Abstract:
A filter assembly including a primary flow passageway having at least one filter deployed therein and a secondary flow passageway having a bypass filter deployed therein is provided. The filter assembly further includes a bypass valve assembly configured to selectively open and close the secondary flow passageway when a fluid pressure is respectively above and below an adjustable, predetermined threshold value. Exemplary embodiments of this invention may be coupled to a drill string and advantageously utilized to filter drilling fluid downhole. Such embodiments tend to advantageously improve the filtering efficiency and safety of drilling operations.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/527,614 entitled Drilling Fluid Filter Assembly Having a Bypass Passageway, filed Dec. 5, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to filtering devices. More particularly, this invention relates to a downhole filtering tool including a filtered, pressure activated, bypass flow passageway.  
       BACKGROUND OF THE INVENTION  
       [0003]     The use of drilling fluids for the drilling of subterranean boreholes is well known. The drilling fluid serves numerous purposes, including, for example, suppression of formation pressure, lubrication of the drill string, flushing drill cuttings away from the drill bit, cooling of the bottom hole assembly, driving turbines that provide power for various downhole tools, and powering downhole progressive cavity motors. In use drilling fluids are typically pumped down through the tubular drill string to the drill bit and circulate back to the surface in the annular region between the drill string and the borehole wall. The circulating drilling fluid typically carries drill cuttings, metal shavings, and other debris to the surface. Large particles, having a size that may damage sensitive downhole tools, such as various measurement while drilling (MWD) or logging while drilling (LWD) tools, or plug drill bit jets are desirably removed from the drilling fluid before recycling back into the borehole.  
         [0004]     Various surface filtering techniques are well known in the industry for removing drill cuttings and other debris from the drilling fluid. For example, shaker tables are commonly used to screen out relatively large particles (e.g., having a diameter greater than {fraction (1/8)} inch). Centrifugal tools, such as desanders and desilters are also commonly used to remove abrasive solids prior to recycling the drilling fluid back into the borehole. However, it is not uncommon for such surface filtering techniques to fail, resulting in large drill cuttings and debris being pumped downhole. Additionally, various “foreign objects”, such as tools, rags, gravel, chunks of plastic from thread protectors, and the like are sometimes introduced into the drilling fluid through human error and inadvertently pumped downhole.  
         [0005]     As a redundant measure, pipe screens are commonly used on the topmost section of drill string with the intention of preventing large particles and debris from being pumped downhole. While such pipe screens have been successfully utilized and are commercially available, they are nevertheless prone to failure in that operator intervention is required to remove, clean, and reposition the screen each time a new length of drill string is added. Furthermore, damaging scale and/or cement particles often originate from locations within the drill string. Scale particulate may result, for example, from corrosion of the drill string components or various mineral deposits, while cement particles are sometimes deposited on the interior of the drill string during cementing operations. Such particles are sometimes freed during drilling operations and are a known source of blockage or damage to downhole tools.  
         [0006]     In an attempt to overcome such difficulties, retrievable downhole filtering tools are known, for example, those disclosed by Beimgraben in U.S. Pat. No. 4,495,073, Taylor in U.S. Pat. No. 6,296,055, and Mashburn in U.S. Pat. No. 6,598,685. Such retrievable filtering tools are intended to be periodically removed from the drill string and cleaned (e.g., when the pressure at the mud pump reaches some predetermined threshold). While such prior art filtering tools may, in certain applications, remove damaging particles from the drilling fluid, their retrieval from the drill string is often problematic. For example, in certain drilling applications, it may be advantageous for various sections of the drill string to include a reduced inner diameter. However, such a reduced inner diameter may render it impossible to retrieve the filtering tools. Furthermore, in deep well applications (e.g., at measured depths greater than 10,000 feet), it is sometimes difficult to generate the impact required to dislodge the filtering tool from the drill string (e.g., to shear one or more shear pins). In such instances it is often necessary to remove at least a portion of the drill string from the borehole (at significant expense and time loss) in order to retrieve the filtering tool. Moreover, the act of retrieving such retrievable filtering tools has been known to cause debris to be freed or dumped in the drill string.  
