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BACKGROUND  
       [0001]     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 using a drill bit attached to the lower end of a drill string. The well penetrates the subsurface formations containing the trapped materials so that the materials can be recovered.  
         [0002]     During drilling or after a well is drilled, various logging instruments are used to collect information about the formation properties. The well may then be completed based on the information collected about the formation to maximize the production efficiency. In the processes of drilling, logging, completion, and production, various tools are used. These tools need to withstand the harsh conditions downhole, which may include temperatures as high as 200° C. and pressures as high as 20,000 psi. Often sensitive parts of the tools are enclosed in chambers (seal housings) that may be filled with liquids (e.g., oil). The part of the tools that exit the enclosed chambers are often protected with seals that isolate the enclosed oil from the outside, while allowing movement (e.g., rotation) of the extruded parts. These seals are often referred to as “dynamic seals” because they seal against a moving part. The following description uses a mud pulse telemetry system as an example to illustrate the present invention.  
         [0003]      FIG. 1  shows a typical drilling system  101 . A drilling rig  102  at the surface is used to rotate a drill bit  107  using a drill string  103 . Using a mud pump  121 , drilling fluid, called “mud,” is pumped to the drill bit  107  through the drill string  103 . The downward flow of mud is represented in  FIG. 1  by downward arrow  104 . The mud lubricates and cools the drill bit  107  and then it carries the drill cuttings back to the surface as it flows upwardly through the annulus. The return flow of mud is represented by the upward arrow  106 .  
         [0004]     The drilling system  101  includes a bottom-hole assembly (“BHA”)  110  at the bottom end of the drill string  103 . The BHA  110  includes the drill bit  107  and any sensors, testers, tools, or other equipment (not shown) used in the drilling process. Such equipment may include formation evaluation tools, directional drilling tools, and control circuitry.  
         [0005]     Communication between the driller and the BHA  110  is typically called “telemetry.” The data that are collected by the sensors in the BHA  110  must be relayed to the surface so that the driller will have the data when making decisions about the drilling process. Additionally, the driller must be able to communicate with the BHA  110  so that commands may be sent to the BHA  110 . A “downlink” is a communication from the surface to the BHA. Likewise, an “uplink” is a communication from the BHA to the surface.  
         [0006]     There are various prior art telemetry methods. One class of telemetry methods is called “mud pulse telemetry.” Mud pulse telemetry uses pulses in the mud flow rate or pressure to communicate between the surface and the BHA.  
         [0007]     One method of downlink mud pulse telemetry uses the mud pumps at the surface to control the mud flow rate to the BHA. The flow rate is detected and interpreted by the downlink system. Methods of uplink mud pulse telemetry typically include a pressure modulator in the downhole tool. The pressure modulator creates pressure pulses in the mud flow that may be detected at the surface. A pressure modulator uses a motor or drive mechanism to operate a flow control device to generate pressure pulses in the mud flow. The drive mechanism is enclosed in a seal housing that includes a dynamic seal to allow the drive shaft to exit the seal housing.  
         [0008]     Dynamic seals on downhole tools need to function in a wide range of ambient pressures—from the atmospheric pressure uphole to the high pressure (up to 20,000 psi) downhole. To overcome such challenges, a seal housing is often equipped with a pressure compensation mechanism that permits the pressure inside the seal housing to adapt to the ambient pressure. Prior art pressure compensation mechanisms typically use a piston that is allowed to move in order to change the volume of the seal housing in response to the ambient pressure.  
         [0009]     Due to the limited diameter (hence, the volume) of the downhole tools, the piston mechanism may have to be placed at a distance from the dynamic seal. The distance between the dynamic seal and the pressure compensation mechanism unnecessarily introduces a delay between pressure pulse generation and compensation. It is therefore desirable to have methods and systems that can provide better pressure compensation.  
       SUMMARY  
       [0010]     In some embodiments the invention relates to a downhole pressure compensation system that includes a seal housing disposed in a downhole tool, a dynamic seal disposed on the seal housing, wherein the dynamic seal seals around a part that is allowed to move relative to the seal housing, and a flexible membrane disposed in a sidewall of the seal housing proximate the dynamic seal.  
         [0011]     In some other embodiments, the invention relates to a method of compensating for a mud pressure signal that includes generating a pressure signal in a mud flow rate, and transmitting the pressure to the inside of a seal housing through a flexible membrane disposed on a seal housing proximate a dynamic seal.  
