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BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to the operation of instrumentation within a wellbore. More particularly, the invention relates to a separable plug for use with a wellbore tool.  
         [0003]     2. Description of the Related Art  
         [0004]     In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is typically filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore. Numerous operations occur in the well after the casing is secured in the wellbore. All operations require the insertion of some type of instrumentation or hardware within the wellbore. For instance, wireline logging tools are employed in the wellbore to determine various formation parameters including hydrocarbon saturation.  
         [0005]     Early oil and gas wells were typically drilled in a vertical or near vertical direction with respect to the surface of the earth. As drilling technology improved and as economic and environmental demands required, an increasing number of wells were drilled at angles which deviated significantly from vertical. In the last several years, drilling horizontally within producing zones became popular as a means of increasing production by increasing the effective wellbore wall surface exposed to the producing formation. It was not uncommon to drill sections of wellbores horizontally (i.e. parallel to the surface of the earth) or even “up-hill” where sections of the wellbore were actually drilled toward the surface of the earth.  
         [0006]     The advent of severely deviated wellbores introduced several problems in the performance of some wellbore operations. Conventional wireline logging was especially impacted. Wireline logging utilizes the force or gravity to convey logging instrumentation into a wellbore. Gravity is not a suitable conveyance force in highly deviated, horizontal or up-hill sections of wellbores. Numerous methods have been used, with only limited success, to convey conventional wireline instrumentation or “tools” in highly deviated conditions. These methods include conveyance using a drill string, a coiled tubing, and a hydraulic tractor. All methods require extensive well site equipment, and often present operational, economic, and logistic problems.  
         [0007]     Another problem that affects both a deviated wellbore and a vertical wellbore occurs when the wellbore contains a high percentage of water relative to the hydrocarbons in the surrounding formations. In this situation, fluid tends to collect and remain static proximate the lowest point of the wellbore because there is not enough hydrocarbon formation pressure to move the fluid. For instance, fluid tends to collect at the junction between the vertical portion and the deviated portion in a deviated wellbore. Without fluid flow, production logging tools can not operate properly to collect data. To overcome this problem, some form of artificial lift is typically employed to move fluids through the wellbore, such as a submersible pump. The increased velocity of the fluid provides an adequate flow rate for the logging tool to operate.  
         [0008]     Generally, the submersible pump is run into the wellbore on production tubing with a Y block between the production tubing and the submersible pump. The Y block allows the pump to be turned on and the well produced while leaving an access point to the wellbore for logging tools. Typically, the access point is a smaller string of tubing attached to the Y block which is run along side the submersible pump. In operation, a logging tool is conveyed through the production tubing attached to a string of coiled tubing. As the logging tool passes through the Y block and the smaller string of tubing, a plug attached to the string of coiled tubing lands in a seat formed in the smaller string of tubing. The plug seals off the smaller string of tubing while allowing the string of coiled tubing and the logging tool to continue to travel into the wellbore. Although coiled tubing may be used in deviated wellbores, the coiled tubing and associated injector equipment are still physically large and present drawbacks similar to those encountered with drill string conveyed systems.  
         [0009]     A need therefore exists for a reliable and operationally efficient system to convey and operate wellbore tools, like logging tools, in wellbores which are deviated from the vertical.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention generally relates to a tool for use in a wellbore. In one aspect, a method of performing an operation in a wellbore is provided. The method includes running a selectively separable plug member accommodating a tool into the wellbore on a continuous rod. Next, a first portion of the plug member is separated from a second portion and then the continuous rod is used to position the second portion with the tool below the first portion to perform the operation.  
         [0011]     In another aspect, a method of logging a wellbore is provided. The method includes running a selectively actuatable plug member into the wellbore on a continuous rod, wherein the plug member accommodates a logging tool. Next the plug member is actuated, thereby separating a first portion of the plug member from a second portion. Thereafter, the continuous rod is used to run the second portion with the logging tool to a predetermined location below the first portion to collect data.  