         [0007]     Therefore, there exists a need for an improved downhole filtering tool for filtering a drilling fluid. In particular there exists a need for a downhole filtering tool that does not generally require retrieval from the drill string.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention addresses one or more of the above-described drawbacks of prior art drilling fluid filtering apparatuses. Exemplary aspects of this invention include a filtering tool configured for deployment in a drill string. The filtering tool typically includes one or more filters configured for capturing large particles, for example, greater than about {fraction (3/8)} inch, from the drilling fluid. The one or more filters may advantageously be fabricated from a hard, wear resistant material, and are configured to hold the large particles until they erode to a sufficiently small size to pass through the filter(s). The filtering tool further includes a filtered bypass flow passageway (also referred to as a secondary flow passageway) in the event that the one or more filters become substantially full of debris. A bypass valve assembly is configured to open, thereby allowing drilling fluid to flow through the bypass flow passageway, when the pressure of the drilling fluid exceeds a predetermined threshold.  
         [0009]     Exemplary embodiments of the present invention advantageously provide several technical advantages. Embodiments of the filtering tool of this invention advantageously provide a filtered secondary flow passageway that may be opened at a predetermined threshold pressure. Further, the threshold pressure may be adjusted at the surface (e.g., by a drilling operator) to meet the requirements of various drilling applications. The use of a filtering tool having a secondary flow passageway may also advantageously improve the safety of drilling operations. In the event that the filter(s) become substantially full of debris and the pressure of the drilling fluid increases, circulation of the drilling fluid may be maintained and the well kept under control, via diverting a portion of the flow through the secondary flow passageway. In many instances, drilling operations may continue. Moreover, exemplary embodiments of this invention may also be configured to be “self-cleaning” in that the filter(s) may trap and hold large particles until they erode to a smaller size, potentially obviating the need to use retrievable filters (as with the above described prior art tools).  
         [0010]     In one aspect the present invention includes a filtering assembly. The filtering assembly includes a housing having a through bore that provides a primary flow passageway through the housing. A bypass flow tube deployed in the through bore provides a secondary flow passageway through the housing. At least one primary filter is deployed in the primary flow passageway, and a bypass filter is disposed to filter fluid flow through the secondary flow passageway. The filtering assembly further includes a bypass valve assembly deployed in the housing and disposed to selectively open the secondary flow passageway when a fluid pressure reaches a predetermined threshold pressure. In certain exemplary embodiments of this invention, first and second primary filters may be deployed about the bypass flow tube. Moreover, in some exemplary embodiments, the bypass filter may be coupled to an upstream end of the bypass flow tube, while the bypass valve assembly may be located proximate to a downstream end of the bypass flow tube.  
         [0011]     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0013]      FIG. 1  is a schematic representation of an offshore oil and/or gas drilling platform utilizing an exemplary embodiment of the present invention.  
         [0014]      FIG. 2  depicts, in cross section, a drilling sub in which embodiments of this invention may be deployed.  
         [0015]      FIG. 3  depicts, in cross section, an exemplary filtering tool embodiment of this invention.  
         [0016]      FIG. 4A  is a cross sectional view as shown on section  4 A- 4 A of  FIG. 3 .  
         [0017]      FIG. 4B  is a cross sectional view as shown on section  4 B- 4 B of  FIG. 3 .  
         [0018]      FIG. 5  depicts, in cross section, the exemplary filtering tool of  FIG. 3  in a compressed configuration.  
     
    
     DETAILED DESCRIPTION  
       [0019]      FIG. 1  schematically illustrates one exemplary embodiment of a downhole filtering sub  100  according to this invention in use in an offshore oil or gas drilling assembly, generally denoted  10 . In  FIG. 1 , a semisubmersible drilling platform  12  is positioned over an oil or gas formation (not shown) disposed below the sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to a wellhead installation  22 . The platform may include a derrick  26  and a hoisting apparatus  28  for raising and lowering the drill string  30 , which, as shown, extends into borehole  40  and includes a drill bit  32  and filtering sub  100 . In the embodiment shown, downhole filtering sub  100  is deployed in the drill string  30  above one or more downhole measurement tools  200  (e.g., MWD or LWD tools). It will be appreciated that filtering sub  100  may be deployed in substantially any location in the drill string  30 . However, in certain applications the filtering sub  100  may be advantageously deployed near the bottom of the drill string  30 , but above sensitive measurement tools, such as measurement tools  200 .  