         [0012]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]      FIG. 1  shows a cross section of a typical drilling system.  
         [0014]      FIG. 2  shows a cross section of a prior art pressure compensation system.  
         [0015]      FIG. 3A  shows one embodiment of a modulator in an open position.  
         [0016]      FIG. 3B  shows one embodiment of a modulator in a closed position.  
         [0017]      FIG. 4  shows a cross section of a seal in a prior art pressure compensation system.  
         [0018]      FIG. 5  shows a cross section of a mud port and a piston in a prior art pressure compensation system.  
         [0019]      FIG. 6  shows a graph of a mud pressure signal and a compensated pressure signal in a prior art pressure compensation system at 24 Hz.  
         [0020]      FIG. 7  shows one embodiment of a pressure compensation system in accordance with one embodiment of the invention.  
         [0021]      FIG. 8  shows a graph of a mud pressure signal and a compensated pressure signal in a pressure compensation system operating at 24 Hz in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]     Embodiments of the invention relate to pressure compensation systems suitable for applications involving high frequency and high amplitude pressure pulses. Certain embodiments of the present invention relate to a system for high frequency/high amplitude pressure compensation. Other embodiments of the invention may relate to a method of compensating a high frequency/high amplitude pressure signal. For clarity, the following description uses a mud pulse telemetry generator to illustrate the present invention. However, one of ordinary skill in the art would appreciate that embodiments of the invention are not limited solely to mud pulse generator. Instead, embodiments of the invention are generally applicable in any pressure compensation applications, particularly for downhole tools. The invention will now be described with reference to the figures.  
         [0023]      FIG. 2  shows a cross section of a mud pulse modulator  201  that may be used to send an uplink signal. The mud pulse modulator  201  includes a rotor  202  and a stator  203 . The rotor  202  rotates with respect to the stator  203  to generate the pressure pulses, as will be explained with reference to  FIGS. 3A and 3B . The rotor  202  is coupled to a shaft  205  that connects the rotor  202  to a drive assembly that includes a gear assembly  206  and a servo motor  207 . The shaft  205  passes through a seal housing  216 , and seals  204  seal around the shaft  205  to isolate the working oil inside the seal housing  216  from the mud that is outside the seal housing  216 . Typically, a servo motor  207  is used to enable precise control of the rotor  202 , although other drive mechanisms may be used.  
         [0024]      FIGS. 3A and 3B  show one example of a modulator  301  that may be used to generate a pressure pulse. In  FIG. 3A , the modulator  301  is in an open position. The stator  304  includes four passages, such as passage  305 , that enable mud to flow through the modulator  301 . In the open position, the rotor  306  is positioned so that it does not cover the openings  305  in the stator  304 . The rotor includes cuts  307  that enable the openings  305  to be uncovered in the open position. In the open position, the modulator  301  enables free flow of mud through the modulator  301 .  
         [0025]      FIG. 3B  shows a modulator  301  in a closed position. Flaps  308  on the rotor  306  partially cover the openings  305  in the stator  304 . This presents an impediment to the flow of mud, and the pressure increases so that a constant flow rate of mud is maintained.  FIGS. 3A and 3B  shows the modulator  301  in open and closed positions, but those having ordinary skill in the art will realize that the rotation of the rotor  306  causes the modulator  301  to modulate between the open and closed positions.  
         [0026]     Referring again to  FIG. 2 , the seal  204  provides a dynamic seal to isolate the oil inside the seal housing  216  from the mud outside. The oil inside the seal housing  216  lubricates and protects the drive mechanisms. In order for the seal  204  to maintain its integrity and proper function under conditions ranging from the atmospheric pressure (when it is uphole) to the downhole pressure (up to 20,000 psi), a pressure compensation mechanism is needed so that the pressure differential across the seal  204  is minimal, regardless of the outside pressure. The pressure compensation mechanism typically comprises a piston that is able to move freely along a cylinder to alter the volume of the oil chamber in response to the outside pressure, ensuring that the pressures on both sides of the piston are substantially the same regardless of the outside pressure. A pressure compensation mechanism typically used in a downhole tool will be described in detail later.  