         [0012]     In yet another aspect, a plug assembly for use in a wellbore is provided. The plug assembly includes a first portion with a pressure activatable ring member for sealing around a continuous rod. The plug assembly further includes a second portion for accommodating a wellbore tool. Additionally, the plug assembly includes a releaseable member disposed between the first portion and the second portion to selectively allow the second portion to separate from the first portion while the first portion maintains a sealing relationship with the continuous rod.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0014]      FIG. 1  is a sectional view illustrating a tool and a plug assembly being lowered into a wellbore on a continuous rod.  
         [0015]      FIG. 2  is a sectional view illustrating the plug assembly being positioned in a receiver member.  
         [0016]      FIG. 3  is a sectional view illustrating the tool being urged through the wellbore after the plug assembly has been actuated.  
         [0017]      FIG. 4  is a sectional view illustrating the tool and the plug assembly being removed from the wellbore. 
     
    
     DETAILED DESCRIPTION  
       [0018]     In general, the present invention relates to a selectively actuated logging plug for use with a continuous rod, such as a COROD string. The COROD string is a means and a method for conveying and operating a wide variety of equipment within a wellbore. The COROD string works equally well in vertical and highly deviated wells. When the COROD string is used in logging operations, the downhole tools record data of interest in memory within the downhole tool rather than telemetering the data to the surface as in conventional wireline logging. Data is subsequently retrieved from memory when the tool is withdrawn from the wellbore. The tool position in the wellbore is synchronized with a depth encoder, which is preferably at the surface near a COROD injector apparatus. The depth encoder measures the amount of COROD string within the well at any given time. Data measured and recorded by the downhole tool is then correlated with the depth encoder reading thereby defining the position of the tool in the well. This information is then used to form a “log” of measured data as a function of depth within the well at which the data is recorded. The COROD can be used for multiple runs into a well with no fatigue as compared to coiled tubing operations. COROD can be run through tubing thereby eliminating the additional cost and time required to deploy a drill string, coiled tubing, or tractor conveyed systems. It is also noted that the COROD string for conveying equipment is not limited to oil and gas well applications. The system is equally applicable to pipeline where pipeline inspection services are run. To better understand the novelty of the apparatus of the present invention and the methods of use thereof, reference is hereafter made to the accompanying drawings.  
         [0019]      FIG. 1  is a sectional view illustrating a tool  180  and a plug assembly  100  being lowered into a deviated wellbore  10  on a continuous string, such as a COROD string  175 . For purposes of discussion, the wellbore  10  is illustrated as a deviated wellbore. It should be understood, however, that the plug assembly  100  may be employed in a vertical wellbore, without departing from principles of the present invention.  
         [0020]     As illustrated, the wellbore  10  is lined with a string of steel pipe called casing  15 . The casing  15  provides support to the wellbore  10  and facilitates the isolation of certain areas of the wellbore  10  adjacent hydrocarbon bearing formations. The casing  15  typically extends down the wellbore  10  from the surface of the well to a designated depth. An annular area  20  is thus defined between the outside of the casing  15  and the wellbore  10 . This annular area  20  is filled with cement  25  pumped through a cementing system (not shown) to permanently set the casing  15  in the wellbore  10  and to facilitate the isolation of production zones and fluids at different depths within the wellbore  10 . Subsequently, a submersible pump  35  is run into the wellbore  10  on a production tubing  40  with a Y-block  30  between the production tubing  40  and the submersible pump  35 . The Y block  30  allows the pump  35  to be turned on and the well produced while leaving an access point to the wellbore  10  for logging tools. Typically the access point is an instrument tube  45  positioned adjacent the submersible pump  35  and attached to the Y block  30 .  
         [0021]     After the submersible pump  35  and the production tubing  40  are positioned in the wellbore  10 , the plug assembly  100  and the tool  180  are lowered through the production tubing  40  on the COROD string  175  in the direction indicated by arrow  95 . Generally, the COROD string  175  is lowered into the wellbore  10  by an injector apparatus (not shown). The injector apparatus typically includes a depth encoder (not shown) to record the amount of COROD string  175  within the wellbore  10  at any given time thereby determining the position of the tool  180  within the wellbore  10 . Additionally, the depth encoder may be used to determine the location of the plug assembly  100  in relation to the instrument tube  45  as the plug assembly  100  is lowered through the production tubing  40 .  
