embodiments, the controller  302  relies on control parameters  304  (e.g., a wireline speed parameter, a wireline tension parameter, and a pump rate parameter) to generate appropriate commands to the wireline unit  306  and pump unit  308 . 
     Data corresponding to the control parameters  304  are received from system sensors, which are arranged to monitor the respective control parameters from appropriate locations on the pumping unit, wireline unit and/or wireline tool, or otherwise on the drilling platform or in the wellbore, and are coupled to the controller  302 . Pressure also may be monitored by the controller  302  to account for pumping limitations. 
     In at least some embodiments, a wireline speed sensor  310 , a wireline tension sensor  312 , and a pump rate sensor  314  provide sensor data to the controller  302 . Other sensor data might be relayed to the controller, for example, relating to the position and/or orientation of the wireline tool in the wellbore. The sensor data from the wireline speed sensor  310  may correspond directly to wireline speed data or to data that enables the wireline speed to be calculated. The sensor data from the wireline tension sensor  312  may correspond directly to wireline tension data or to data that enables the wireline tension to be calculated. The sensor data from the pump rate sensor  314  may correspond directly to pump rate data or to data that enables the pump rate to be calculated. 
     During pump down operations, such as pump-and-log or pump-and-perf, the controller  302  analyzes new sensor data from the sensors  310 ,  312 ,  314  and is configured to automatically direct the pump unit  308  to adjust its pump rate in response to changes in a monitored wireline speed and/or monitored wireline tension. Additionally, the controller  302  may automatically direct the wireline unit  306  to adjust its wireline speed in response to changes in a monitored pump rate. For example, the controller  302  may direct the pump unit  308  to increase its pump rate in response to a decrease in the monitored wireline speed in order to maintain the speed at which the tool is advanced. Of course, this action assumes the wireline tension to be unchanging, or changing proportional to speed. If, to the contrary, the wireline tension is decreasing at a non-proportional rate to the rate at which the speed is decreasing, this would likely indicate that the tool is entering debris, and the appropriate action would then be to decrease the pump rate, or shut off the pump altogether, in order to prevent the tool getting stuck. It will therefore be appreciated that control of the pump rate in dependence on the wireline speed will preferably also be dependent upon the wireline tension. Additionally or alternatively, the controller  302  may direct the wireline unit  306  to reduce its wireline speed and/or direct the pump unit  308  to reduce its pump rate in response to an increase in the monitored wireline tension. In at least some embodiments, comparisons of control parameter values to predetermined threshold values (e.g., greater than or less than comparisons) for wireline speed, wireline tension, and pump rate may be considered by the controller  302  in addition to (or instead of) directional changes (an increase/decrease) for the control parameters. 
       FIGS. 4A-4B  illustrate other control systems which may be distributed between the wireline unit  306 , pumping unit  308 , the wireline sensors  410 A, and a separate controller, as desired. The distributed control systems are suitable for controlling pump down operations, such as pump-and-perf and pump-and-log, in accordance with embodiments of the disclosure. 
     In system  400 A of  FIG. 4A , distributed control of a wireline unit  406 A and a pump unit  408 A are illustrated. In other words, the wireline controller  402 A and the pump controller  404 A perform the functions described for the controller  302 , except in a distributed manner. More specifically, wireline controller  402 A directs commands to the wireline unit  406 A, while pump controller  404 A directs commands to the pump unit  408 A. In order to account for changes that may occur in the control parameters (e.g., wireline speed, wireline tension, and pump rate), the wireline controller  402 A and the pump controller  404 A are configured to communicate. Such changes may be detected based on sensor data gathered from wireline sensors  410 A coupled to the wireline controller  402 A. Additionally, the pump controller  404 A may gather sensor data from pump sensors  412 A coupled thereto. The amount of information exchanged between wireline controller  402 A and pump controller  404 A may vary for different embodiments. For example, wireline controller  402 A and pump controller  404 A may be configured to exchange sensor data periodically. Additionally or alternatively, wireline controller  402 A and pump controller  404 A may be configured to send requests as needed (e.g., the wireline controller  402 A may request that the pump controller  404 A reduce the pump rate or the pump controller  404 A request that the wireline controller  402 A reduce the wireline speed). The amount of reduction related to each request may be communicated with the request, deduced, or preset for each controller  402 A,  404 A. Increases in pump rate and wireline speed are likewise possible and may be requested between distributed controllers such as controllers  402 A and  404 A. 