         [0027]     Referring to  FIG. 2  again, the modulator  201  creates pressure pulses that travel uphole, or to the left in  FIG. 2 . For example, when the modulator  201  is in a closed position (e.g., as shown in  FIG. 3B ), a high pressure pulse will travel up hole. In the closed position, a reduction in pressure is experienced on the downhole side of the modulator  201 . Conversely, when the modulator  201  is in an open position (e.g., as shown in  FIG. 3A ), a reduction in pressure is experienced uphole, and an increase in pressure is experienced on the downhole side of the modulator  201 .  
         [0028]      FIG. 4  shows a close-up of the shaft  205  that drives the rotor ( 202  in  FIG. 2 ) and a seal assembly  404 ,  406  that seal around the shaft  205 . The outer seal  404  is a rotating seal that rotates with the shaft  205 , and inner seal  406  is a stationary seal that also seals around the shaft  205 , but it remains fixed with the seal housing  216 . In operation, the rotor  202  is driven by the drive shaft  205  to rotate with respect to the stator  203 , generating pressure pulses in the mud. These pressure pulses are experienced on the outboard side of the inner seal  406 , in area  410 , for example. The pressure pulses created by the modulator can have an adverse effect on seal performance and seal life. Thus, it is often desirable to use a pressure compensation system to balance the oil pressure on the inboard side of the seal  406 .  
         [0029]     A pressure compensation system balances the oil pressure inside the seal housing  216  (i.e., in area  412 ) so that is will fluctuate with the borehole hydrostatic pressure and the mud pressure signal outside the seal housing  216  (i.e., in area  410 ). This will ensure that the pressure differential across the inner seal  406  will remain close to zero at all times. A balanced pressure will reduce the leakage across the seal  406  and, more importantly, increase the life of the seal.  
         [0030]     Referring back to  FIG. 2 , a pressure compensation system provides pressure compensation using a port  208 , a mud chamber  210 , and a piston  212  to achieve pressure compensation inside the drive housing  209  that is in fluid communication with the stator seal  406 . The piston  212  is free to move along the length of the mud chamber  210  so that the pressures on both sides of the piston  212  are substantially the same, which in turns ensures that the pressures across the stator seal  406  are substantially the same, regardless of the outside pressure. The pressure compensation system is placed at a distance to the seal  406  due to the limited diameter (volume) of the downhole tool. The distance between the pressure compensating piston and the seal  406  necessarily creates a time delay between the pulse generation and compensation. The pressure pulses from the mud pulse modulator  201  need to travel through the mud outside the tool between the modulator  201  and the mud port  208 . At the mud port  208 , the change in pressure may enter the mud chamber  210  in the drive housing  209 . Typically, the pressure compensation piston  212 , located inside the drive housing  209 , is able to move (e.g., along the length of the mud chamber  210 ) in response to pressure differences between the mud in the mud chamber  210  and the oil pressure inside the drive chamber  209 . The oil pressure behind the piston  212  is then relayed to the seal  406  to counter (compensate) the change in pressure on the other side of the seal  406 . However, due to the time needed for the change in pressure to travel this distance, the pressures across the seal  406  are not equalized during the delay. If the pressure on the outside is greater than the pressure on the inside, then the fluid on the outside (e.g., mud) may leak into the oil housing, resulting in damages to the parts to be protected.  
         [0031]      FIG. 5  shows a close-up view of the mud port  208 , the mud chamber  210 , and the pressure compensation piston  212 . A change in pressure enters the drive housing  209  through the port  208  and is transmitted into the mud chamber  210 . The change in pressure then acts on the piston  212 , causing a corresponding change in the oil pressure. An increase in mud pressure will cause the piston  212  to move upwardly and increase the oil pressure. Similarly, a decrease in mud pressure will cause the piston  212  to move downwardly and decrease the oil pressure.  
         [0032]     In some embodiments, a piston  212  may be coupled to a spring  214 . The spring  214  applies a force to the piston  212  that would create a slightly higher pressure in the oil chamber than the pressure in the mud chamber  210 . Thus, if there were to be any leakage across the inner seal ( 406  in  FIG. 4 ), the leakage would be of oil out of the seal housing ( 216  in  FIG. 4 ) and not of mud into the seal housing.  
