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RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/692,153 filed Jun. 20, 2005. 

   FIELD OF THE INVENTION 
   The present invention relates in general to conducting coil tubing operations in wellbores and more specifically to maintaining depth control during the operations. 
   BACKGROUND 
   In a cased oil or gas well, the hydrocarbon in the formation can be accessed by perforating the casing with a high-energy shape charge or by abrasively cutting holes or slots in the casing with a jetting tool. In the latter application, slurry is pumped down a tubular and through a small jetting nozzle. This abrasive mixture exits the jetting tool at a high velocity, impinges on the casing wall and abrades or cuts holes in the casing. Abrading holes in casing is performed by technologies such as the Abrasijet™ tool introduced by Schlumberger. 
   Conventional jetting assemblies are lowered on drillpipe. Some drillpipe conveyed jetting assemblies include slip-type mechanisms to limit the vibration of the bottom hole assembly (BHA) in the wellbore, however, these slips are not designed to stop axial movement of the BHA in the wellbore. 
   Recently, jetting tools have been attached to coiled tubing and this has introduced new challenges. The primary issue facing coiled tubing deployed jetting is depth control. Knowing exactly where the BHA is during a job and maintaining the BHA in a desired location during operations is difficult. The coiled tubing length is susceptible to axial compression and tension forces, internal pressure, flow rate down the tubing or annulus, high temperatures, coiled tubing friction with casing wall, etc. During jet cutting and other wellbore operations, many of the forces mentioned act on the tubing and BHA. The result is that the overall length of the coiled tubing changes and the tool moves during the operation. Movement of the jetting tool during cutting operations results in slots or incomplete cutting of the casing. In a worst-case scenario, the jetting tool can move as much as ten ft (3 m), which can be enough to jet holes into the wrong formation behind the reservoir. 
   Conventional techniques for maintaining depth control of coiled tubing include devices that monitor how much tubing has been fed into the wellbore, however these techniques do not provide the extent of buckling, stretch, etc. Enhancements to these methods include the step of using forward modeling or knowledge of the tubing properties to predict this buckling, stretch, etc. 
   Depth control during abrasion cutting has conventionally included the step of using a mechanical casing collar locator (CCL) that activates a hammer to “strike” the coiled tubing each time the CCL crosses a casing collar. The sound of the hammer striking the coil can (sometimes) be picked up by listening to the coil at the surface. 
   Therefore, there is a desire to provide methods and systems for controlling the depth of a coiled tubing conveyed tool during wellbore operations. 
   SUMMARY OF THE INVENTION 
   Accordingly, depth control systems and methods for maintaining a tubing conveyed tool at a desired depth in a cased wellbore during wellbore operations is provided. An embodiment of a depth control system for maintaining a tubing conveyed tool in a desired location in a cased wellbore during wellbore operations performed with the tool includes a bottom hole assembly carried by a tubing, the bottom hole assembly including a tool and an anchoring device. 
   An embodiment of a method for maintaining a tool at a desired depth in a cased wellbore while performing wellbore operations with the tool includes the steps of conveying a tool and an anchoring device on a tubing to a desired depth in a wellbore having a casing, operating the tool to perform a wellbore operation and actuating the anchoring device to engage the casing and maintain the tool at the desired depth. 
   The foregoing has outlined the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of an embodiment of the depth control system of the present invention; 
       FIG. 2A  is perspective view of an anchoring device of the present invention in a retracted position; 
       FIG. 2B  is a perspective view of the anchoring device of  FIG. 2A  in the extended or engaged position; and 
       FIG. 3  is a perspective view of another embodiment of an anchoring device of the present invention. 
   