         [0022]      FIG. 2  is a sectional view illustrating the plug assembly  100  being positioned in a receiver member  55 . The plug assembly  100  generally comprises a first portion  105  and a second portion  110 . The first and second portions  105 ,  110  are operatively attached to each other by a selectively actuated release member  115 . The release member  115  is a device that operates at a predetermined pressure or force. In one embodiment, the release member  115  is a shear bolt or shear pin disposed between the first portion  105  and the second portion  110  as illustrated in  FIG. 2 . The shear bolt is constructed and arranged to fail at a predetermined axial force. Generally, the shear bolt is a short piece of brass or steel that is used to retain sliding components in a fixed position until sufficient force is applied to break the bolt. Once the bolt is sheared, the components may then move to operate the tool.  
         [0023]     Alternatively, other forms of shearable members may be employed in the release member  115 , as long as they are capable of shearing at a predetermined force. For example, a threaded connection (not shown) may be employed between the first portion  105  and the second portion  110 . Generally, the threads machined on the first portion  105  are mated with threads machined on the second portion  110  to form the threaded connection. The threads on the first portion  105  and the second portion  110  are machined to a close fit tolerance. The threads are constructed and arranged to fail or shear when a predetermined axial force is applied to the plug assembly  100 . The desired axial force required to actuate the release member  115  determines the quantity of threads and the thread pitch.  
         [0024]     The first portion  105  includes a pressure activated ring  120  substantially enclosed in a housing  125  at an upper end thereof. The pressure activated ring  120  creates and maintains a seal around the COROD string  175  during deployment of tool  180 . The ring  120  is pressure activated, whereupon the application of a predetermined pressure in the production tubing  40  a sealing relationship is formed between the plug assembly  100  and the COROD string  175 . In one embodiment, the ring  120  is constructed from an elastomeric material.  
         [0025]     Adjacent the housing  125  is an upper mandrel  130  with a ring member  135  disposed around the outer surface thereof. The ring member  135  secures and seals the first portion  105  within the instrument tube  45 . The ring member  135  includes a plurality of profiles formed on the outer surface thereof that mate with a receiver member  55  formed in the instrument tube  45 . After the ring member  135  mates with the receiver member  55 , a sealing relationship is formed between the plug assembly  100  and the instrument tube  45 . If there is no sealing relationship between the plug assembly  100  and the instrument tube  45 , the pump  35  will only circulate fluid around the Y-block  30  rather than pumping fluid up the production tubing  40 . In one embodiment, the ring member  135  is constructed from a fiber material.  
         [0026]     The first portion  105  further includes a lower mandrel  140  attached to the upper mandrel  130  through a connection member, such as a lock nut assembly. Additionally, the lower mandrel  140  is operatively attached to a housing  145 , on the second portion  110  by the selectively actuated release member  115 .  
         [0027]     Adjacent the housing  145  in the second portion  110  is a connector  150 . The connector  150  includes a first threaded portion that mates with a threaded portion on the COROD string  175  to form a threaded connection  155  which connects the plug assembly  100  to the COROD string  175 . The connector  150  includes a second threaded portion that mates with a threaded portion on the tool  180  to form a threaded connection  160  which connects the plug assembly  100  to the tool  180 . It should be understood, however, that COROD string  175  and the tool  180  may be connected to the plug assembly  100  by any type of connection member, without departing from principles of the present invention.  
         [0028]     As illustrated in  FIG. 2 , the plug assembly  100  is urged through the production tubing  40  and the Y-block  30  into instrument tube  45  until the ring member  135  contacts the receiver member  55  formed in the instrument tube  45 . At that point, the ring member  135  mates with the receiver member  55  to form a seal between the plug assembly  100  and the instrument tube  45 . As the COROD string  175  continues to be urged downward, a force is created on the release member  115 . At a predetermined force, the release member  115  actuates, thereby allowing the second portion  110  of the plug assembly  100  and the tool  180  to move in relation to the first portion  105  of the plug assembly  100  which is secured in the instrument tube  45 .  