         [0033]     Referring again to  FIG. 2 , an increase of pressure in the mud chamber  210  will cause the piston  212  to move, thereby transmitting the pressure increase through the drive chamber  209  and to the inboard side of the seal ( 406  in  FIG. 4 ). This type of pressure compensation system requires that a pressure pulse travel from the modulator  201  to a port  208  in the drive housing, before returning through the interior of the drive housing  209 .  
         [0034]     The time delay, t, between the mud pressure pulse and the resulting pulse in the oil is related to the distance that the pulse must travel and the speed of sound in the particular fluid through which the pulse is traveling. The time delay may be quantified as shown in Equation 1:  
             t   =         (       d   o     +     d   m       )       C   m       +       d   m       C   m       +       d   o       C   o                 Eq   .           ⁢   1             
 
 where d o  is the length of the oil cavity in the tool (shown in  FIG. 2 ), d m  is the length of the mud cavity in the tool (shown in  FIG. 2 ), C o  is the speed of sound in the oil, and C m  is the speed of sound in the mud. 
 
         [0035]     The first term in Equation 1 represents the time it takes the mud pressure pulse to travel through from the seal and mud pulse modulator area to the mud port (e.g.,  208  in  FIG. 2 ). This length is represented by the sum of the length of the oil chamber d o  and the length of the mud chamber d m . The sum is divided by the speed of sound in mud C m , the medium through which the signal travels in this direction. The middle term represents the time it takes the pressure pulse to travel back through the mud chamber (e.g.,  210  in  FIG. 2 ) inside the drive housing. This time is represented by the length of the mud chamber d m  divided by the speed of sound in mud C m . The last term in Equation 1 represents the time it takes the pressure pulse to travel through the oil chamber of the drive mechanism—the length of the oil chamber d o  divided by the speed of sound in oil C o .  
         [0036]     More sophisticated mud pulse telemetry systems use higher pulse frequencies to increase and optimize the data transmission rate of the telemetry system. These can range from less than 1 Hz to 24 Hz. The higher frequencies have created problems with the response time of pressure compensation systems. At higher frequencies, the time that it takes for the pressure signal to travel to the mud port (e.g.,  208  in  FIG. 2 ), travel back through the mud chamber (e.g.,  210  in  FIG. 2 ), and travel back through the oil chamber to the inboard side of the seal (e.g.,  204  in  FIG. 2 ) may be a significant portion of one cycle. The time delay creates a compensated pressure that is out of phase with the modulator pressure.  
         [0037]      FIG. 6  shows a graph of the mud pressure signal  601  along with the compensated pressure signal  602  in the oil on the inboard side of the seal in a prior art pressure compensation system. The signal shown in  FIG. 6  is a 24 Hz signal. As shown in  FIG. 6 , there is a phase shift between the mud signal  601  and the oil signal, or compensated pressure signal  602 . The compensated signal  602  is delayed from the mud signal  601 , making the compensated signal  602  out of phase with the mud signal  601 . The difference between the mud signal  601  and the compensated signal  602  is plotted at  603 . The pressure difference  603  shown in  FIG. 6  may cause the seal (e.g.,  406  in  FIG. 4 ) to oscillate with the pressure fluctuations (represented by the pressure difference curve  603 ). Oscillation of the seal may cause damage to the seal that will reduce seal life. Additionally, when the pressure on the outside of the seal is higher than that on the inside, mud may leak into the housing, leading to damages of the seal and the drive mechanism.  
         [0038]      FIG. 7  shows one embodiment of a pressure compensation system in accordance with one embodiment of the invention. The seal housing  716  includes a flexible membrane  710  that enables pressure to be transmitted to the interior of the seal housing  716 . When the pressure outside of the seal housing  716  increases, the flexible membrane  710  flexes inwardly, thereby increasing the pressure on the inboard side of the seal  706 . Conversely, when the pressure outside of the seal housing  716  decreases, the flexible membrane  710  flexes outwardly, thereby decreasing the pressure on the inboard side of the seal  706 .  
         [0039]     The flexible membrane  710  is located in the seal housing  716  to be proximate the seal  706 . This significantly reduces the distance over which the pressure signal must be transmitted to compensate the pressure on the inboard side of the seal  706 . By reducing the distance over which the signal must travel, the response time of the pressure compensation system is significantly increased.  
