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FIELD 
     Embodiments described relate to oilfield well operations. In particular, applications for cutting and removing a well access line from a well that has been stuck downhole for any number of reasons. The well access line may be wireline, slickline, coiled tubing or any of a host of downhole conveyance mechanisms, generally with a tool or toolstring disposed at a downhole end thereof. 
     BACKGROUND 
     Exploring, drilling, completing, and operating hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. In recognition of these expenses, added emphasis has been placed on well access, monitoring and management throughout its productive life. Well intervention and ready access to well information may play critical roles in maximizing the life of the well and total hydrocarbon recovery. As a result, downhole tools are frequently deployed within a given hydrocarbon well throughout its life. These tools may include logging tools to provide well condition information. Alternatively, these tools may include devices for stimulating hydrocarbon flow, removing debris or scale, or addressing a host of other well issues. 
     The above noted downhole tools are generally delivered to a downhole location by way of a well access line. A well access line may include a wireline or slickline cable, coiled tubing, and other forms of downhole conveyance line. Regardless, once delivered downhole, a well application may proceed employing the tool. Subsequently, a winch-driven drum at the surface of the oilfield may be used to withdraw the well access line and tool from the well. Unfortunately, however, the well access line and/or tool often become stuck in place downhole. This may be due to the presence of an unforeseen obstruction, unaccounted for restriction, differential sticking of the tool against the well wall, or a host of other reasons. 
     In the case of wireline cable, a weak-point is generally built into the cable head where the tool and cable are joined. Thus, when sticking occurs, the winch may continue to pull uphole on the line until a break occurs at the weak-point. Subsequently, a fishing operation may ensue to retrieve the stuck tool from the well. Unfortunately, slickline, coiled tubing, and other conveyances often lack a built-in weak-point. Thus, at best, continued pulling on the line will only result in an uncontrolled break, generally nearer the oilfield surface. Such an uncontrolled break may leave the well obstructed by thousands of feet of line that will only add to the time, effort, and expense of the follow-on fishing operation. Furthermore, even where a weak-point is built into the assembly, break failure of the weak-point often occurs. This may be due to a design or manufacturing flaw, or other reasons. Regardless the reason, failure of the weak-point to break may result in an uncontrolled break as noted above. 
     In the case of wireline or other non-tubing conveyances, cutting bars are often employed in an attempt to avoid uncontrolled breaking of the line. A cutting bar is a pipe equipped with an internal cutting mechanism. The bar may be positioned over the line and dropped vertically into the well. In theory, the bar will drop until it reaches the sticking location, at which point the sudden stopping of the bar will actuate the cutting mechanism and induce a break in the line. 
     Unfortunately, employing a cutting bar may still result in breaking the line at a location uphole of the sticking location. This is due to the fact that the described cutting bar technique proceeds blindly. So, for example, in the case of a deviated well, the cutting bar will stop dropping and cut the line as soon as a bend or deviation is encountered which may be nowhere near the targeted sticking location. Similarly, a slight narrowing in the well, or minimal obstruction unrelated to the sticking of the line, may be enough to stop the fall of the cutting bar. Either way, the cutting bar may stop uphole of the sticking location, induce a break in the line, and add tremendous time and expense to the follow-on fishing operation. 
     As an alternative to the cutting bar, a timed cutter may be deployed within the well. That is, a cutter equipped with a cutting mechanism that is activated based on a timer may be dropped into the well. In this way, temporary stopping of the cutter, for example, upon encountering a minor obstruction, may not result in activation of the cutting mechanism. Rather, the cutting mechanism may be activated only after a set period of time, presumably after bypassing any such minor temporary obstructions. 
