Abstract:
A tool for moving within a passage comprises an elongated body; a closed system for converting a circulating flow of a fluid into movement of the tool within the passage, and a pump-powering assembly configured to power the pump, the pump-powering assembly comprising one of a turbine and an E-line controlled motor. The closed system comprises a gripper assembly on the body, a barrel surrounding and engaged with the body, a piston longitudinally fixed with respect to the body, a valve assembly, and a pump configured to circulate the fluid through the closed system. The gripper assembly is configured to utilize fluid pressure to grip onto an inner surface of the passage. The barrel is longitudinally movable with respect to the body, and the gripper assembly is longitudinally fixed with respect to the barrel. The barrel and the body define an annular space therebetween, wherein one or more interfaces between the barrel and the body are sealed to substantially prevent escape of fluid from the annular space to an exterior of the barrel. The piston is positioned within the barrel and fluidly separates the annular space into aft and forward chambers of the barrel, wherein sizes of the aft and forward chambers of the barrel vary as the piston moves longitudinally within the barrel. The valve assembly is configured to direct fluid to and from (1) the gripper assembly and (2) the aft and forward chambers of the barrel to produce movement of the body within the passage.

Description:
INCORPORATION BY REFERENCE  
       [0001]     The present application incorporates by reference the entire disclosures of U.S. Pat. Nos. 6,003,606 (entitled “PULLER-THRUSTER DOWNHOLE TOOL”); 6,347,674 (“ELECTRICALLY SEQUENCED TRACTOR”); 6,241,031 (“ELECTRO-HYDRAULICALLY CONTROLLED TRACTOR”); 6,679,341 (“TRACTOR WITH IMPROVED VALVE SYSTEM”); 6,464,003 (“GRIPPER ASSEMBLY FOR DOWNHOLE TRACTORS”); and 6,715,559 (“GRIPPER ASSEMBLY FOR DOWNHOLE TRACTORS”). The present application also incorporates by reference the entire disclosures of U.S. Patent Application Publication Nos. 2004/0168828 (“TRACTOR WITH IMPROVED VALVE SYSTEM”); and 2005/0247488 (“ROLLER LINK TOGGLE GRIPPER AND DOWNHOLE TRACTOR”). The present application also incorporates by reference the entire disclosure of U.S. Provisional Patent Application No. 60/781,885, filed Mar. 13, 2006 (“EXPANDABLE RAMP GRIPPER”).  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to tools for conducting operations within passages, and specifically to tools for borehole intervention and/or drilling.  
         [0004]     2. Description of the Related Art  
         [0005]     U.S. Pat. No. 6,003,606, entitled “Puller-Thruster Downhole Tool,” discloses an innovative self-propelled tool or tractor for drilling, completion, stimulation, and intervention that pulls a drill string and simultaneously thrusts itself and its payload downhole and/or into a casing or borehole formation. The &#39;606 patent discloses a tractor that includes one or more gripper assemblies (e.g., bladders or packerfeet) that grip onto an inner surface of a borehole or casing, and one or more propulsion assemblies that propel the tractor body forward when at least one of the gripper assemblies is gripping the borehole. A valve system directs a fluid (e.g., drilling mud, intervention fluid, hydraulic fluid) to and from the gripper assemblies and propulsion assemblies to power movement of the tractor.  
         [0006]     The &#39;606 patent discloses two basic types of tractor configurations—open loop and closed loop. The open loop system uses an externally provided fluid as a medium of hydraulic communication within the tractor. The open loop consists of a ground surface pump, tubing extending from the pump into a borehole, a tractor within the borehole and connected to the tubing, and an annulus between the exterior of the tractor and an inner surface of the borehole. The fluid is pumped down through the tubing to the tractor, used by the tractor to move and conduct other downhole operations, and then forced back up the borehole through the annulus. The tractor is powered by differential pressure—the difference of the pressure at the point of intake of fluid to the tractor and the pressure of fluid ejected from the tractor into the annulus. In the open loop system, a portion of the fluid is used to power the tractor&#39;s movement and another portion of the fluid flows through the tractor for various downhole purposes, such as hole cleaning, sand washing, acidizing, and lubricating of a drill bit (in drilling operations). Both portions of the fluid return to the ground surface through the annulus.  
         [0007]     The &#39;606 patent also discloses a closed loop configuration in which a hydraulic fluid is circulated through the gripper assemblies and propulsion assemblies to power the tractor&#39;s movement within the borehole. In particular, FIG. 19 of the &#39;606 patent discloses a downhole motor that powers the recirculation of the hydraulic fluid.  