         [0029]      FIG. 3  is a sectional view illustrating the tool  180  being urged through the wellbore  10  after the plug assembly  100  has been actuated. For purposes of discussion, assume the tool  180  is a logging tool. It is to be understood, however, that the tool  180  may be any type of wellbore tool without departing from principles of the present invention, such as a casing perforating “gun” for perforating the casing  15  in a formation zone of interest. The tool  180  may also be a casing inspection tool, or a production logging tool to measure the amount and type of fluid flowing within the casing  15  or within production tubing  40 . The tool  180  can also be a fishing tool that is used to retrieve unwanted hardware from the wellbore  10 , such as an overshot or a spear. It should be further noted that the tool  180  need not be retrieved when the COROD string  175  is withdrawn from the wellbore  10 . As an example, the tool  180  could be a packer or a plug, which is left positioned within the borehole when the COROD string  175  is withdrawn. Thus, the COROD string  175  is suitable for delivering or operating completions tools.  
         [0030]     As shown in  FIG. 3 , the COROD string  175  continues to urge the second portion  110  along with the tool  180  through the deviated portion of the wellbore  10  to conduct a logging operation. At the same time, the pressure activated ring  120  maintains a seal around the COROD string  175  and the ring member  135  maintains a seal between the plug assembly  100  and the instrument tube  45 .  
         [0031]     In one embodiment, the tool  180  contains a sensor package (not shown) which responds to formation and wellbore parameters of interest. The sensors can be nuclear, acoustic, electromagnetic, or combinations thereof. Response data from the sensor package is recorded in a memory member (not shown) for subsequent retrieval and processing when the tool  180  is withdrawn from the wellbore  10 . A power supply (not shown), which is typically a battery pack, provides operational power for the sensor package and memory member. As the data is retrieved from the memory, it is correlated with the depth encoder response to form a “log” of measured parameters of interest as a function of depth within the wellbore  10 .  
         [0032]     In another embodiment, the invention is equally usable with more traditional wireline logging methods dependent upon a conductor to transmit data as logging operations are taking place. The COROD string  175  can be manufactured with a longitudinal bore therethrough to house a conductor (not shown) suitable for transmitting data. In one example, the conductor is placed within the bore of the COROD string  175  prior to rolling the COROD string  175  on a transportation reel (not shown). As the tool  180  and the plug assembly  100  are assembled at one end of the COROD string  175 , a mechanical and electrical connection is made between the conductor housed in the COROD string  175  and the tool  180  connected to the end of the COROD string  175  prior to insertion into the wellbore  10 . In this manner, the COROD string  175  is used to both carry the tool  180  downhole and transmit data from the tool  180  to the surface of the wellbore  10 .  
         [0033]     In another embodiment, the COROD string  175  itself can act as a conductor to transmit data to the surface of a wellbore  10 . For example, COROD string  175  can be covered with a coating of material (not shown) having the appropriate conductive characteristics to adequately transmit signals from the tool  180 . In this manner, no additional conductor is necessary to utilize the tool  180  placed at the end of the COROD string  175 .  
         [0034]     Additionally, the COROD string  175  can be used to transport logging tools (not shown) that are capable of real time communication with the surface of the well without the use of a conductor. For example, using a telemetry tool and gamma ray tool disposed on the COROD string  175  having various other remotely actuatable tools disposed thereupon, the location of the tools with respect to wellbore zones of interest can be constantly monitored as the telemetry tool transmits real time information to a surface unit. At the surface, the signals are received by signal processing circuits in surface equipment (not shown), which may be of any suitable known construction for encoding and decoding, multiplexing and demultiplexing, amplifying and otherwise processing the signals for transmission to and reception by the surface equipment. The operation of the gamma ray tool is controlled by signals sent downhole from surface equipment. These signals are received by a tool programmer which transmits control signals to the detector and a pulse height analyzer.  