     Unfortunately, the use of a timed cutter fails to overcome uncontrolled line breaks in circumstances of deviated wells or in the face of significant well obstructions. In such cases, the activation of the cutting mechanism is still likely to take place well uphole of the sticking location. That is, the mode of cutting remains blind and thus, susceptible to breaking the line well uphole of the targeted sticking location. Furthermore, in the case of coiled tubing, similar cutting mechanisms may be employed that generally involve the initial deployment of a cable interior of tubing so that follow-on cutting techniques may be carried out. However, such techniques remain blind and susceptible to inducing coiled tubing breaks uphole of the targeted sticking location. In fact, in the case of coiled tubing, the cutting techniques generally require cutting of the coiled tubing at the location of the drum in order to deploy the interior cable. As a result, large amounts of coiled tubing are rendered ineffective for future use. Thus, in many cases, the operator may ultimately be left with no better option than to run a blind attempt at cutting the line which runs a significant likelihood of adding several hundred thousand dollars of expense to future fishing and other operations. 
     SUMMARY 
     A cutting tool is provided for cutting a well access line downhole in a well. The tool includes a housing which accommodates an active propulsion mechanism for driving the tool along the well access line to a cut location thereof. A cutting mechanism is also accommodated by the housing in order to achieve cutting of the well access line at the cut location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side overview of an oilfield with an embodiment of a cutting tool thereat for cutting a non-tubular well access line of a tool stuck in a well. 
         FIG. 2  is a side cross-sectional view of the cutting tool of  FIG. 1 . 
         FIG. 3A  is a side cross-sectional view of the cutting tool of  FIG. 2  dropped into the well of  FIG. 1  about the well access line therein. 
         FIG. 3B  is a side cross-sectional view of the cutting tool of  FIG. 2  striking a bend in the well of  FIG. 1 . 
         FIG. 3C  is a side cross-sectional view of the cutting tool of  FIG. 2  propelling along the well access line in a lateral section of the well of  FIG. 1 . 
         FIG. 4  is a side cross-sectional depiction of the cutting tool of  FIG. 2  interfacing a cable head of the tool of  FIG. 1 . 
         FIG. 5  is an enlarged view of the cutting tool taken from  5 - 5  of  FIG. 4  depicted cutting the well access line of  FIG. 1 . 
         FIG. 6  is a side overview of the oilfield of  FIG. 1  with the cutting tool and well access line retrieved from the well thereof. 
         FIG. 7  is a depiction of an alternate embodiment of a cutting tool for cutting a well access line in the form of coiled tubing. 
         FIG. 8  is a flow-chart summarizing embodiments of employing cutting tools as described in  FIGS. 1-7  for cutting well access lines in a well. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are described with reference to certain downhole tool operations at an oilfield. For example, primarily wireline based tractor driven logging operations are described throughout. However, alternate downhole operations employing different types of well access line, including coiled tubing, may utilize embodiments of cutting tools as described herein. Of particular note, these cutting tool embodiments may be equipped with a propulsion mechanism configured to actively drive the cutting tools along the well access line to a deliberately targeted cut location. 
     Referring now to  FIG. 1 , a side overview of an oilfield  105  is shown with a well  180  running through a formation  190  thereat. The well  180  includes a vertical section  181  that transitions into a lateral section  182  as it rounds a bend  195 . In the embodiment shown, a downhole logging tool  130  is driven through the well  180  by way of a downhole tractor  120  to obtain diagnostic information relative to the well  180 . For example, pressure, temperature, flow and other readings may be obtained through such an application. 
     The above noted tractor  120  and logging tool  130  are delivered to the depicted downhole location by way of a well access line in the form of a wireline cable  110 . The wireline cable  110  may provide telemetric and powering capacity between the tractor  120  and/or logging tool  130  and surface equipment, such as a processing unit  178  and power unit  179 . As shown, the wireline cable  110  is delivered to the oilfield  105  by way of a wireline truck  175  accommodating the noted equipment along with a drum  177  about which the wireline cable  110  is wound. Additionally, as described in greater detail below, a cutting tool  100  is provided in the event that that the logging tool  130  and/or tractor  120  become stuck downhole in the well  180 . 