         [0008]     The &#39;606 patent further discloses, in FIG. 24, an embodiment in which an electrical line (referred to herein as an “E-line”) is provided within the coiled tubing. The E-line can be utilized to send electrical signals from the ground surface to the tractor to control the position of a start/stop valve that regulates the inflow of drilling fluid into the tractor&#39;s valve assembly, in an open loop system.  
         [0009]     U.S. Pat. Nos. 6,347,674; 6,241,031; and 6,679,341, as well as U.S. Patent Application Publication No. 2004/0168828, disclose alternative valve systems and methods for directing fluid to and from a downhole tractor&#39;s gripper assemblies and propulsion assemblies for moving the tractor.  
       SUMMARY  
       [0010]     In one aspect, a tool for moving within a passage is provided. The tool comprises an elongated body; a closed system for converting a circulating flow of a fluid into movement of the tool within the passage, and a pump-powering assembly configured to power the pump, the pump-powering assembly comprising one of a turbine and an E-line controlled motor. The closed system comprises a gripper assembly on the body, a barrel surrounding and engaged with the body, a piston longitudinally fixed with respect to the body, a valve assembly, and a pump configured to circulate the fluid through the closed system. The gripper assembly is configured to utilize fluid pressure to grip onto an inner surface of the passage. The barrel is longitudinally movable with respect to the body, and the gripper assembly is longitudinally fixed with respect to the barrel. The barrel and the body define an annular space therebetween, wherein one or more interfaces between the barrel and the body are sealed to substantially prevent escape of fluid from the annular space to an exterior of the barrel. The piston is positioned within the barrel and fluidly separates the annular space into aft and forward chambers of the barrel, wherein sizes of the aft and forward chambers of the barrel vary as the piston moves longitudinally within the barrel. The valve assembly is configured to direct fluid to and from (1) the gripper assembly and (2) the aft and forward chambers of the barrel to produce movement of the body within the passage.  
         [0011]     For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.  
         [0012]     All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic diagram of a conventional coiled tubing tractor system.  
         [0014]      FIG. 2  is a schematic diagram of a turbine-powered motor for a closed loop system for powering a downhole tractor, according to an embodiment of the invention.  
         [0015]      FIG. 3  is a more detailed schematic diagram of the closed loop system of  FIG. 2 .  
         [0016]      FIG. 4  is a schematic diagram of an E-line powered motor for a closed loop system for powering a downhole tractor, according to an embodiment of the invention.  
         [0017]      FIG. 5  is a more detailed schematic diagram of the closed loop system of  FIG. 4 .  
         [0018]      FIG. 6  is a schematic diagram of a turbine-powered pump for a closed loop system for powering a downhole tractor, according to an embodiment of the invention.  
         [0019]      FIG. 7  is a more detailed schematic diagram of the closed loop system of  FIG. 6 .  
         [0020]      FIG. 8  is a schematic diagram of a system in which a positive displacement motor powers a pump for a closed loop system for powering a downhole tractor, according to an embodiment of the invention.  
         [0021]      FIG. 9  is a more detailed schematic diagram of the closed loop system of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]      FIG. 1  illustrates a conventional coiled tubing tractor or tool for conducting downhole operations such as intervention and drilling. The illustrated system is an open loop configuration. The coiled tubing system  100  typically includes a power supply  102  for powering ground-level equipment, a tubing reel  104 , a tubing guide  106 , and a tubing injector  110 , which are well known in the art. The illustrated system includes a bottom hole drilling assembly  120  for drilling a borehole  132  with a drill bit  130 . However, other types of bottom hole assemblies  120  can alternatively be provided, such as those for intervention operations like hole cleaning, sand washing, acidizing, and the like. As known, coiled tubing  114  is inserted into the borehole  132 , and a fluid (e.g., drilling mud, intervention fluid) is typically pumped through the inner flow channel of the coiled tubing  114  towards the drill bit  130  located at the end of the drill string. Positioned between the drill bit  130  and the coiled tubing  114  is a tool or tractor  112 . The illustrated bottom hole assembly  120  includes a number of elements known to those skilled in the art, such as a downhole motor  122  and a Measurement While Drilling (MWD) system  124 . The tractor  112  is preferably connected to the coiled tubing  114  and the bottom hole assembly  120  by connectors  116  and  126 , respectively, as known in the art. In this system, the fluid is pumped through the inner flow channel of the coiled tubing  114  and through the tractor  112  to the drill bit  130 . The fluid and drilling debris return to the surface in the annulus defined between the exterior surface of the tractor  112  and the inner surface of the borehole  132 , and also defined between the exterior surface of coiled tubing  114  and the inner surface of the borehole  132 .  