         [0020]     It will be understood by those of ordinary skill in the art that the deployment illustrated on  FIG. 1  is merely exemplary for purposes of describing the invention set forth herein. It will be further understood by those of ordinary skill in the art that the filtering sub  100  of the present invention is not limited to use with a semisubmersible platform  12  as illustrated in  FIG. 1 . Downhole filtering sub  100  is equally well suited for use with any kind of subterranean drilling operation, either offshore or onshore.  
         [0021]     It will be further understood that the present invention is also not limited to subterranean drilling applications. Embodiments of the invention include a filter assembly that has a filtered, pressure activated, bypass flow passageway.  
         [0022]     Referring to  FIGS. 2 through 5 , it will be understood that features or aspects of the embodiments illustrated may be shown from various views. Where such features or aspects are common to particular views, they are labeled using the same reference numeral. Thus, a feature or aspect labeled with a particular reference numeral on one view in  FIGS. 2 through 5  may be described herein with respect to that reference numeral shown on other views.  
         [0023]     With reference now to  FIG. 2 , an exemplary filter sub  100  according to this invention is shown in longitudinal cross section. Filter sub  100  includes a tubular tool body  102  having threaded ends  104  and  106  (commonly referred to as a box  106  and pin  104 ). The tool body  102  is typically sized and shaped for coupling to a conventional drill string and may be fabricated from substantially any suitable material (e.g., a high strength stainless steel). The tool body includes a center bore  108  that provides a suitable passageway for the flow of drilling fluid. In the embodiment shown, filtering sub  100  includes an internal filtering module  110  having one or more filters  120 ,  130  deployed in a wear sleeve  112 . While the embodiment shown in  FIG. 2  includes upper and lower filters  120 ,  130 , it will be appreciated that filter sub  100  may include substantially any number of filters. Terms used in this disclosure such as “upper” and “lower” are intended merely to show relative positional relationships of various components in the described exemplary embodiments and are not limiting of the invention in any way. As described in more detail below, filtering module  110  includes primary  115  and secondary  145  flow passageways. A bypass valve assembly  150  is disposed to control the flow of drilling fluid through the secondary flow passageway  145 .  
         [0024]     Turning now to  FIG. 3 , exemplary embodiments of an internal filtering module  110  according to this invention are described in more detail. As described above, the embodiment shown includes upper and lower filters  120 ,  130  deployed in a substantially annular primary flow passageway  115 . Exemplary internal filtering module  110  further includes a bypass flow tube  140  deployed coaxially with the wear sleeve  112  and the upper and lower filters  120 ,  130 . The bypass flow tube  140  provides a secondary flow passageway  145  (also referred to as a bypass flow passageway) and is positioned such that an upper end  143  thereof is disposed upstream of the upper filter  120  and a lower end  147  thereof is positioned downstream of the lower filter  130 .  
         [0025]     With continued reference to  FIG. 3 , the upper and lower filters  120 ,  130  may, for example, be slidably received about bypass flow tube  140 . In the exemplary embodiment shown, upper filter  120  is received on the upper end  143  of the bypass flow tube  140  and abuts a first shoulder portion  141  thereof. The lower filter  130  is received on the lower end  147  of the bypass flow tube  140  and abuts a second shoulder portion  142  thereof. Screen cap  116  is threadably received in wear sleeve  112  and holds upper filter  120  securely against shoulder portion  141 . Lower filter  130  is held securely in place between shoulder portion  142  and a shoulder portion  117  of wear sleeve  112 .  