     The wireline cable  110  is run from the drum  177  to a rig  150  where it is strung about sheaves  152 ,  154  and ultimately directed through well access and regulation equipment  155 , often referred to as a ‘Christmas tree’. This equipment  155  includes blowout prevention and other valve mechanisms to allow for the coupling downhole tools  120 ,  130  to a cable head  115  at the end of the cable  110 . Such tools  120 ,  130  may then be advanced through the well  180 . Indeed, as shown in  FIG. 1 , the tractor  120  may be employed to drive the logging tool  130  to the location shown. Thus, the cable  110  traverses the well  180 , eventually terminating at the in the lateral section  182  thereof. 
     However, in the embodiment of  FIG. 1 , the logging tool  130  is shown stuck in debris  197 . In certain circumstances, this sticking may reach a point that the combined efforts of the tractor  120  and winch-powered drum  177  remain unable to dislodge the logging tool  130 . Thus, cutting of the cable  110  followed by fishing out of the downhole tools  120 ,  130  may be in order. However, in cutting the cable  110 , it may be of significance that the cut take place as close to the cable head  115  as possible. In this manner, the well  180  may be substantially free of cable  110  during the subsequent fishing operation. Therefore, in order to help ensure that the cable  110  is cut close to the cable head  115 , the cutting tool  100  may be positioned about the cable  110  and directed into the well  180  toward the cable head  115  as detailed herein-below. 
     With added reference to  FIG. 2 , a side cross-sectional view of the cutting tool is depicted. The cutting tool  100  is equipped with a line or cable space  215  running there-through to allow the tool  100  to be positioned about the cable  110  and dropped into the well  180 . A blade  240  for cutting the cable  110  is provided for use once the tool  100  is properly positioned downhole. Along these lines the tool  100  is also equipped with an active propulsion mechanism in order to help properly position the tool  100  for the cutting. That is, as shown, the tool  100  includes wheels  200  disposed at the end of extension arms  201 . Thus, at the appropriate time, the wheels  200  may grab onto the cable  110  in the space  215  and drive the tool  100  to the proper downhole location for cutting. 
     Continuing with reference to  FIG. 2 , the above noted propulsion mechanism is housed within a main housing  250  of the tool  100  along with a clamping mechanism  230  as described further below. Additionally, a power source  225  and locator housing  275  are each coupled to the main housing  250 . The power source  225  may be a conventional battery such as an off-the-shelf lithium battery casing. In one embodiment, up to about 12 volts of power may be provided to the propulsion mechanism from the power source  225  so as to adequately drive the tool  100  downhole as described. Also, as detailed below, the clamping mechanism  230  may be activated to secure the tool  100  to the cable  110  in advance of the cutting thereof. Actuation of this clamping may be powered by the power source  225  or mechanically. Regardless, once clamping of the cable  110  is achieved at the location of the clamping mechanism  230 , cutting of the cable  110  downhole thereof will result in securing of the tool  100  to a portion of the cable  110  that is now retractable about the drum  177  at surface. 
     The above noted locator housing  275  may house a locator mechanism such as bearings  277  which are biased by springs  278 . As described below, the locator housing  275  may interface a cable head  115  as the tool  100  reaches a targeted location for cutting the cable  110 . As this interfacing of the locator housing  275  and the cable head  115  occurs, the bearings  277  may be laterally displaced in a manner that effects compression of the springs  278 . In the embodiments described herein-below, this compression of the springs  278  may be utilized as an indicator of tool location. Thus, signaling may be sent by conventional means throughout the tool  100  indicative of tool location. For example, spring compression may be employed as a trigger for actuation of the clamping mechanism  230 , immediately followed by actuation of the cutting of the cable  110  by the blade  240 . 
