You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method for casing wellbores drilled within the earth. More particularly, the present invention relates to a method for casing wellbores drilled within the earth by a composite coiled tubing drilling apparatus. More particularly still, the present invention relates to a method for drilling and casing a wellbore with a composite coiled tubing drilling system in a single trip downhole. 
     2. Background of the Invention 
     Traditional drilling rigs include large structures that are erected upon land or offshore. The rigs typically support each length of drill string as it is fed into the well and provide rotational motion to a drill bit at the end of the string of drill pipe. Often, when starting a new well, a large diameter drill bit is used for the first several hundred feet of borehole. Once this borehole is complete, the bit is retrieved and a string of metal pipe, known as casing, is placed in the newly drilled borehole. The casing string is slightly smaller in outer diameter than the drilled borehole. Once in the well, the casing is cemented in place and provides a well-defined and fixed reference for subsequent drilling operations. With the first section of casing string installed, a smaller drill bit is lowered through it and is used to drill another, narrower borehole for the next section of casing string to be installed. As each borehole section is drilled, the gage of the drill bit and diameter of the subsequent casing string are reduced until the entire string of casing resembles an extended, inverted telescope. These lengths of casing serve to isolate the drilling and production fluids from the formation surrounding the casing, thereby preventing loss of these fluids into the formation, cross-contamination of the drilling fluids and formation fluids, and degradation of the surrounding formation. 
     Recent developments in drilling technology have led to the replacement of conventional drill pipe, which is assembled from relatively short lenghts of rigid pipe, with coiled tubing, which is a single length of flexible pipe, typically of steel or a composite. Systems of this type have the ability to operate without conventional pipe-handling equipment, and are capable of drilling much deeper into the earth&#39;s crust and with much more directional capability than was previously achievable. In a composite tubing drilling system, a drilling apparatus is deployed downhole at the end of a long string of composite tubing or hose, the hose being deployed from a large spool on a specialty rig or truck located at the surface. Because no kelly or rotary table is used, all of the mechanical energy to rotate an attached drill bit is created downhole by a downhole drilling motor, with a tractor device being used to maintain the proper amount of weight on bit and torque. As the density of the tubing can be adjusted during manufacture to allow the string to be buoyant in the column drilling fluid, the maximum depth achievable with a drilling system of this type is not limited by the tensile strength of the tubing. Furthermore, because of the relative short tool length and increased flexibility of the composite tubing compared to conventional drill pipe systems, the drilling apparatus is capable of making directional changes with much smaller turning radii than are achievable with rigid drill strings. Additionally, because the composite tubing is preferably manufactured from an electrically insulating material, communications conduits (wire pairs, fiber optic lines) can be incorporated into the sidewall of the tubing during manufacture. Such features enable drillers to send and receive real-time data and commands to and from the drilling apparatus, rather than rely on traditional forms of telemetry. 
     One drawback to systems of this type is that with the elimination of the conventional drilling rig and the complex directional movement of the composite coiled tubing drilling system, conventional casing strings are not easily deployed into a well created by such a system. Also, because the same apparatus is used to drill an entire well from start to finish, the telescoping casing technique is impractical. Furthermore, because the drilling apparatus may execute complex directional changes along its drillpath, conventional steel casing is not pliable enough to follow such a well contour. To prevent the loss of drilling and production fluids through formation leaching, a casing methodology applicable to wells drilled with composite coiled tubing drilling systems is highly desirable. 
     The present invention overcomes the deficiencies of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     The deficiencies of the prior art are overcome using a method that includes delivering an expandable casing string to an uncased borehole coaxially upon a composite coiled tubing drilling string. Once drilling operations are complete in one portion of the borehole, pressure is supplied between the drill string and the coaxially mounted casing string so as to expand the casing string to the full gage of the borehole. Preferably, the outer surface of the casing string includes an adhesive agent that is designed to be activated by the expansion of the casing string against the borehole wall. Alternatively, the mechanical structure of the casing string itself may be configured to prevent the casing string from collapsing once it has been expanded. Once expanded, the casing string is left behind to isolate the well formation from drilling and production fluids that may subsequently flow through the wellbore. 
     These and other advantages of the present invention will become apparent on reading the detailed description of the invention in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
     FIG. 1 is a schematic representation of a casing while drilling system constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a schematic representation of the casing while drilling system of FIG. 1 being run in a borehole; 
     FIG. 3 is a schematic representation of the casing while drilling system of FIG. 1 being run in the borehole of FIG. 2 with the drill bit and drive assembly detached; 
     FIG. 4 is a schematic representation of the casing while drilling system of FIG. 3 with the casing string shown in activated form; 
     FIG. 5 is a schematic representation of the casing while drilling system of FIG. 3 shown during the activation of the casing string by a preferred means; and 
     FIG. 6 is a schematic representation of the casing while drilling system of FIG. 3 shown during the activation of the casing string through a second preferred means. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, a casing while drilling system  10  preferably includes a flexible drillstring  12 , a drill bit  14 , and an attachment and drive package  16 . 