         [0026]     Turning now to  FIGS. 4A and 4B , exemplary embodiments of upper and lower filters  120 ,  130  are described in more detail. In the embodiments shown, upper and lower filters  120 ,  130  include substantially disk shaped screen portions  124 ,  134 , each having a plurality of radial slots  122 ,  132  formed therein. While screen portions including perforations of substantially any shape (e.g., holes, slots, and the like) may be utilized, the use of radial slots  122 ,  132  may be advantageous in that filtered debris are typically less likely to fully block the flow path through the filter  120 ,  130 . In the exemplary embodiment shown, the diameter  125  of the radial slots  122  in the upper filter  120  is greater than the diameter  135  of the radial slots  124  in the lower filter  130 , however, the invention is not limited in this regard. It will be appreciated that filters having substantially any slot size may be utilized. For example, in various exemplary downhole embodiments, diameter  125  may advantageously be in the range of from about ⅜ to about {fraction (5/8)} inch, while diameter  135  may advantageously be in the range of from about ¼ to about {fraction (1/2)} inch. It will likewise be appreciated that filters  120 ,  130  may include substantially any slot pattern.  
         [0027]     Filters  120  and  130  may be advantageously fabricated from a highly wear resistant material, such as a high strength stainless steel, to minimize erosion thereof in the high velocity, abrasive drilling fluid. Preferred embodiments include Rockwell C hardness values of greater than about 55. In one embodiment, screens  124  and  134  are fabricated from a D2 tool steel (a high strength, nonmagnetic, alloy steel) available from Diehl Steel in Dallas, Tex. Such highly wear resistant materials may advantageously withstand drilling fluid velocities of up to about 80 feet per second. It will be appreciated that other components, such as the bypass flow tube  140 , bypass valve stem  154 , bypass filter  160 , and wear sleeve  112  may be advantageously fabricated from a highly wear resistant material, such as a D2 tool steel, to minimize erosion thereof.  
         [0028]     With reference again to  FIG. 3 , internal filtering module  110  further includes a bypass valve assembly  150  deployed therein. Bypass valve assembly  150  includes a valve stem  154  deployed (e.g., slidably received) in a bypass valve housing  156 . The valve stem  154  is typically secured in the bypass valve housing  156  via a retainer nut  155 . Valve stem  154  is further disposed to slide longitudinally in housing  156  such that compression of pressure setting spring  158  permits a range of longitudinal motion d 1 . Comparison of  FIGS. 3 and 5  shows valve stem  154  in opposing end positions within sliding range d 1 . In the first position (as shown in  FIG. 3 ), a tapered end  152  of valve stem  154  is biased into contact with a valve seat  146  on the lower end  147  of the bypass flow tube  140  via pressure setting spring  158 , thereby effectively closing the secondary flow passageway. In the fully displaced position (shown in  FIG. 5 ), pressure setting spring  158  is substantially fully compressed, thereby opening the secondary flow passageway.  
         [0029]     Exemplary embodiments of pressure setting spring  158  may be fabricated from substantially any suitable material such as an ELGILOY® spring steel available from Elgiloy, Incorporated, Elgin, Ill. In one exemplary embodiment, pressure setting spring  158  may advantageously be rated in the range of from about 100 to about 200 pounds per compressed inch (e.g., a nominal 150 pounds per compressed inch). In such an embodiment, spring  158  may be pre-compressed, for example, about one inch to exert about 150 pounds of force when holding tapered end  152  against valve seat  146 . The application of such a force on the valve stem in the rest position tends to prevent the flow of drilling fluid through the bypass flow passageway  145  under normal operating conditions (as described in more detail below). Moreover, the pressure exerted by spring  158  on valve stem  154  advantageously prevents the bypass valve assembly  150  from inadvertently opening due to mechanical forces experienced downhole, such as impact and shock.  
         [0030]     It will be appreciated that the magnitude of the force holding the tapered end  152  of valve stem  154  against valve seat  146  may be readily adjusted at a drilling site. For example, spring  158  may be replaced with a spring member having a different spring constant (e.g., increasing the spring constant which increases the force) or a spring having another longitudinal dimension (e.g., increasing the length of the spring which increases the amount of pre-compression and thus the force). Alternatively, spacers (e.g., conventional washers) may be inserted (or removed from) between the spring  158  and the base of the bypass valve housing  156 , effectively changing the amount of spring pre-compression.  