     As shown in  FIG. 2 , the blade  240  is retained within a chamber  242  by a membrane  450  (see  FIG. 4 ). However, once the tool  100  reaches the cutting location as indicated by the above-noted interfacing, the blade  240  may be fired from the chamber  242  to achieve cutting of the cable  110 . That is, a firing mechanism  244  such as an explosive charge, compressed gas or other conventional source may be employed to fire the blade  240  toward the cable  110  in order to attain cutting thereof. Once this process occurs as detailed below, the cable  110  with tool  100  clamped thereto may be retrieved from the well  180  and a follow-on fishing operation may ensue for retrieval of the cable head  115  and other downhole tools  120 ,  130 . 
     Referring now to  FIGS. 3A-3C , enlarged depictions of the cutting tool  100  making its way down the well  180  and through tortuous sections thereof are shown in greater detail. Of note is the fact that the tool  100  is guided through such well sections without prematurely triggering cutting of the cable  110 . Rather, as traversing the well  180  becomes more challenging, the propulsion mechanism is employed to drive the tool  100  therethrough and toward a proper cut location as shown in  FIG. 4 . 
     With particular reference to  FIG. 3A , the cutting tool  100  is shown dropped through the vertical section  181  of the well  180 . At this point, the tool  100  freely drops with the cable  110  running through the cable space  215 . There is no engagement of the clamping mechanism  230  or the wheels  200  relative to the cable  110 . Indeed, in the embodiment shown, the tool  100  traverses the vertical section  181  of the well  180  without draining any power from the power source  225  (see  FIG. 2 ). 
     As shown in  FIG. 3B , the tool  100  eventually reaches the bend  195  in the well  180 . In the embodiment shown, the impact of reaching the bend  195  may act as a trigger to activate the extension arms  201  of the propulsion mechanism. In this manner, the wheels  200  may engage the cable  110  and begin driving of the cutting tool  100  further through the well  180 . That is, as opposed to triggering a cut of the cable  110  as in the case of a conventional cutting tool, the impact of the sudden stoppage of the depicted cutting tool  100  is to activate engagement of the propulsion mechanism. That is, a conventional motion sensor  202 , best seen in  FIG. 2 , within the tool  100  may be employed to trigger engagement of the propulsion mechanism in lieu of cutting. Thus, premature cutting of the cable  110  may be avoided. 
     As shown in  FIG. 3C , the wheels  200  of the propulsion mechanism may be powered by the power source  225  sufficiently to drive the tool  100  around the bend  195  of  FIG. 3B . In fact, it is worth noting that no downhole powering of the tool  100  is generally required for dropping the tool  100  through the vertical section  181  of the well  180  or for removing the tool  100  from the well entirely (see  FIG. 6 ). Thus, a conventionally available battery pack may sufficiently serve as the only downhole power source  225  for driving the tool  100 . 
     Eventually, as depicted in  FIG. 4 , the cutting tool  100  may come to the cable head  115 . Thus, a targeted location for cutting of the cable  110  has been reached. That is, a cut of the cable  110  made while the cutting tool  100  interfaces the cable head  115  may avoid leaving any significant amount of cable  110  in the well  180  following the cutting and retrieval operation. As described above, the wheels  200  may act to drive the tool  100  to interface the cable head  115 . 
     As shown in  FIG. 4 , the cable  110  may terminate at an extension  400  of the cable head  115 . Thus, the extension  400  may be received by the locator housing  275  at the cable space  215  thereof. When this occurs, the bearings  277  may be displaced as described above such that the springs  278  are compressed. As such, locating of the tool  100  at the cut location may be communicated throughout the tool  100  by conventional means. In particular, clamping of the cable  110  by the clamping mechanism  230  may be initiated followed by actuation of cutting. As shown in  FIG. 5 , this may include firing of the blade  240  from the chamber  242  and through a retaining membrane  450  toward the cable  110 . Such firing may be achieved through a firing mechanism  244  as described above. In an alternate embodiment, however, firing may be actuated when the propulsion mechanism is prevented from continued downhole advancement (e.g. when sticking is uphole of the cable head  115 ). Nevertheless, the firing takes place following driving by the propulsion mechanism and thus, in a less blind manner than conventional cutting. 
