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 APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 60/683,756, entitled “Method and Apparatus for Wellbore Communication” filed on May 23, 2005, which is hereby incorporated in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to telemetry systems for use in wellbore operations. More particularly, the present invention relates to telemetry systems for providing power to downhole operations and/or for passing signals between a position in a wellbore penetrating a subterranean formation and a surface unit 
         [0003]    Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth&#39;s crust. A well is typically drilled by advancing a drill bit into the earth. The drill bit is attached to the lower end of a “drill string” suspended from a drilling rig. The drill string is a long string of sections of drill pipe that are connected together end-to-end to form a long shaft for driving the drill bit further into the earth. A bottom hole assembly (BHA) containing various instrumentation and/or mechanisms is typically provided above the drill bit. Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the borehole wall. 
         [0004]    During conventional measurement while drilling (MWD) or logging while drilling (LWD) operations, signals are passed between a surface unit and the BHA to transmit, for example commands and information. Typically, the surface unit receives information from the BHA and sends command signals in response thereto. Communication or telemetry systems have been developed to provide techniques for generating, passing and receiving such signals. An example of a typical telemetry system used involves mud-pulse telemetry that uses the drill pipe as an acoustic conduit for mud pulse telemetry. With mud pulse telemetry, mud is passed from a surface mud pit and through the pipes to the bit. The mud exits the bit and is used to contain formation pressure, cool the bit and lift drill cuttings from the borehole. This same mud flow is selectively altered to create pressure pulses at a frequency detectable at the surface and downhole. Typically, the operating frequency is in the order 1-3 bits/sec, but can fall within the range of 0.5 to 6 bits/sec. An example of mud pulse telemetry is described in U.S. Pat. No. 5,517,164, the entire contents of which are hereby incorporated. 
         [0005]    In conventional drilling, a well is drilled to a selected depth, and then the wellbore is typically lined with a larger-diameter pipe, usually called casing. Casing typically consists of casing sections connected end-to-end, similar to the way drill pipe is connected. To accomplish this, the drill string and the drill bit are removed from the borehole in a process called “tripping.” Once the drill string and bit are removed, the casing is lowered into the well and cemented in place. The casing protects the well from collapse and isolates the subterranean formations from each other. After the casing is in place, drilling may continue or the well may be completed depending on the situation. 
         [0006]    Conventional drilling typically includes a series of drilling, tripping, casing and cementing, and then drilling again to deepen the borehole. This process is very time consuming and costly. Additionally, other problems are often encountered when tripping the drill string. For example, the drill string may get caught up in the borehole while it is being removed. These problems require additional time and expense to correct. 
         [0007]    The term “casing drilling” refers to the use of a casing string in place of a drill string. Like the drill string, a chain of casing sections are connected end-to-end to form a casing string. The BHA and the drill bit are connected to the lower end of a casing string, and the well is drilled using the casing string to transmit drilling fluid, as well as axial and rotational forces, to the drill bit. Upon completion of drilling, the casing string may then be cemented in place to form the casing for the wellbore. Casing drilling enables the well to be simultaneously drilled and cased. Examples of such casing drilling are provide in U.S. Pat. No. 6,419,033, US Patent Application No. 20040104051 and PCT Patent Application No. WO00/50730, all of which are incorporated herein by reference. 
         [0008]    Despite the advances in casing drilling technology, current casing drilling systems are unable to provide high speed communication between the surface and the bottom hole assembly. Therefore, what is needed is a system and method to provide a casing drilling system with high speed, low attenuation rate and/or enhanced band width signal capabilities. 
       SUMMARY OF INVENTION 
       [0009]    In at least one respect, the present invention includes a communication system and method for a casing while drilling system. The casing while drilling system is adapted to advance a into a subsurface formation via a casing. The communication system includes a high frequency modulator and a transducer. The modulator is positioned in the bottom hole assembly and adapted to generate a mud pulse by selectively restricting the mud flow passing therethrough. The transducer is adapted to detect the mud pulse generated by the modulator. 
         [0010]    In another aspect, the invention relates to a method of communicating with a bottom hole assembly of a casing while drilling system. The casing while drilling system is adapted to advance the bottom hole assembly into a subsurface formation via a casing. The method includes generating mud pulses at predefined frequencies by selectively restricting a mud flow passing through a modulator of the bottom hole assembly and detecting the mud pulses at the surface. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    So that the above recited features and advantages 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 the embodiments thereof that 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. 