     Flexible drillstring  12  and attachment and drive package  16  are shown in a simplified schematic form. It is to be understood that drillstring  12  is preferred to exhibit the qualities of a composite coiled tubing system wherein drillstring  12  is constructed as a composite tubing with integral communications members. Likewise, it is to be understood that attachment and drive package  16  includes an axial and rotational drive system to thrust and rotate bit  14  within a borehole. For example, attachment and drive package  16  can comprise a bottom hole assembly (BHA), such as are well known in the art. The BHA preferably includes propulsion and steering equipment, including but not limited to a downhole motor and a bent sub. 
     Still referring to FIG. 1, flexible drillstring  12  includes a flowbore  18  and is surrounded by an expandable casing layer  20 . Expandable outer layer  20  (shown in FIG. 1 in its contracted state) is preferably bonded to flexible drillstring  12  by a lightly adhesive layer  22  and is affixed to drillstring  12  at the bit end by an expandable anchor  24 . Furthermore, a casing securing agent  26  is preferably applied to the outer diameter of the contracted casing layer  20 . 
     Referring now to FIG. 2, casing while drilling system  10  is shown being run in an uncased borehole  28  of formation  30 . Attachment and drive package  16  preferably includes a tractor (not shown) to apply axial downhole force while a drive package (not shown) provides a rotational force to bit  14 . The tractor preferably provides enough counter-rotational force such that drillstring  12  does not rotate as bit  14  rotates. Preventing rotation of drillstring  12  with respect to borehole  28  is preferred to prevent damage to the casing layer  20  or the adhesive agent  26 . 
     When casing of borehole  28  is desired, a signal is sent to attachment and drive package  16 . With bit  14  detached, casing while drilling system  10  is retracted slightly as shown in FIG. 3 to allow room for the casing operation to be performed. Alternately, casing while drilling system  10  may be constructed so that casing operations may be carried out with bit  14  attached, although it may be preferable to detach bit  14  to allow casing while drilling system  10  room to actuate. 
     Referring now to FIG. 4, borehole  28  is cased when an activation event A occurs to overcome lightly adhesive layer  22  and expand casing  20  out to the borehole  28 . With casing layer  22  expanded to borehole wall by event A, adhesive agent  26  is activated and securely bonds and seals casing layer  22  to borehole  28  wall. 
     Following casing of borehole  28 , an annulus  32  is formed between casing  20  and drillstring  12 , thus allowing drillstring  12  to operate and rotate independently of casing string  20 . Afterward, attachment and drive package  16  may be re-coupled to drillstring  12  and either retracted to the surface or used to drill deeper into formation  30 . 
     To accomplish the task of casing borehole  28 , various methods and devices may be used by casing while drilling system  10  for activation event A, and many forms of adhesive structures may be employed for adhesive agent  26 . Particularly, referring now to FIG. 5, a mandrel  50  may be employed to expand casing  20  fully to the borehole  28  throughout the length of drillstring  12  of apparatus  10 . In using mandrel  50 , operators at the surface prepare drillstring  12  for the delivery of mandrel  50  by cutting the tubing and stretching casing  20  away from drillstring  12  to allow mandrel  50  to be “started” along its path. Once started, hydrostatic pressure P is applied from the surface behind mandrel  50 , effectively pushing mandrel  50  down the length of drillstring  12 , breaking adhesive layer  22  and expanding casing  20  as mandrel  50  travels downhole. In FIG. 5, portion  52  is fully expanded, portion  54  is in the process of being expanded, and portion  56  is not yet expanded. The expansion of casing  20  away from drillstring  12  and against borehole  12  continues until mandrel  50  reaches expandable anchor  24 . Once mandrel reaches anchor  24 , an increase in pressure P can be generated to enable mandrel  50  to rupture anchor  24  and pass through, thereby allowing free flow of drilling and production fluids in the newly created annulus ( 32  from FIG.  4 ). An advantage of mandrel  50  is that its use allows a large contact pressure to be applied between casing  20  and borehole  28  through adhesive agent  26 . One drawback to using mandrel  50  for activation event A is that it may not be completely effective in the event that a portion of the borehole  28  is significantly washed out or non uniform. In the event of such an occurrence, pressure P may wash around mandrel  50 , thereby preventing it from traveling completely downhole. 
     The features and operation of activation event A can include a variety of different concepts. For example, the type of activation event A described in the preceding paragraph is a mechanical operation, inasmuch as the mandrel  50  applies a mechanical force to expand an expandable casing. Other types of activation events A that are contemplated herein include but are not limited to: thermal, pH, electronic, acoustic etc. 