         [0040]     In the embodiment shown, the flexible membrane  710  is coupled to a passageway  712  that leads to the interior of the seal housing  716 . In other embodiments, a flexible membrane may be in contact with both the mud outside the seal housing and with the oil inside the seal housing without the need for a passage way, i.e., the flexible membrane  710  may form part of a wall of a seal housing.  
         [0041]     The flexible membrane  710  may be made of any material that will flex enough to transmit pressure to the interior of the seal chamber  716 . For example, the flexible membrane  710  may be constructed of an elastomer or a thin piece of metal. Additionally, the geometry (i.e., the shape and size) of the membrane  710  may be selected based on the particular application or operating condition. For example, the membrane  710  may extend around the entire circumference of the seal housing  716 , forming a frustoconical shape. In other embodiments, the membrane  710  may form a window over only a portion of the seal housing  716 . The geometry and the material of the membrane  710  may be selected for specific applications and design considerations.  
         [0042]     Those having ordinary skill in the art will realize that any number of variations of a flexible membrane may be possible without departing from the scope of the invention. For example, this description makes reference to a “seal housing,” which houses and protects the seals, and a “drive housing,” which houses and protects the drive mechanisms for the modulator. In practice, however, these may not be separate components. That is, a drive mechanism housing may also house and protect the seals.  
         [0043]     Additionally, a flexibly membrane may be constructed of a material having enough strength that the flexible membrane may be in direct contact with both the mud on the outside of the seal housing and the oil on the inside of the seal housing. In such an embodiment, a passage (i.e., passage  712 ) between the flexible membrane and the interior of the seal housing may not be necessary. Other variations of a flexible membrane may be devised that do not depart from the scope of the invention.  
         [0044]      FIG. 8  shows a graph of a mud pressure signal  801  along with a compensated pressure signal  802 , using a pressure compensation system in accordance with the invention. The compensated pressure signal  802  closely matches the mud pressure signal  801  created by the modulator (e.g.,  202  in  FIG. 2 ). Plot  803  shows the difference between the mud pulse signal  801  and the compensated pressure signal  802 . The difference  803  shows a constant, slight excess pressure on the inside of the seal housing. This slight excess pressure is typically provided by a spring mechanism to ensure that no mud will leak into the housing.  
         [0045]     Embodiments of the invention use flexible members close to the dynamic seals to provide better pressure compensation and improved seal lives. One of ordinary skill in the art would appreciate that the flexible membrane pressure compensation mechanism in accordance with the invention may be used together with the prior art piston pressure compensation mechanism. For downhole tools, the combined use of these two types of pressure compensation mechanisms is particularly beneficial—the piston pressure compensation mechanism ensures that the protected oil chamber can be used in a wide range of pressure (e.g., from the atmospheric pressure to the downhole pressure), while the flexible membrane mechanism ensures that high frequency and/or high magnitude pressure pulses are effectively compensated.  
         [0046]     It is noted that a piston arrangement is one possible prior art pressure compensation system that could be used with embodiments of the invention. Other pressure compensation systems may include a bellows system or a bladder system. Those having ordinary skill in the art will be able to devise other types of pressure compensation systems that may be used with embodiments of the invention.  
         [0047]     Certain embodiments of the present invention may present one or more of the following advantages. A pressure compensation system in accordance with the invention may decrease the phase shift of a compensated pressure pulse. At a high modulator frequency, the reduced phase shift may reduce the pressure differential across a seal in the modulator system.  
         [0048]     Advantageously, a pressure compensation system in accordance with the invention may reduce or prevent oscillations of a seal in the modulator system. Reduced oscillation may decrease seal leakage and increase seal life. The ability of a pressure compensation system to compensate for high frequency pressure telemetry signals enables the use of still yet higher frequencies in a telemetry. Advantageously, a pressure compensation system in accordance with the invention may enable faster communication in a telemetry system. Similarly, embodiments of the invention may provide benefits to other tools that include pressure compensation mechanisms. It is noted that there are devices that can emit high frequency and high magnitude pressure changes other than the telemetry devices described above and the scope of this invention should not be limited as such.  
         [0049]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A downhole pressure compensation system includes a seal housing disposed in a downhole tool, a dynamic seal disposed on the seal housing, wherein the dynamic seal seals around a part that is allowed to move relative to the seal housing, and a flexible membrane disposed in a sidewall of the seal housing proximate the dynamic seal.