     With reference to  FIG. 5 , an enlarged view taken from  5 - 5  of  FIG. 4  is shown, revealing the cutting of the cable  110  by the blade  240 . In this view, the membrane  450  of  FIG. 4  is eliminated as the blade  240  is fired from the chamber  242 . The firing results in the cutting of the cable  110  within the cable space  215  as defined by the main housing  250  of the tool  100 . Thus, while a small segment of cable  110  downhole of the cut may be left, the vast majority of the cable  110  is now free of any downhole sticking (see  FIGS. 1 and 6 ). 
     Referring now to  FIG. 6 , the drum  177  may be employed to remove the severed cable  110  from the well  180 . As such, the well  180  is cleared of any significant cable obstruction. With added reference to  FIG. 4 , the removal of the severed cable  110  also removes the cutting tool  100  from the well  180  due to the clamping of the clamping mechanism  230  about the cable  110 . By the same token, the engagement between the extension  400  and the locator housing  275  may be of a matching, however, not a locked fashion. Thus, pulling on the cable  110  by the winding drum  177  may be sufficient to disengage the extension  400  and locator housing  275  so as to allow cable  110  and cutting tool  100  removal from the well  180 . As such, follow-on fishing operations may proceed to remove the stuck downhole tools  120 ,  130  without concern over cable interference. 
     Referring now to  FIG. 7 , an alternate embodiment of a cutting tool  700  is shown. In this embodiment, the tool  700  is particularly configured for cutting well access line in the form of coiled tubing  710 . That is, due to the larger diameter and hallow nature of the coiled tubing  710 , the tool  700  is deployed within the tubing  710  as opposed to being deployed thereabout. In fact, the cutting tool  700  may be configured small enough to allow for introduction to the coiled tubing  710  at a coiled tubing reel at the surface of the oilfield  105 . In this manner, cutting of the coiled tubing  710  at the surface may be avoided, thereby salvaging potentially several thousand feet of tubing  710  for future use. 
     Continuing with reference to  FIG. 7 , the main housing  725  is coupled to a drop line  711  and positioned within the coiled tubing  710  as shown. In the embodiment shown, the line  711  may have power delivering capacity built therein so as to meet power requirements of the tool  700 . Additionally, given the generally unobstructed nature of a coiled tubing interior, pump assisted driving of the tool  700  may be employed. Indeed, the generally unobstructed nature of the coiled tubing  710  may make premature cutting due to locating error less of a concern. Nevertheless, the main housing  725  is equipped with a propulsion mechanism in the form of tracks  750  which extend outward and engage the interior walls of the coiled tubing  710 . As such, the tool  700  may be stably driven to the downhole cut location. 
     Similar to the cutting tool  100  of  FIGS. 1-6 , the tool  700  may be advanced through the coiled tubing  710  in a relatively passive manner. For example, depending on the architecture of the well  180 , pump assistance and gravity alone may be employed to drive the tool  700  through the majority thereof. However, motion sensing and/or other conventional mechanisms may also be employed such that the noted tracks  750  are deployed at some point in advance of the downhole cut location. 
     In one embodiment, the tool  700  is driven in this manner until a coiled tubing connector head is reached. At this point, an interfacing may be achieved similar to that detailed above for the cutting tool  100  of  FIGS. 1-6 . For example, a smaller diameter or other recognizable feature of the connector head may be encountered and employed as a location indicator. Thus, cutting as described below may ensue. 