     In system  400 B of  FIG. 4B , another embodiment of distributed controllers for pump down operations is illustrated. As shown, wireline controller  402 B and wireline sensors  410 B are incorporated into wireline unit  406 B. Similarly, pump controller  404 B and pump sensors  412 B are incorporated into pump unit  408 B. In at least some embodiments, the wireline unit  406 B and the pump unit  408 B are configured to communicate to each other to automate control of a pump rate and wireline speed during pump down operations. Wireline tension also may be considered and may affect the control of both the pump rate and the wireline speed during pump down operations. Similar to the discussion of  FIG. 4A , the amount of information exchanged between wireline controller  402 B and pump controller  404 B may vary for different embodiments. In various embodiments, sensor data, notifications, and/or requests may be sent from one distributed controller to the other. 
     The controller  302  of  FIG. 3  and/or the controllers  402 A, B and  404 A, B of  FIGS. 4A-4B  may correspond to any of a variety of hardware controllers. In some embodiments, such controller may correspond to hardware/firmware/software systems. As an example,  FIG. 5  illustrates a computer system  500  used with pump down operations in accordance with an embodiment of the disclosure. The computer system  500  comprises a computer  502  with one or more processors  504  coupled to a system memory  506 . Some embodiments of the computer  502  also include a communication interface  526  and I/O devices  528  coupled to the processor  504 . The computer  502  is representative of a desktop computer, server computer, notebook computer, handheld computer, or smart phone, etc. 
     The processor  504  is configured to execute instructions read from the system memory  506 . The processor  504  may, for example, be a general-purpose processor, a digital signal processor, a microcontroller, etc. Processor architectures generally include execution units (e.g., fixed point, floating point, integer, etc.), storage (e.g., registers, memory, etc.), instruction decoding, peripherals (e.g., interrupt controllers, timers, direct memory access controllers, etc.), input/output systems (e.g., serial ports, parallel ports, etc.) and various other components and sub-systems. 
     The system memory  506  corresponds to random access memory (RAM), which stores programs and/or data structures during runtime of the computer  502 . For example, during runtime of the computer  502 , the system memory  506  may store a pump down control application  514 , which is loaded into the system memory  506  for execution by the processor  504 . 
     The system  500  also may comprise a computer-readable storage medium  505 , which corresponds to any combination of non-volatile memories such as semiconductor memory (e.g., flash memory), magnetic storage (e.g., a hard drive, tape drive, etc.), optical storage (e.g., compact disc or digital versatile disc), etc. The computer-readable storage medium  505  couples to I/O devices  528  in communication with the processor  504  for transferring data/code from the computer-readable storage medium  505  to the computer  502 . In some embodiments, the computer-readable storage medium  505  is locally coupled to I/O devices  528  that comprise one or more interfaces (e.g., drives, ports, etc.) to enable data to be transferred from the computer-readable storage medium  505  to the computer  502 . Alternatively, the computer-readable storage medium  505  is part of a remote system (e.g., a server) from which data/code may be downloaded to the computer  502  via the I/O devices  528 . In such case, the I/O devices  528  may comprise networking components (e.g., a network adapter for wired or wireless communications). Regardless of whether the computer-readable storage medium  505  is local or remote to the computer  502 , the code and/or data structures stored in the computer-readable storage medium  505  may be loaded into system memory  506  for execution by the processor  504 . For example, the pump-and-perf control application  514  or other software/data structures in the system memory  506  of  FIG. 5  may have been retrieved from computer-readable storage medium  505 . 
     The I/O devices  528  also may comprise various devices employed by a user to interact with the processor  504  based on programming executed thereby. Exemplary I/O devices  528  include video display devices, such as liquid crystal, cathode ray, plasma, organic light emitting diode, vacuum fluorescent, electroluminescent, electronic paper or other appropriate display panels for providing information to the user. Such devices may be coupled to the processor  504  via a graphics adapter. Keyboards, touchscreens, and pointing devices (e.g., a mouse, trackball, light pen, etc.) are examples of devices includable in the I/O devices  528  for providing user input to the processor  504  and may be coupled to the processor by various wired or wireless communications subsystems, such as Universal Serial Bus (USB) or Bluetooth interfaces. 