   DETAILED DESCRIPTION 
   Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
   As used herein, the terms “up” and “down”; “upper” and “lower”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, these terms relate to a reference point as the surface from which drilling operations are initiated as being the top point and the total depth of the well being the lowest point. 
   The present invention relates to controlling and maintaining the depth of a tubing conveyed tool during wellbore operations. The present invention is described herein in relation to jet cutting and stimulation operations, however, it should be recognized that the depth control systems and methods of the present invention may be utilized in conjunction with other wellbore operations. It should further be noted, that although the invention is particularly suited for coiled tubing operations, the system and method may be utilized with other tubulars including drillpipe. 
     FIG. 1  is a perspective view of an embodiment of the depth control system of the present invention, generally denoted by the numeral  10 . Depth control system  10  includes a tool  12  and anchoring mechanism  14  conveyed by tubing  16  into a wellbore  18  having casing  20 . Tool  12  and anchoring mechanism  14  are referred to herein as the bottom hole assembly (BHA) and generally designated by the numeral  5 . Depth control system  10  may further include a depth management system  22 . 
   A first step in conducting wellbore operations is to position tool  12  at the desired depth in wellbore  18 . In the illustrated embodiment, it is desired to cut hole  24  proximate formation  26  and then to stimulate formation  26  for production or injection. Depth management system  22  is utilized to accurately convey tool  12  via tubing  16  to the desired depth at formation  26  by identifying the location of BHA  5  in wellbore  18 . In one embodiment of the present invention, depth management system  22  includes one or more sensors  28  carried by BHA  5  operationally connected to a surface unit  30  for displaying depth readings of BHA  5 . Sensor  28  may be connected to surface unit  30  via a cable  32 , such as but not limited to optical fibers, monocable or heptacable. Sensor  28  may be operationally connected to surface unit  30  via wireless telemetry. Sensors  28  may further be adapted to measure and provide additional data, including pressure, temperature and BHA  5  telemetry information such as axial and azimuthal data to surface unit  30 . It should further be noted that surface unit  30  may be in operational connection with tool  12  and/or anchoring mechanism  14  to provide electronic control of their operation. 
   Anchoring mechanism  14  is adapted to engage casing  20  so as to limit or prevent longitudinal movement of BHA  5  in wellbore  18  when engaged. Examples of anchoring mechanisms  14  include (i) pressure, flow, or mechanically activated gripping slips that engage casing  20  during tool  12  operation or (ii) spring, pressure, flow or mechanically activated drag blocks that simply use friction to hold tool  12  in place during operation of tool  12 . 
   Referring now to  FIGS. 2A and 2B , anchoring mechanism  14  is illustrated as a button type slip. Anchoring mechanism  14  includes a button slip  34  moveable between a retracted position shown in  FIG. 2A  and an extended or engaged position, shown in  FIG. 2B . Anchoring mechanism  14  may further including shoulders  36  extending from button slips  34  and a matable lip  38  to limit the extension of button slip  34 . 
   Operation of button slips  14  is further described with reference to  FIGS. 1 ,  2 A and  2 B. When wellbore operations are commenced, fluid, such as an abrasive fluid, is pumped through the internal bore  40  of coiled tubing  16 , tool  12  and anchoring mechanism  14 . As the pressure increases in bore  40  over the pressure in the annulus  42  between BHA  5  and casing  20 , button slip  34  extends outward from BHA  5  and engages casing  20 . When the wellbore operations cease and the pressure in bore  40  equalizes with pressure in annulus  42 , button slip  34  is biased back to the retracted position of  FIG. 2A . 
     FIG. 3  is a perspective view of another embodiment of anchoring device  14 . In this embodiment, anchoring device  14  includes a drag block  44 . Drag block  44  is extended from anchoring device  14  and engages casing  20 . Drag block  44  utilizes friction to minimize the movement of BHA  5 . Drag block  44  may be actuated via pressure in bore  40  and/or by biasing means such as, but not limited to, springs  46 . 
   Depth control of BHA  5  may further include the step of adjusting or controlling the location of tool  12  to enable adjustment of its axial location or its azimuthal location. As previously indicated, depth management system  22  may provide BHA  5  telemetry information and operator control of tool  12  operation. In the case of adjusting the axial location of a jet tool  12 , an injector control may be utilized. In the case of adjusting the azimuthal location, a gravity-sensor, such as a hanging weight  48  may be added to BHA  5  and the jets  50  oriented with respect to hanging weight  48 . A combination of these techniques could be used to create spirals, ovals, etc in casing  20 . 
   Downhole measurement data can be obtained and transmitted during the stimulation via depth management system  22  using optical telemetry, wireless telemetry and telemetry along a cable. A preferred embodiment is optical telemetry, in which case optical devices exist to transmit temperature and pressure. Downhole pressure can also be used to derive flow-rate, foam-quality and viscosity or dedicated sensors can be used. 
   In an embodiment of the present invention, formation  26  is stimulated utilizing hydraulic fracturing via tool  12 . Measured data, via sensors  28 , is pressure and the method includes the step of monitoring the downhole pressure to give an indication of at least one of: screen-out, radial fracture extent, vertical fracture extent, and perforation friction. The measured data can be transmitted up cable  32  and plotted on a chart of log-time versus log-pressure. If the slope of this approaches one then this is indicative of a screen-out, wherein the formation cannot absorb any more proppant. In such a case, the pumping operation needs to be quickly switched to stop wellbore  18  from completely filling with sand. Having a downhole measurement gives many minutes of advance warning. Other slopes on the log/log plot are indicative of either the fracture growing radially or vertically. 
   During wellbore operations such as jetting, downhole measurement data can be transmitted to optimize the procedure, e.g., adjusting the flow rate to maintain a constant pressure drop across jets  50  in cutting operations. As the abrasive cutting material passes through jets  50 , the jets will lower the impinging pressure on casing  20 . By monitoring this, the flow-rate can be increased in accordance so as to maintain a constant pressure on the casing surface, resulting in a cleaner and faster cut hole  24 . 
   The present invention covers both pumping down a tubular and into annulus  42  between tubular  16  and casing  20 . For example, coiled tubing  16  can be introduced into wellbore  18  and stimulation fluid is pumped down annulus  42 . 
   Alternatively, the stimulation fluid can be pumped down coiled tubing  16 . In older wells the stimulation fluid is forced into jetted holes  24  via a zonal isolation apparatus (not shown) straddling those holes. Typically such apparatus include cups and inflatable packers. 
   Once holes  24  have been jetted and reservoir formation  26  stimulated, the reservoir will be allowed to flow-back, sometimes kicked off with nitrogen to initiate the flow. In the case of hydraulic fracturing, this initiation can allow a significant amount of sand to return into the well-bore. This sand coming at high-speed through the jetted holes will then itself act as a sort of abrasive jet and can cut holes in the tubular used to convey the bottom hole assembly. Consequently, it is a preferred feature of this method to pull the tubular up above the incoming fluid, so as to avoid abrading that tubular. 
   From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a depth control system and method for maintaining and controlling a tubing conveyed tool during wellbore operations that is novel has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow.

Summary:
A depth control system for maintaining a tubing conveyed tool in a desired location in a cased wellbore during wellbore operations performed with the tool includes a bottom hole assembly carried by a tubing, the bottom hole assembly including a tool and an anchoring device. A method for maintaining a tool at a desired depth in a cased wellbore while performing wellbore operations with the tool includes the steps of conveying a tool and an anchoring device on a tubing to a desired depth in a wellbore having a casing, operating the tool to perform a wellbore operation and actuating the anchoring device to engage the casing and maintain the tool at the desired depth.