         [0023]     When operated, the tractor  112  is configured to move within the borehole  132 . This movement allows, for example, the tractor  112  to maintain a pre-selected force on the bottom hole assembly  120  such that the rate of movement or drilling can be controlled. The tractor  112  can be used to move various types of equipment through the borehole  132 . For example, it will be understood that the tractor  112  can be connected with or include, without limitation, a downhole motor (for rotating a drill bit), steering system, instrumentation sub (an instrumented package that controls various aspects of downhole operation, including shock vibration, weight on bit, torque at bit, rate of penetration, downhole motor rpm, and differential pressure across motor), Measurement While Drilling apparatus (an apparatus for measuring gyroscopic data such as azimuth, inclination, and measured depth), drill bit, mechanical and hydraulic disconnect for intervention, jetting tools, production logging tools (including apparatus for measuring and recording, without limitation, temperature, annulus pressure, and various flow rates), drilling logging tools (for measuring and recording, without limitation, resistivity measurements, magnetic resonance (MRI), sonic neutron density, density, fluid identification, and gamma ray measurements), perforation guns, casing collar locators, and torque limiting tools (for drilling).  
         [0024]     A closed loop configuration has relevant differences from an open loop system that operates on differential pressure (the difference in pressure between the bore of the tractor and the exterior of the tractor). With an open system, a restriction in the system is required to produce a pressure difference (decrease) between the interior and exterior of the tractor. Typically, the restriction is an orifice such as a fixed diameter nozzle, and is not capable of being adjusted from the surface. For typical coiled tubing rig operations, the effective means of control is to control the surface pump output flow rate. However, the differential pressure available at the tractor is a quadratic (non-linear) function of the surface pump output flow rate. Thus, doubling the surface pump output flow rate will increase the differential pressure through an in-series fixed orifice by a factor of four. This makes power control of the tractor more difficult as normal operational changes can have non-linear impact on tractor power, requiring additional features to be incorporated into the open loop powered tractor to restrict the amount of pressure delivered to the gripper assemblies, for example. Further, this has a disadvantage in that the normal operating range of the surface pump output flow rate required for various operations may have to be restricted, thus reducing cleaning efficiency during the operation.  
         [0025]     Described below are four embodiments of closed loop power systems for powering a tractor: (1) a turbine-powered motor, (2) an E-line powered motor, (3) a turbine-powered pump; and (4) a pump powered by a positive displacement motor. Any one of these configurations can be used for circulating a given tractor&#39;s closed system fluid (e.g., hydraulic fluid) through the tractor&#39;s valve system, gripper assemblies, and propulsion assemblies. The difference between these configurations is how power is delivered to the downhole pump that circulates the fluid.  
         [0000]     Turbine-Powered Motor  
         [0026]      FIG. 2  is a schematic illustration of a turbine-powered motor for circulating hydraulic fluid in a closed loop for powering a downhole tool or tractor, according to an embodiment of the present invention. In this configuration, a first fluid (typically drilling/intervention fluid) that is externally pumped into the coiled tubing usually at the ground surface flows through the tractor and passes through a turbine  150  on its way to the remaining bottom hole assembly (typically secured to the distal end of the tractor). The turbine  150  drives a generator  152  that produces electricity, as known in the art of turbine power generation. The electricity produced by the generator  152  powers an electric motor  154  that in turn powers a pump  156 . The pump  156  circulates a second fluid (typically hydraulic fluid) in a closed system loop  155 . Box  158  represents a valve system, gripper assemblies, and propulsion assemblies as known in the art. For example, the valve system, gripper assemblies, and propulsion assemblies can be substantially as shown and described in U.S. Pat. Nos. 6,003,606; 6,347,674; 6,241,031; and 6,679,341, as well as U.S. Patent Application Publication No. 2004/0168828. Also, the gripper assemblies can be substantially as shown and described in U.S. Pat. Nos. 6,464,003 and 6,715,559; U.S. Patent Application Publication No. 2005/0247488; and U.S. Provisional App. No. 60/781,885. The second fluid provides hydraulic force for operation of the gripper assemblies and propulsion assemblies, and in some cases the valves.  