           [0012]      FIG. 1  is a schematic view, partially in cross-section, of a rig having a casing drilling system for drilling a wellbore, the casing drilling system provided with a casing drilling communication system. 
           [0013]      FIG. 2A  is a detailed view of the casing drilling system of  FIG. 1 , the casing drilling system can entail a drilling, measurement, and/or formation evaluation assembly such as a rotary steerable (RSS), measurement while drilling (MWD) and/or logging while drilling (LWD) system and a modulator. 
           [0014]      FIG. 2B  is a detailed view of the casing drilling system of  FIG. 1 , wherein the casing drilling communication system is run with a mud motor or turbo drill and the communication system is located uphole relative to the mud motor. 
           [0015]      FIG. 3  is a detailed, exploded view of the modulator of  FIG. 2  having a stator and a rotor. 
           [0016]      FIG. 4A  is a detailed view of the modulator of  FIG. 2  with the rotor in the open position relative to the stator. 
           [0017]      FIG. 4B  is a detailed view of the modulator of  FIG. 2  with the rotor in the closed position relative to the stator. 
           [0018]      FIGS. 5A-D  are schematic views of the rotor and stator of  FIG. 3  depicting the movement of the rotor relative to the stator. 
           [0019]      FIGS. 6A-D  are graphs depicting the relationship between pressure versus time for the rotors and stators depicted in  FIGS. 5A-D , respectively. 
           [0020]      FIG. 7  is a graph depicting signal strength versus depth at a first frequency and bit rate. 
           [0021]      FIG. 8  is a graph depicting signal strength versus depth at a second frequency and bit rate. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Referring to  FIG. 1 , a casing drilling system  100  includes a rig  102  with a bottom hole assembly (BHA)  104  deployed into a borehole  106  via a casing  108 . The rig  102  has a traveling hook/block  126 , top drive  128 , guide rail and top drive/block dolly  130  and draw works  131 . A casing drive head/assembly  132  operatively connects the casing to the top drive  128 . The casing  108  extends through a conductor pipe  134 . Casing slips  136  are used to suspend the casing  108  string when adding a new joint of casing as drilling depth increases. 
         [0023]    In one embodiment, the BHA  104  includes a drill bit  118  at a downhole end thereof, a rotary steerable (RSS), measurement while drilling (MWD) and/or logging while drilling (LWD) assembly  125 , and an under reamer  122 . A BHA latch &amp; seal assembly  124  operatively connects the BHA  104  to the casing  108 . Preferably, the latch &amp; seal assembly  124  and the BHA  104  are retrievable through the casing  108 . The MWD/LWD assembly  125  preferably includes or communicates with a telemetry system or modulator, which is described in detail below, for communication with an acquisition and demodulation unit  127 . The acquisition and demodulation unit  127  typically resides in a surface unit, cabin or enclosure (not shown). 
         [0024]    A surface mud pit  110  with a mud  112  therein is positioned near the rig  102 . Mud  112  is pumped through feed pipe  114  by pump  116  and through the casing  108  as indicated by the arrows. Mud  112  passes through the BHA  104 , out the drill bit  118  and back up through the borehole  106 . Mud  112  is then driven out an outlet pipe  120  and back into mud pit  110 . 
         [0025]    The drill bit  118  advances into a subterranean formation F and creates a pilot hole  138 . The under reamer  122  advances through the borehole  106 , expands the pilot hole  138  and creates an under-reamed hole  140 . The BHA  104  is preferably retrievable through the casing  108  on completion of the drilling operation. The under reamer  122  is preferably collapsible to facilitate retrieval through the casing  108 . 
         [0026]    Referring now to  FIG. 2A  depicts a portion of the casing drilling system  100  of  FIG. 1  in greater detail. As mud  112  is pumped from feed pipe  114  through pump  116 , it passes by a pressure transducer  142  and down through the casing  108  to an RSS, MWD, and/or LWD assembly  125  as indicated by arrows  148 ,  150 , and  152 . The mud  112  passes through the BHA  104 , exits the drilling bit  118  and returns through borehole  106  as indicated by arrows  154 ,  156  and  158 . 