         [0035]     The surface equipment includes various electronic circuits used to process the data received from the downhole equipment, analyze the energy spectrum of the detected gamma radiation, extract therefrom information about the formation and any hydrocarbons that it may contain, and produce a tangible record or log of some or all of this data and information, for example on film, paper or tape. These circuits may comprise special purpose hardware or alternatively a general purpose computer appropriately programmed to perform the same tasks as such hardware. The data/information may also be displayed on a monitor and/or saved in a storage medium, such as disk or a cassette.  
         [0036]     The electromagnetic telemetry tool generally includes a pressure and temperature sensor, a power amplifier, a down-link receiver, a central processing unit, and a battery unit. The electromagnetic telemetry tool is selectively controlled by signals from the surface unit to operate in a pressure and temperature sensing mode, providing for a record of pressure versus time or a gamma ray mode which records gamma counts as the apparatus is raised or lowered past a correlative formation marker. The record of gamma counts is then transmitted to surface and merged with the surface system depth/time management software to produce a gamma ray mini log which is later compared to the wireline open-hole gamma ray log to evaluate the exact apparatus position. In this manner, components, including packers and bridge plugs can be remotely located and actuated in a wellbore using real time information that is relied upon solely or that is compared to a previously performed well log.  
         [0037]      FIG. 4  is a sectional view illustrating the tool  180  and the plug assembly  100  being removed from the wellbore  10 . After the logging operation is complete, the COROD string  175 , tool  180  and second portion  110  are urged toward the surface of the wellbore  10  until the second portion  110  of the plug assembly  100  contacts the first portion  105 . At that time, the housing  145  of the second portion  110  aligns with the lower mandrel  140  of the first portion  105 . Thereafter, the plug assembly  100  comprised of the first and the second portions  105 ,  110  acts as one unit. As the COROD string  175  continues to be urged toward the surface of the wellbore  10 , the ring member  135  disengages from the receiver member  55 , thereby removing the sealing relationship between the plug assembly  100  and the instrument tube  45 . Subsequently, the plug assembly  100 , the tool  180  and COROD string  175  are pulled out of the wellbore  10  in the direction indicated by arrow  60 . At the surface of the wellbore  10 , the ring member  135  may be replaced and the plug assembly  100  may be once again transported into the wellbore  10  with another logging tool at the lower end of a COROD string.  
         [0038]     In operation, a logging tool and a plug assembly are urged though a production tubing into a deviated wellbore on a COROD string. Generally, the plug assembly comprises a first portion and a second portion operatively connected to each other by a selectively activated release member. The logging tool and plug assembly are urged through the production tubing until the first portion of the plug assembly seats in the receiver member formed in an instrument tube at the lower end of the production tubing. As the COROD string continues to be urged downward, a force is created on the selectively activated release member. At a predetermined force, the release member is activated, thereby allowing the second portion of the plug assembly and the logging tool to move in relation to the first portion of the plug assembly which is secured in the instrument tube. Thereafter, the COROD string continues to urge the second portion along with the logging tool through the deviated portion of the wellbore to conduct a logging operation. After the logging operation is complete, the COROD string urges the logging tool and second portion toward the surface of the wellbore until the second portion of the plug assembly contacts and aligns with the first portion. Thereafter, the plug assembly comprised of the first and the second portions acts as one unit. Subsequently, the plug assembly, the logging tool and COROD string are pulled out of the wellbore.  
         [0039]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
The present invention generally relates to a tool for use in a wellbore. In one aspect, a method of performing an operation in a wellbore is provided. The method includes running a selectively separable plug member accommodating a tool into the wellbore on a continuous rod. Next, a first portion of the plug member is separated from a second portion and then the continuous rod is used to position the second portion with the tool below the first portion to perform the operation. In another aspect, a method of logging a wellbore is provided. In yet another aspect, a plug assembly for use in a wellbore is provided.