     As shown in  FIG. 5 , the pump down control application  514  comprises wireline control instructions  516 , pump control instructions  518  and control parameters  520 . When executed, the wireline control instructions  516  operate to generate commands for a wireline unit  536  coupled to the computer  502  via the communication interface  526 . Likewise, the pump control instructions  518 , when executed, operate to generate commands for a pump unit  534  coupled to the computer  502  via the communication interface  526 . The generation of commands by the wireline control instructions  516  and the pump control instructions  518  may be based on monitored control parameters  520  such as wireline speed, wireline tension and/or pump rate. The monitored control parameters  520  may be received during pump down operations from sensors  532  coupled to the communication interface  526 . Alternatively, the sensors  532  provide wireline data and pump data from which the monitored control parameters  520  are calculated. In either case, the received or derived control parameters  520  are stored in the computer  502  for access by the pump down control application  514 . 
     In at least some embodiments, the commands generated by the pump control instructions  518  for the pump unit  534  cause the pump unit  534  to change its pump rate. For example, the pump control instructions  518  may generate a reduce pump rate command for the pump unit  534  in response to an increase in the monitored wireline speed and/or an increase in the monitored wireline tension. Alternatively, the pump control instructions  518  may generate an increase pump rate command for the pump unit  534  in response to a decrease in the monitored wireline speed and/or a decrease in the monitored wireline tension. Further, the wireline control instructions  516  may generate a decrease wireline speed command for the wireline unit  536  in response to a decrease in the monitored pump rate. In this manner, efficiency of pump down operations is improved while also considering safety thresholds. 
       FIG. 6  illustrates a method  600  in accordance with an embodiment of the disclosure. 
     Though depicted sequentially as a matter of convenience, at least some of the actions shown can be performed in a different order and/or performed in parallel. Additionally, some embodiments may perform only some of the actions shown. In some embodiments, the operations of  FIG. 6 , as well as other operations described herein, can be implemented as instructions stored in a computer-readable storage medium (e.g., computer-readable storage medium  505 ) and executed by a processor (e.g., processor  504 ). 
     The method  600  starts by monitoring a wireline speed (block  602 ) and monitoring a wireline tension (block  604 ). The monitoring may be performed by sensors in communication with a hardware controller or a computer running software. In some embodiments, pressure and rate sensors could be monitored, if need be, from a transducer and flowmeter in the line rather than from the pump directly. A pump rate for pump down operations is then set based on the monitored wireline speed and monitored wireline tension (block  606 ). If changes to control parameters occur during pump down operations (determination block  608 ), the pump rate is automatically updated in response to the changes (block  610 ). In at least some embodiments, the control parameters correspond to the monitored wireline speed and the monitored wireline tension. For example, the pump rate may be decreased during pump down operations in response to a reduction in the monitored wireline speed. The amount of decrease in the pump rate may correspond to the difference between the monitored wireline speed and a predetermined threshold. The method  600  may additionally comprise receiving sensor data and determining the wireline speed and the wireline tension from the sensor data. Further, the method  600  may additionally comprise changing a wireline speed in response to a monitored pump rate during pump down operations. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Further, the method  600  may include fewer steps than those illustrated or more steps than those illustrated. In addition, the illustrated steps of the method  600  may be performed in the respective orders illustrated or in different orders than that illustrated. As a specific example, the method  600  may be performed simultaneously (e.g., substantially or otherwise). Other variations in the order of steps are also possible. Accordingly, other implementations are within the scope of the following claims.

Summary:
A system for pump down operations in a wellbore includes a tool string disposed in a wellbore, the tool string including a cable head having an upper end coupled to an electric wireline cable, a downhole tool coupled to the cable head, the downhole tool selected from the group consisting of perforating tools and downhole logging tools. In this embodiment, the system further includes a downhole tension sensor located proximal to the upper end of the cable head, the sensor adapted to obtain downhole wireline tension data and transmit the downhole wireline tension data via the electric wireline cable, a fluid pump with a fluid output operatively connected to the wellbore above the tool string, and a controller adapted to selectively adjust a pump fluid output rate of the fluid pump during pump down operations based on the downhole wireline tension data received from the downhole tension sensor.