         [0027]    The RSS, MWD, and/or LWD assembly  125  uses a mud pulse system, such as the one described in U.S. Pat. No. 5,517,464, which is incorporated herein by reference. The RSS, MWD, and/or LWD assembly  125  includes a modulator  162  adapted to communicate with a surface unit (not shown). As mud  112  passes through the modulator  162 , the modulator  162  restricts the flow of the mud  112  and hence the pressure to generate a signal that travels back through the casing  108  as indicated by arrows  160  and  163 . The pressure transducer  142  detects the changes in mud pressure caused by the modulator  162 . The acquisition and demodulation unit  127  processes the signal thereby allowing the  104  to communicate to the surface through the unit  127  for uphole data collection and use. 
         [0028]    Referring now to  FIG. 2B , an alternative embodiment is shown wherein a BHA  204  includes a drilling, measurement, and/or formation evaluation assembly  225 , such as RSS, MWD, and/or LWD, a mud motor or turbo-drill  210 , a drill bit  218 , an under-reamer  222 , and a data transmission module  224 . The mud motor  210  is located downhole or below a casing drilling modulator  262 , which is similar to the modulator  162  of  FIG. 2A . Using a mud or drilling motor, such as the mud motor  210 , provides the advantage of reducing the amount of rotations on the casing  108 . In one embodiment, the modulator  262  communicates with the transmission module  224 , which is in communication with other components or elements of the BHA  204 . In an alternative embodiment, the modulator  262  communicates directly with the other elements in the BHA  204  including the RSS, MWD, and/or LWD assembly  225  through various means including wired or wireless such as electromagnetic or ultrasonic methods. The scope of the present invention is not limited by the mean used for communication, which includes but is not limited to transmission through wired methods or wireless methods, which could include electromagnetic, ultrasonic or other means, or a combination thereof, such a wired and wireless or ultrasonic and electromagnetic combined with wired communication. Positioning the mud motor  210  downhole relative to the modulator  262  is the present embodiment which limits signal attenuation and produces the higher data rate and depth capability. 
         [0029]    Referring now to  FIG. 3 , the modulator  162  of  FIG. 2A  and modulator  262  of  FIG. 2B  are depicted in greater detail. In each of the embodiments set forth herein, the modulator are similar in operation. Accordingly, even though the operation of one of the modulators is discussed in detail, the operation and results are applicable to similar types of modulators shown in alternative embodiments. The modulator  162  includes a stator  164 , rotor  166  and turbine  167 . The modulator  162  may be, for example, of the type described in U.S. Pat. No. 5,517,464, already incorporated herein by reference. In one embodiment, the modulator  162  is preferably a rotary or siren type modulator. Such modulators are typically capable of high speed operation, which can generate high frequencies and data rates. Alternatively, in another embodiment conventional “poppet” type or reciprocating pulsers may be used, but they tend to be limited in speed of operation due to limits of acceleration/deceleration and motion reversal with associated problems of wear, flow-erosion, fatigue, power limitations, etc. 
         [0030]    As the mud flow passes through the turbine  167 , the mud flow turns the turbine  167  and the rotation of the turbine  167  caused by the flow of mud generates power that can be used to power any required part of portion the BHA  104 , including the rotor  166  of modulator  162 . 
         [0031]      FIGS. 4A and 4B  show the position of the rotor  166  and stator  164 . In  FIG. 4A , the rotor  166  is in the open position. In other words, the rotor  166  is aligned with the stator  164  to permit fluid to pass through apertures  168  therebetween. 
         [0032]    In  FIG. 4B , the rotor  166  is in the closed position, such that the apertures  168  are blocked, at least partially. In other words, the rotor  166  is mis-aligned with respect to the stator  164  to block at least a portion of the fluid passing through apertures  168  therebetween. The movement between the open and closed position creates a ‘pressure pulse.’ This pressure pulse is a signal detectable at the surface, and is used for communication. 
         [0033]    Referring now to  FIGS. 5A-D , the flow of fluid past the rotor  166  and stator  164  is shown in greater detail in  FIGS. 5A-D . In the open position ( FIG. 5A ), fluid passes with the least amount of restriction past stator  164  and rotor  166 . 
         [0034]    As the rotor  166  rotates and blocks a portion of the aperture  168  ( FIG. 5B ), fluid is partially restricted, thereby causing a change in pressure over time. The rotor  166  then rotates to a more restricted or closed position ( FIG. 5C ) and restricts at least a portion of the fluid flow. The rotor  166  advances further until it returns to the unobstructed position ( FIG. 5D ). 