         [0027]     Commercially available turbine-generators are sold by Spring Electronics of Worcestershire, United Kingdom. One turbine-generator sold by Spring Electronics comprises a three-phase alternator, rectifier, and switch mode power supply producing about 70 Watts at 50 volts. Larger versions of turbine-generators are commercially available.  
         [0028]      FIG. 3  is a more detailed schematic illustration of the closed loop system of  FIG. 2  adapted for use with a variation of the Puller-Thruster Downhole Tool (also referred to as “Puller Thruster Assembly” or “PTA”) described in U.S. Pat. No. 6,003,606. As the first fluid is pumped through the turbine  150 , the turbine powers the motor  154  and in turn the pump  156  that circulates the second fluid through the illustrated valve assembly. The second fluid flows from a supply line  228  through a start/stop valve  160  (also known as an “idler valve”) into the valve system. A six-way control valve  162  shuttles back and forth to direct the fluid to and from an aft gripper assembly  180  (illustrated as a deflated packerfoot) and a forward gripper assembly  182  (illustrated as an inflated packerfoot), and also to and from an aft propulsion assembly  184  and a forward propulsion assembly  186  (each propulsion assembly comprising barrels and internal pistons, as taught in the &#39;606 patent). Valves  164  and  166  (also known as “directional control valves”) control the shuttling and position of the six-way control valve  162 . Packerfeet valves  168  and  170  regulate the flow of fluid into the packerfeet  180  and  182 . A reverser valve  172  controls the direction of tractor movement (i.e., uphole or downhole). The operation of these valves is understood from the teachings of the aforementioned patents incorporated by reference. A sump  157  is preferably provided to store a reservoir of the second fluid. The circulating second fluid returns to the sump  157  via a return line  230 .  
         [0029]      FIG. 3  shows an embodiment of a tool  200  (illustrated as a Puller-Thruster Assembly) positioned within a drilled hole  205  inside a rock formation  212 . The tool  200  includes an elongated body formed of central coaxial cylinders  207 . The aft gripper assembly  180 , aft propulsion assembly  184 , forward gripper assembly  182 , and forward propulsion assembly  186  are engaged on the central coaxial cylinders  207 . The aft propulsion assembly  184  includes annular pistons  218  secured to the cylinders  207 . Similarly, the forward propulsion assembly  186  includes annular pistons  220  secured to the cylinders  207 . The number of pistons can vary (e.g., up to 20 pistons) and depends on the desired thrust and pull loads.  
         [0030]     The tool body defines an internal mud flow passage  224  inside the cylinders  207 . The aft end of the tool body has an inlet  201  connected to coiled tubing  114  via a coiled tubing connector  206  (connection can be threaded or snapped together). While  FIG. 3  shows coiled tubing  114 , the tool  200  can also be used with rotary drill rigs instead (and the same is also true for the embodiments of  FIGS. 4-9 ). The forward end of the tool body is connected to a bottom hole assembly (BHA)  204 . The illustrated tool includes a female coiled tubing connector  208  and stabilizers  210 . The valve control pack  214  is positioned between the forward and aft gripper assemblies and also between the forward and aft propulsion assemblies. Splines  216  can optionally be incorporated between the central coaxial cylinders  207  and the gripper assemblies to prevent the transmission of torque from the BHA  204  to the coiled tubing  114 .  
         [0031]     In use, drilling/intervention fluid flows from the coiled tubing  114  into the inlet  201  of the tool body, and downhole (toward the bottom of the hole) through the mud flow passage  224 . The fluid flows through the turbine  150 , turning the motor  154 . The fluid continues through the passage  224  into the BHA  204 , exiting the BHA  204  through an outlet  203 . The inlet  201  and outlet  203  are also shown in relation to the turbine  150  on the bottom right hand side of  FIG. 3 . The drilling/intervention fluid that exits via the outlet  203  then flows uphole to the ground surface through an annulus defined between the tool  200  and the drilled hole  205 .  
         [0032]     The upper right hand side of  FIG. 3  includes a cross-sectional view of the inflated packerfoot  182 , taken along line A-A. The illustrated packerfoot  182  includes three inflated sections. Three mud flow return paths  222  are defined between the three inflated sections of the packerfoot. These return paths  222  allow drilling fluid that exits via the outlet  203  to flow back uphole past the inflated packerfoot. It will be understood that the aft packerfoot  180  can be substantially identical to the forward packerfoot  182 . The illustrated packerfoot cross section shows the packerfoot inflated radially beyond the outside diameter  226  of the tool  200 .  