     Referring now to FIG. 6, an alternative means for expanding casing string  20  away from drillstring  12  is shown. In FIG. 6, casing string  20  is expanded away from drillstring  12  through the use of an expanding medium  60  between the compressed layers to overcome adhesive layer  22 . Expanding medium  60  may take the form of any number of fluids but is preferably either drilling mud or water. By injecting fluid  60  between layers  12  and  20 , casing  20  can be expanded completely within borehole  28  with little chance of failure, even if borehole  28  is washed out or otherwise non-uniform in cross section. In the event of a washed out portion of borehole  28 , hydrostatic pressure P created by the injection of fluid  60  will allow casing  20  to expand farther outward than was possible with the fixed-diameter mandrel  50  of FIG.  5 . In order for such a fluid injection arrangement to work properly, the hydrostatic pressure required to expand casing  20  and activate adhesive  26  must be lower than the pressure required to break the relatively weak adhesive layer  22 . To accomplish this, the strength of adhesive layer  22  for the technique of FIG. 6 may have to be increased from the strength required in the expansion technique used in FIG.  5 . If the strength of layer  22  is left weak, it may be possible for injected fluid  60  to separate casing  20  from the entire length of drillstring  12  prior to making proper adhesive contact with borehole wall  28 . 
     It is further contemplated that two or more layers could be used in combination on a single length of tubing. In one embodiment, the layers are concentric, with one layer surrounding another. In this embodiment, the layers preferably have different constructions and different activation events, so that activation of the outermost layer does not result in the undesired activation of any of the inner layers. A multilayer expandable casing can be used to case successive portions of a borehole, or to provide multiple or overlapping casings in one or more portions of the borehole. 
     Once casing string  20  is expanded, it may be held in place by strong adhesive agent  26 . Agent  26  can be any of several adhesives, but preferably is designed only to adhere to borehole  28  after casing  20  is expanded away from drillstring  12 . To accomplish this, adhesive agent  26  may be constructed as a pressure, chemical, or temperature sensitive substance, requiring secondary activation to make a permanent bond. For example, agent  26  may be delivered as a binary adhesive, including an exposed resin with a hardener component contained in pressure sensitive capsules (not shown). When casing  20  is expanded against borehole  28 , the expansion force ruptures the capsules, thereby mixing resin and hardener components to create an active adhesive. Alternatively, adhesive agent  26  may be constructed such that an elevated temperature, such as are typically found downhole, will activate the adhesive or rupture the hardener-containing capsules. 
     Additionally, adhesive agent  26  may also be constructed as a chemically or pH sensitive gel containing an emulsion of epoxy monomers. By being delivered in gel form, adhesive  26  can be adhered to the outside of collapsed casing string  20  throughout drilling. During drilling, if the gel were to become abraded, scraped, or otherwise removed in places, the pieces of the removed gel would be removed to the surface with the used drilling mud. Upon completion of drilling operations, the chemical makeup or pH of the circulating mud can be altered to activate the gel, thereby releasing the epoxy monomers and resulting in an epoxy coated casing  20  and stabilized wellbore  28 . 
     Finally, casing string  20  may be constructed in such a fashion that its mechanical structure keeps it in place without the need for an adhesive agent. For example, components of casing string  20  may be designed such that once the structure is opened, it cannot be closed, as is with the case of a ratchet, for example. The “one way” structure of casing  20  may be on a microscopic or a macroscopic magnitude and mechanically prevents casing  20  from retracting once opened. Alternatively, casing string  20  may be delivered with an adhesive layer  26  attached thereupon that is activated by the expansion of casing string  20  from drillstring  12  to the borehole wall  28 . An example of such a structure include microencapsulated adhesive materials that become adhesive upon being “stretched” by the expansion of casing string  20 . Subsequent activation would further transform the adhered casing string  20  to give it desirable attributes such as increased tensile strength, hardness, and wear resistance. 
     Using the above described system, a borehole created with a composite coiled tubing drilling system can be effectively and efficiently cased to isolate the drilling and production fluids from the surrounding formation. Using the system described above, drilling and casing operations can be completed on a single trip into the wellbore, thereby saving the operating company time and cost. Furthermore, using the casing system of the present invention, numerous wells that would otherwise have to remain uncased, would be allowed some form of isolation from the formation throughout their depth. 
     In some embodiments, it is preferred that either or both of attachment and drive package  16  and bit  14  have an outside diameter that is smaller than the inside diameter of the expanded casing so that the BHA and/or the bit can be pulled up out of the well after it has been cased. In instances where this is desired, either the BHA or the bit, or both, are mounted on the drillstring  12  by means of a releasable collar. 
     The above discussion is meant to be illustrative of the principles of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Summary:
A method to deliver an expandable casing string to an uncased borehole coaxially upon a composite coiled tubing drilling string is presented. Once the drilling operation is completed, the casing string is expanded by supplying pressure between the coaxially positioned strings to expand the casing string to the borehole. Preferably, the outer diameter of the casing string contains an adhesive agent designed to be activated by the expansion of the casing string against the borehole wall. Alternatively, the mechanical structure of the casing string itself may be configured to prevent the casing string from collapsing once it has been expanded. Once expanded, the casing string is left behind to isolate the well formation from drilling and production fluids that may subsequently flow therethrough.