         [0035]    Referring now to  FIGS. 6A-D , the change in pressure over time is displayed in graphs of pressure-versus-time plots of the fluid flow for each of the rotor positions of  FIGS. 5A-D , respectively. 
         [0036]    The following equations show the general effect of various parameters of the mud pulse signal strength and the rate of attenuation: 
         [0000]        S=S   o exp[−4 πF ( D/d ) 2 (μ/ K )] 
         [0000]    where
   S=signal strength at a surface transducer;   S o =signal strength at the downhole modulator;   F=carrier frequency of the MWD signal expressed   D=measured depth between the surface transducer and the downhole modulator   d=inside diameter of the drill pipe (same units as measured depth);   L plastic viscosity at the drilling fluid; and   K=bulk modulus of the volume of mud above the modulator,
 
and by the modulator signal pressure relationship
   
 
         [0000]        S   o ∞(ρ mud   ×Q   2 )/ A   2    
         [0000]    where
   S o =signal strength at the downhole modulator;   ρ mud =density of the drilling fluid;   Q=volume flow rate of the drilling fluid; and   A=the flow area with the modulator in the “closed” position   
 
         [0048]    The foregoing relationships demonstrate that a larger diameter of pipe, such as the casing  108 , makes higher carrier frequencies and data rates possible since the attenuation rate is lower for larger pipe diameters. Thus, for the specific application of casing drilling, the effect of the inside diameter “d”, as shown in  FIG. 2 , makes higher carrier frequencies (hence, data rates) possible since the rate of attenuation is much less compared to conventional drill pipe. Accordingly, the ability to transmit at high frequencies and, hence the scope of the present invention, is determined by the foregoing relationships. The specific data rates provided below are for illustration purposes and not intended as a limiting example. 
         [0049]    Referring now to  FIGS. 7 and 8 , graphs comparing the signal strength (y-axis) at various depths (x-axis) for a drill pipe in comparison to a casing.  FIG. 7  shows the signal strength for a 5″ drill pipe ( 170 ) and a 7″ casing ( 172 ). A minimum level ( 174 ) for detecting signal strength is also depicted. The graph illustrates the effect diameter has on signal strength in a 24 hz-12 bit/second deep water application using synthetic oil based mud. This shows that with the larger internal diameter of casing, 12 bit/sec telemetry rate is possible to about 20000 feet as compared to the smaller drill pipe diameter where 12 bit/sec is limited to about 13000 feet. Thus, the communication system described herein in this example can operate in the range of 1 bit/sec up to 12 bits/sec depending on the casing diameter and depth. 
         [0050]      FIG. 8  shows the signal strength for a 5″ drill pipe ( 180 ) and a 7″ casing ( 182 ). A minimum level ( 184 ) for detecting signal strength is also depicted. The graph illustrates the effect diameter has on signal strength in a 1 hz-1 bit/second deep water application using synthetic oil based mud. Typically, telemetry with drill pipe will be limited to 1 bit/sec, hence there is one order of magnitude higher data rate possible in these conditions with casing as compared to drill pipe. There is also an approximately four-fold increase in signal amplitude with casing as compared to drill-pipe for 1 Hz telemetry. 
         [0051]    It should be noted that both of the examples illustrated in  FIGS. 7 and 8  are for comparison purpose only and that by changing the relevant parameters in the previously stated relationships, an increase in depth and/or data rate capability is possible. 
         [0052]    It will be understood from the foregoing description that various modifications and changes may be made in the preferred and alternative embodiments of the present invention without departing from its true spirit. Furthermore, this description is intended for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be determined only by the language of the claims that follow. The term “comprising” within the claims is intended to mean “including at least” such that the recited listing of elements in a claim are an open set or group. Similarly, the terms “containing,” having,” and “including” are all intended to mean an open set or group of elements. “A” or “an” and other singular terms are intended to include the plural forms thereof unless specifically excluded.

Summary:
A communication system for a casing while drilling system is provided. The casing while drilling system is adapted to advance a bottom hole assembly into a subsurface formation via a casing. The communication system comprises a high frequency modulator and a transducer. The modulator is positioned in the bottom hole assembly and adapted to generate a mud pulse by selectively restrict mud flow passing therethrough. The transducer is adapted to detect the mud pulse generated by the modulator.