         [0033]     An advantage of the system using a turbine-powered motor as illustrated is that the system is flow-based, meaning that the downhole tractor can be more easily controlled by the surface pump that pumps fluid down into the coiled tubing toward the turbine. With a flow-based system, any change in the surface pump output volume flow rate linearly changes the power available to the tractor. Since the surface pump output flow rate can be relatively easily adjusted dynamically during tractor operation, the resulting adjustment of the power to the tractor provides enhanced control over the tractor&#39;s speed and pulling force. This enhanced control is available over a substantial operating range of surface pump output flow rates. This is convenient for some types of operations. For example, during sand washing it is desirable to provide a maximum amount of fluid into the borehole while the tractor continues its forward movement, usually at near-maximum pulling capacity.  
         [0034]     While the illustrated turbine-powered motor system disclosed in  FIGS. 2 and 3  offers enhanced control over prior systems, one limitation of the system is a loss of efficiency. With each energy conversion, the overall machine efficiency is reduced. For example, the conversion from fluid flow of the drilling/intervention fluid in the coiled tubing into mechanical rotation of the turbine results in some energy loss. Similarly, the conversion of mechanical rotation of the turbine into electrical power from the generator also results in some energy loss.  
         [0035]     Another limitation of the turbine-powered pump system is that the turbine requires relatively high flow rates to generate significant amounts of electrical power. For some tractor operations, such as delivering perforation guns, it may be desirable to limit the amount of flow that gets delivered to the bottom hole assembly, for environmental protection reasons. For these types of applications, a turbine-powered motor system may be less preferable than other embodiments disclosed herein.  
         [0000]     E-Line Powered Motor  
         [0036]      FIG. 4  is a schematic illustration of an E-line powered motor for circulating hydraulic fluid in a closed loop for powering a downhole tool or tractor, according to an embodiment of the present invention. In this configuration, an E-line  190  preferably extends from the tractor upward to a control box  191 , typically located at the ground surface. As used herein, “control box” is a broad term and incorporates a wide variety of controls, including controls in a very small housing and those including wireless features. The illustrated E-line  190  extends to the downhole electric motor  154  that in turn powers the pump  156 , it being understood that the motor  154  and pump  156  are preferably housed within or on the tractor. Compared to the embodiment of  FIGS. 2 and 3 , this embodiment does not include a turbine. The control box  151  preferably includes at least a portion of an electronic control system adapted to send electrical control signals through the E-line  190  for powering and controlling the motor  154 . The pump  156  circulates a fluid (typically hydraulic fluid) in a closed system loop  155 . Box  158  represents a valve system, gripper assemblies, and propulsion assemblies as known in the art and preferably as described above.  
         [0037]     In one embodiment, the E-line  190  is provided within coiled tubing that also delivers a fluid to the tractor in an open system loop. For example, in drilling operations it is typically desirable to deliver fluid to the drill bit to lubricate the bit and carry drill cuttings back up to the ground surface through the annulus between the borehole inner surface and the exterior of the tractor. In other operations, it may be desirable to deliver an intervention fluid to the bottom hole assembly (e.g., sand washing, acidizing, hole cleaning, etc.). The drilling or intervention fluid preferably passes through an internal passage of the tractor to the bottom hole assembly, and then flows up through the annulus.  
         [0038]     In an alternative embodiment, the E-line  190  is provided within a wireline that does not include a lumen for the delivery of fluid. In other words, there is no coiled tubing. In this embodiment, the tractor is completely electrically powered and controlled. This configuration is useful for operations that do not require the delivery of fluid into the borehole, for example logging operations.  
         [0039]      FIG. 5  is a more detailed schematic illustration of the closed loop system of  FIG. 4  adapted for use with the variation of the Puller-Thruster Downhole Tool shown in  FIG. 3 . The E-line  190  provides power and electrical control for the motor  154 , which in turn powers the pump  156  that circulates a fluid (typically hydraulic fluid) in a closed loop through the illustrated valve assembly. The E-line  190  extends along with the coiled tubing  114  for delivering drilling/intervention fluid to a BHA  204 . As noted below, other embodiments omit the coiled tubing  114  and only provide a wireline. In use, drilling/intervention fluid flows from the coiled tubing  114  into the inlet  201  of the tool body, and downhole (toward the bottom of the hole) through the mud flow passage  224 . The fluid flows into the BHA  204  and ultimately exits the BHA  204  through the outlet  203 . The drilling/intervention fluid that exits via the outlet  203  then flows uphole to the ground surface through an annulus defined between the tool  200  and the drilled hole  205 .  