     The cutting tool  700  of  FIG. 7  is also equipped with a cutting extension  742  and blade  740  for extending outward and cutting the coiled tubing  710  (see cut  720 ). Due to the secure nature of the tracks  710  compressed against the tubing  710 , a stable cut  720  may be made therein as the extension  742  and blade  740  are rotated about the tool  700 . In an alternate embodiment, the blade  740  serves as a scoring device for scoring of the tubing  710  as opposed to complete cutting. Nevertheless, follow-on uphole pulling on the coiled tubing  710  may be employed to induce a coiled tubing break at the scoring location. Indeed, a corrosive chemical from a source  741  of corrosive chemical in the main housing  725  for example, may be sprayed from the extension  742  to enhance the breaking in the coiled tubing  710 . In yet another embodiment, a corrosive alone, without any prior scoring or cutting, may be employed in a manner sufficient to allow uphole pulling to induce the break in the tubing  710 . 
     Referring now to  FIG. 8 , a flow-chart is shown which summarizes embodiments of employing cutting tools as detailed hereinabove. The cutting tools are initially coupled to a well access line to be cut as indicated at  810  and then passively advanced into the well as indicated at  830 . In the case of wireline or other non-tubular well access this may involve coupling the cutting tool about the line and manipulating well access and regulation equipment such as blow out prevention valving. Thus, the cutting tool may then be dropped into a vertical portion of the well. In the case of coiled tubing, on the other hand, this may involve positioning the cutting tool within the tubing at a coiled tubing reel and employing pump assistance to advance the tool to the vertical portion of the well. Regardless, at this point, the advancement of the tool may be achieved without any active propulsion from the tool itself and thus, is considered herein as ‘passive’ advancement. 
     At some point, the tool may reach a bend in the well or other obstruction sufficient to halt passive advancement thereof. A conventional motion sensor within the cutting tool may be employed to detect such a halt. When this occurs, a propulsion mechanism of the tool may be deployed as indicated at  850  to engage the line. As noted above the propulsion mechanism may engage the line by either outward or inward extension, for example, depending upon the type of line and cutting tool involved. Regardless, the propulsion mechanism may thus be employed to drive the tool further downhole as indicated at  870 . 
     The tool may be advanced as described above until reaching a cut location. In the case of non-tubing access such as wireline, confirmation of the tool reaching the cut location may be particularly beneficial as detailed hereinabove. Thus, as indicated at  880 , such cut location may be confirmed, for example, based on an interface achieved between the cutting tool and a cable head. Of course, similar location confirmation techniques may also be employed where the well access line is coiled tubing. In any case, once the cut location is attained by the cutting tool, a break may be induced in the line as indicated at  890 . 
     Embodiments detailed hereinabove provide cutting tools and techniques that may be employed in manners that enhance certainty and accuracy of well access line cutting. The cutting tools may be employed in manners that need not rely exclusively on timers, motion sensors, or other blind mechanisms for triggering cutting of a well access line. This may be particularly beneficial in the case of non-tubular access cutting where actuation of cutting based on such mechanisms is prone to trigger cutting as a response to downhole obstructions or at a point in time that the cutting tool is caught on such an obstruction. Additionally, in the case of coiled tubing, cutting tools and techniques are detailed which may avoid the cutting of the tubing at the well surface, thereby saving potentially several thousand feet of coiled tubing. 
     The preceding description has been presented with reference to presently preferred embodiments. Persons skilled in the art and technology to which these embodiments pertain will appreciate that alterations and changes in the described structures and methods of operation may be practiced without meaningfully departing from the principle, and scope of these embodiments. For example, a cutting tool for severing a non-tubular well access line may be employed with an outward extending propulsion mechanism similar to that described for use on coiled tubing. In such an embodiment, the propulsion mechanism may engage a well wall as opposed to the line interior thereof. By the same token, space permitting, a cutting tool for coiled tubing may be employed about the coiled tubing with inwardly extending propulsion mechanism similar to that described herein for use on non-tubular access lines. With modifications such as these in mind, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope.

Summary:
A cutting tool for cutting a wireline, slickline, coiled tubing, or other well access line stuck downhole in a well. The tool includes a host of features including a propulsion mechanism to aid in delivering the tool to a predetermined cut location of the line. In this manner, the risk of unintended uphole cutting of the line may be minimized.