         [0031]     In the exemplary embodiment shown, bypass valve housing  156  is fitted with a plurality of stabilizer fins  114  that extend radially outward and into contact with an inner surface of wear sleeve  112 . The stabilizer fins  114  are intended to stabilize the bypass valve assembly  150  coaxially in the wear sleeve  112 . In the exemplary embodiment shown, the bypass valve assembly  150  is slidably received in wear sleeve  112 . As the bypass valve assembly  150  is received into the wear sleeve  112 , the tapered end  152  of the valve stem  154  contacts the valve seat  146 . The bypass valve assembly continues to be received into the wear sleeve  112 , partially compressing spring  158  and increasing the force holding valve stem  154  against the valve seat  146 , until stabilizer fins  114  contact shoulder portion  119  of wear sleeve  112 . A screen cap  118  is threadably received in wear sleeve  112  and holds the stabilizer fins  114  securely against shoulder portion  119 .  
         [0032]     With continued reference to  FIG. 3 , internal filtering module  110  further includes a bypass filter housing  162 , having a bypass filter  160 , coupled (e.g., threadably coupled) to the upper end  143  of the bypass flow tube  140 . It will be appreciated that bypass filter  160  may be integral with or coupled to bypass filter housing  162 . Exemplary embodiments of the bypass filter  160  include a plurality of longitudinal slots  164 . Longitudinal slots  164  may advantageously reduce the tendency of the bypass filter  160  to become plugged with debris as the filtered particles are typically swept past the bypass filter  160  to the upper filter  120  by the flow of the drilling fluid.  
         [0033]     In operation, filtering sub  100  ( FIG. 2 ) is coupled to a drill string (e.g., as shown in  FIG. 1 ). As drilling fluid is pumped down through the drill string, it flows through the primary flow passageway  115  as shown at  180  on  FIG. 3 . As drill cuttings and/or various other debris are trapped in filters  120  and/or  130  the pressure of the drilling fluid increases, thereby increasing its local velocity. In general, debris continues to accumulate until the local fluid velocity becomes great enough (e.g., about 50 feet per second) to erode the debris. Such erosion of the debris reduces its size until it passes through the filters  120 ,  130 . In the embodiment shown, in which upper and lower filters  120 ,  130  are employed, debris may be trapped at the upper filter  120  until it erodes sufficiently to pass there through. Such debris may then be trapped at the lower filter  130  until it erodes further and passes there through.  
         [0034]     As the pressure of the drilling fluid increases, the pressure in the secondary flow passageway  145  (in bypass flow tube  140 ) also increases, thereby increasing the force of the drilling fluid against the bypass valve stem  154 . In the event that the pressure increases above a predetermined threshold, the force of the drilling fluid begins to overcome the force applied by the pressure setting spring  158 . As such, the bypass valve stem  154  is displaced longitudinally from its rest position, thereby allowing drilling fluid to flow through the secondary flow passageway  145  as shown at  190  on  FIG. 5 . As the pressure of the drilling fluid increases further, the bypass valve stem  154  is further displaced from its rest position towards a fully displaced position at which spring  158  is substantially fully compressed (as shown in  FIG. 5 ).  
         [0035]     As described above, the use of a filtered, secondary flow passageway often enables drilling to continue even after the upper and lower filters  120 ,  130  are substantially plugged with debris. Bypass filter  160  typically prevents debris from passing through the secondary flow passageway. Further, as described above, bypass filter arrangements having longitudinal slots  164  (as shown on  FIGS. 3 and 5 ) tend to advantageously prevent clogging as debris are often swept past the bypass filter  160  to upper filter  120 .  
         [0036]     After the secondary flow passageway  145  is opened (as described above), a portion of the drilling fluid typically continues to flow through the primary fluid passageway. Such flow through the primary flow passageway, with locally high velocities owing to the high pressure, typically continues to erode the debris lodged in the upper and lower filters  120 ,  130 . It is often the case that such continued erosion enables the debris to eventually pass through the upper and lower filters  120 ,  130  (as described above). In such cases the pressure of the drilling fluid decreases as the debris passes through the upper and lower filters  120 ,  130 . As the pressure decreases, the bypass valve stem  154  displaces longitudinally back towards its rest position, thereby decreasing the flow through the secondary flow passageway  145 . When the pressure decreases below the predetermined threshold value, the bypass valve stem  154  returns to its rest position (in contact with bypass valve seat  146 ), thereby substantially closing the secondary flow passageway.  
         [0037]     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.