         [0040]     An advantage of an E-line powered motor as described herein is that the tractor&#39;s performance is independent of any fluid flow pumped down to the tractor from a ground surface pump. In the illustrated embodiment, the power to operate the tractor comes from surface electricity. Hence, the tractor is completely controllable with electrical power transmission and control equipment. The power can be delivered to the motor via an E-line or wireline, without using any coiled tubing. Advantageously, for operations that do not require an intervention or drilling fluid (e.g., logging), the tractor can be operated with wireline equipment alone. This makes the system easily transportable because the costs and time associated with assembly and disassembly of coiled tubing equipment are completely circumvented. Thus an advantage of the disclosed embodiment is the ability to be rapidly deployed.  
         [0041]     The disclosed system is useful for a variety of operations. For example, the disclosed configuration is useful if multiple tractors are employed in series, as may be necessary to traverse a “washout” in the borehole. A washout is a portion of the borehole having a relatively larger diameter than the rest of the borehole. The washout diameter may be larger than the expansion capability of the tractor&#39;s gripper assemblies, making it impossible to grip the borehole wall within the washout. However, the washout can be traversed if two tractors are employed in series and both tractors employ a closed loop hydraulic fluid circuit powered by an E-line and electric motor as disclosed above. In particular, the first tractor&#39;s motor can be electrically powered until the first tractor encounters the washout, at which point its gripper assemblies are unable to contact the borehole wall. When this condition is detected, power to the first tractor&#39;s motor can be turned off and power to the second tractor&#39;s motor can be turned on. The second tractor will then move until it encounters the washout, at which point the second tractor can be turned off and the first tractor again turned on to resume movement in a portion of the borehole having a contactable hole diameter. It will be understood that the separation between the tractors typically controls the maximum length washout that can be traversed. Separate E-lines can be provided for each tractor. Alternatively, a single E-line and a downhole control system can be provided to control which tractor receives the electrical power. In some operations, it may be desirable to simultaneously power both tractors.  
         [0042]     Even in embodiments in which the E-line is provided within coiled tubing, skilled artisans will recognize that the fluid delivery through the coiled tubing can be selectively provided or shut off (simply by turning on or off the surface pumps) depending upon the type of operation conducted by the tractor. For operations that require tractor movement but do not require fluid for other purposes (e.g., logging), tractor control becomes easier and less expensive due to the ability to shut off the fluid delivery through the coiled tubing.  
         [0043]     Another advantage of the E-line powered motor system, compared to the turbine-powered motor system of  FIGS. 2 and 3 , is that there is no efficiency loss associated with converting turbine rotation into electricity with a generator, or in converting a fluid flow into motor rotation. The motor is controlled entirely electrically. Still another advantage of the E-line powered motor system, compared to the turbine-powered motor system, is that it is possible to generate significant amounts of electrical power without any fluid input to the tractor, let alone an undesirably high rate of fluid input. As mentioned above, in certain tractor operations, such as delivering perforation guns, it is desirable to limit fluid flow to the bottom hole assembly. In these applications, an E-line powered motor system may be preferable.  
         [0044]     While the illustrated E-line powered motor system may be more efficient in some cases than the turbine-powered motor system described above, the E-line system nonetheless still involves some energy losses associated with the transmission of electrical power through the E-line, as well as efficiency loses in the electric motor and the downhole pump.  
         [0045]     In one embodiment, the control box  191  comprises a power supply, switches, connectors, displays (e.g., LED, other types of lights), a NEMA (National Electrical Manufacturers Association) box, and electrical wires. In this embodiment, the control box  191  is designed for simple on/off toggling for the delivery of power to the motor  154 . In addition, various types of power regulation devices may be included, such as a rheostat to adjust power to the tractor and hence tractor speed.  
         [0046]     In another embodiment, in addition to using the E-line  190  to deliver power and control signals to the motor  154 , the control box  191  delivers power and control signals through the E-line  190  to other components of the tractor. The control box  191  can also receive signals from such other components and use the received signals to make control decisions. In this embodiment, the control box  191  preferably comprises a power supply, electrical switches, electrical connectors, power converters, a computer server or personal computer with CPU board, display panel, data storage capability, user interface (preferably graphical), software operating system, high speed mouse, and keyboard. Software for running the control box  191  can be custom-developed. Alternatively, the software can be a modification of a commercially available program (such as “Lab View” made by National Instruments of Austin, Tex.).  
         [0047]     In this embodiment, the control box  191  can deliver electrical power and control signals through the E-line  190  to various instruments, tools, and apparatuses on the tractor. The control box  191  can also be configured to present and store data collected from such instruments, tools, and apparatuses. For example, for intervention and completion operations, the tractor can include logging tools (e.g., pressure sensors, flow rate sensors, and temperature sensors), casing collar collectors, and/or gyroscopic-based positioning instruments electrically connected to the control box  191  through the E-line  190 . As another example, for drilling operations, the tractor can include a Measurement While Drilling apparatus (e.g., for measuring inclination, azimuth, and depth), tension compression sub, instrumented downhole drilling motor, and/or Logging While Drilling apparatus (e.g., drilling logging tools for detecting resistivity, magnetic resonance (MRI), sonic, neutron density, density, fluid identification, gamma ray measurements) electrically connected to the control box  191  through the E-line  190 . Furthermore, sensors such as speedometers, temperature sensors, pressure sensors and the like can be included within the tractor and in electrical communication with the control box  191  through the E-line  190 .  
         [0000]     Turbine-Powered Pump  
         [0048]      FIG. 6  is a schematic illustration of a turbine-powered pump for circulating hydraulic fluid in a closed loop for powering a downhole tool or tractor, according to an embodiment of the present invention. In this configuration, a first fluid (typically drilling/intervention fluid) that is externally pumped into the coiled tubing at the ground surface flows through the tractor and passes through a turbine  150  on its way to the remaining bottom hole assembly (typically secured to the distal end of the tractor). The flow through the turbine  150  produces rotation of the turbine&#39;s output shaft, which drives the pump  156  through a gearbox  192 . The pump  156  circulates a second fluid (typically hydraulic fluid) in a closed system loop  155 . Box  158  represents a valve system, gripper assemblies, and propulsion assemblies as known in the art and preferably as described above.  
         [0049]      FIG. 7  is a more detailed schematic illustration of the closed loop system of  FIG. 6  adapted for use with the variation of the Puller-Thruster Downhole Tool shown in  FIG. 3 . As the first fluid is pumped through the turbine  150 , the turbine output shaft rotates to power the pump  156  via the gearbox  192  (not shown), and the pump  156  in turn circulates the second fluid through the illustrated valve assembly. In use, drilling/intervention fluid flows from the coiled tubing  114  into the inlet  201  of the tool body, and downhole (toward the bottom of the hole) through the mud flow passage  224 . The fluid flows through the turbine  150 , powering the pump  156 . The fluid continues through the passage  224  into the BHA  204 , exiting the BHA  204  through the outlet  203 . The inlet  201  and outlet  203  are also shown in relation to the turbine  150  on the bottom right hand side of  FIG. 7 . The drilling/intervention fluid that exits via the outlet  203  then flows uphole to the ground surface through an annulus defined between the tool  200  and the drilled hole  205 .  
         [0050]     A relevant advantage of using a turbine-powered pump as illustrated is that the system is flow-based, as described above. In other words, the downhole tractor can be more easily controlled by the surface pump that pumps fluid down into the coiled tubing toward the turbine. With a flow-based system, any change in the surface pump output volume flow rate linearly changes the power available to the tractor. Since the surface pump output flow rate can be relatively easily adjusted dynamically during tractor operation, the resulting adjustment of the power to the tractor provides enhanced control over the tractor&#39;s speed and pulling force. This enhanced control is available over a substantial operating range of surface pump output flow rates.  
         [0051]     Another relevant advantage of the turbine-powered pump system is that the downhole pump is desirably directly powered by the rotating output of the turbine/gearbox combination, without any intermediate steps (e.g., electrical power generation from the turbine output, and use of such electrical power to drive an electric motor that drives the pump). As explained above, the provision of such intermediate steps can introduce a risk of a loss of efficiency in converting the kinetic energy of the first fluid pumped into the turbine into power for driving the operation of the downhole pump. While the turbine-powered pump system still involves some efficiency losses associated with converting the first fluid&#39;s flow into mechanical rotation of the turbine, the disclosed turbine/gearbox combination advantageously provides a highly efficient conversion of the first fluid&#39;s kinetic energy.  
         [0000]     Pump Powered by Positive Displacement Motor  
         [0052]      FIG. 8  is a schematic illustration of a pump powered by a positive displacement motor (PDM) for circulating hydraulic fluid in a closed loop for powering a downhole tool or tractor, according to one embodiment of the present invention. In this configuration, a first fluid (typically drilling/intervention fluid) that is externally pumped into the coiled tubing typically at the ground surface flows through the tractor and passes through a positive displacement motor  250  (sometimes referred to as a “mud motor”) on its way to the remaining bottom hole assembly (typically secured to the distal end of the tractor). The flow through the positive displacement motor  250  produces rotation of the motor&#39;s output shaft  251 , which drives the pump  156 , typically through a gearbox  252 . The pump  156  circulates a second fluid (typically a different type of fluid than the first fluid, such as, for example, hydraulic fluid) in a closed system loop  155 . Box  158  represents a valve system, gripper assemblies, and propulsion assemblies as known in the art and preferably as described above.  
         [0053]     Positive displacement motors are well known. A positive displacement motor typically comprises a stator that defines a fluid flow enclosure, a rotor that revolves within the stator, and an output shaft that rotates with the rotor. The rotor typically includes a plurality of lobes, i.e., curved or rounded projections that absorb the kinetic energy of fluid flowing through the stator, causing the rotor to revolve within the stator. Numerous suppliers sell positive displacement motors in a wide variety of sizes and performance capabilities. For example, Weatherford&#39;s (www.weatherford.com) “High Performance PDM” and a “MacDrill High Temperature PDM” are suitable, as is the “Navi-Drill Ultra Series” motors sold by Baker Hughes (www.bakerhughes.com). Positive displacement motors are also sold by numerous smaller suppliers, and are commercially available in small diameter sizes that produce significant torque at acceptable RPM levels.  
         [0054]      FIG. 9  is a more detailed schematic illustration of the closed loop system of  FIG. 8  adapted for use with the variation of the Puller-Thruster Downhole Tool shown in  FIG. 3 . As the first fluid is pumped through the positive displacement motor  250 , the motor&#39;s output shaft  251  rotates to power the pump  156  via the gearbox  252 , and the pump  156  in turn circulates the second fluid through the illustrated valve assembly. In use, drilling/intervention fluid flows from the coiled tubing  114  into the inlet  201  of the tool body, and downhole (toward the bottom of the hole) through the mud flow passage  224 . The fluid flows through the positive displacement motor  250 , which drives the pump  156  through the gearbox  252 . The fluid continues through the passage  224  into the BHA  204 , exiting the BHA  204  through the outlet  203 . The inlet  201  and outlet  203  are also shown in relation to the positive displacement motor  250  on the bottom right hand side of  FIG. 9 . The drilling/intervention fluid that exits via the outlet  203  then flows uphole to the ground surface through an annulus defined between the tool  200  and the drilled hole  205 .  
         [0055]     A relevant advantage of using a pump  156  powered by a positive displacement motor  250  as illustrated is that the system is flow-based, as described above. In other words, the downhole tractor can be more easily controlled by the surface pump that pumps fluid down into the coiled tubing  114  toward the motor  250 . With a flow-based system, any change in the surface pump output volume flow rate linearly changes the power available to the tractor. Since the surface pump output flow rate can be relatively easily adjusted dynamically during tractor operation, the resulting adjustment of the power to the tractor provides enhanced control over the tractor&#39;s speed and pulling force. This enhanced control is available over a substantial operating range of surface pump output flow rates. The pump  156  powered by a positive displacement motor  250  also allows an operator to more quickly and easily shut off the tractor simply by stopping the pumping of the open system fluid down through the coiled tubing  114  to the motor  250 , or by reducing the fluid&#39;s flow rate to a level that is less than a level required to maintain operation of the pump  156 .  
         [0056]     Another advantage of a positive displacement motor  250  is that several design aspects of the motor can be varied to allow some tuning of the expected operational torque and RPM delivered to the gearbox  252 . Design aspects that can be varied include the rotor pitch angle, the number of stages, and the number of lobes of the rotor. This makes it easier to optimize the range of operation of the pump  156 . Still another advantage is that positive displacement motors are a proven, reliable, and relatively inexpensive technology for utilizing the kinetic energy of a fluid.  
         [0057]     Yet another advantage of this system is that the pump  156  can be directly powered by the rotating output shaft  251  of the motor/gearbox combination, without any intermediate steps (e.g., electrical power generation from the motor output, and use of such electrical power to drive an electric motor that drives the pump). The provision of such intermediate steps would introduce a risk of a loss of efficiency in converting the kinetic energy of the first fluid pumped through the positive displacement motor  250  into power for driving the operation of the pump  156 . The disclosed motor/gearbox combination advantageously provides a highly efficient conversion of the first fluid&#39;s kinetic energy.  
         [0058]     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications thereof. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above.