Patent Publication Number: US-6698538-B2

Title: Drill bit having adjustable total flow area

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This invention claims priority from U.S. Provisional Application Ser. No. 60/304,555, filed on Jul. 11, 2001. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention is related to the field of drill bits used to drill wellbores through earth formations. More specifically, the invention is related to types of nozzles, jets and other devices which affect the hydraulic properties of a drill bit. 
     2. Background Art 
     Drill bits used to drill wellbores through earth formations include, for example, fixed cutter bits, such as polycrystalline diamond compact (“PDC”) bits, and roller cone or “rock” bits. Generally, these drill bits include a bit body which can be coupled to a drill string to rotate the bit, and various forms of cutting elements attached to the bit. PDC bits include PDC cutters affixed to the bit body, while roller cone bits include at least one roller cone rotatably mounted to the bit body. The roller cone includes cutting elements thereon, such as milled steel teeth or various forms of inserts. 
     Most of these drill bits include at least one, and typically a plurality of, “nozzles” or “jets” which are hydraulically coupled to the interior of the bit body. During drilling operations, as the drill bit is rotated, a drilling fluid (“drilling mud”) is pumped through the interior of the drill string, where it is discharged through the jets. The drilling mud then travels upward through the annular space between the drill string and the wellbore. The drilling fluid cools and lubricates the cutting elements and the bit body, and cleans cuttings from the bottom of the wellbore as it is drilled. The drilling fluid also lifts the cuttings from the wellbore and transports them to the earth&#39;s surface. 
     The number of, flow area (or orifice size) of, and placement of the jets on any particular drill bit depend on, among other factors, the hydraulic characteristics needed to drill a particular formation at a particular depth in a wellbore, and the type of bit being used. Typically, the wellbore operator desires to have a selected total flow area (“TFA”) of all the jets on the bit so that the drilling fluid circulation system will provide a selected pressure drop in the drilling fluid at a selected drilling fluid flow rate. 
     In certain circumstances, it is desirable to change the TFA of a bit during drilling of a particular wellbore. These circumstances may include, for example, that as the depth of the wellbore increases, the fluid pressure loss due to friction increases. Flow rates of the drilling fluid typically must be increased in order to maintain the necessary flow through the jets on the bit. More recently, specialized directional drilling tools, known as “rotary steerable” systems have been developed for enabling wellbore operators to control the trajectory of the wellbore while rotating the drill string. When rotary steerable systems are used, an amount of pressure drop in the drill string may be limited by the pressure drop capacity of the rotary steerable system. In such cases, it is desirable to change the TFA of the drill bit to reduce fluid pressure drop along the entire drill string. 
     Changing TFA in a typical drill bit includes changing a flow area of one or more of the jets, or replacing a plug in a port therefore in the bit body with an orifice or jet. However, changing the TFA of the bit requires removing the entire drill string from the wellbore to make the jet, plug or orifice change. Removing the drill string can be expensive and time consuming. It is desirable to have a drill bit which can have the TFA changed during drilling without removing the bit from the wellbore. 
     SUMMARY OF INVENTION 
     One aspect of the invention is a drill bit which includes a bit body having a plurality of ports therein arranged to provide a flow path between an interior of a drill string and the exterior of the bit body. At least one flow relief is disposed in one of the ports. The at least one flow relief is adapted to provide an increase in total flow area of the bit upon application to the bit of a selected fluid flow condition. 
     Another aspect of the invention is a method for changing a total flow area of a drill bit, which includes pumping drilling fluid through the drill bit and operating a flow relief disposed in the bit to change the total flow area of the bit. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a drill string in a wellbore as used to turn a drill bit. 
     FIG. 2 shows an oblique view of one embodiment of a drill bit which can be made according to the invention. 
     FIG. 3 shows an end view of the example bit in FIG.  2 . 
     FIG. 4 shows one embodiment of a rupture disk which can be used in a bit according to the invention. 
     FIG. 5 shows an example of a pressure relief valve which can be used in a bit according to the invention. 
     FIG. 6 shows another type of pressure relief valve which can be used in a bit according to the invention. 
     FIG. 7 shows another embodiment of a flow relief according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a drill bit  10  which may be any one of a number of various embodiments of the invention as it is used to drill a wellbore  5  through earth formations  8 . The drill bit  10  is coupled to the lower end of a drill string  4 , which typically includes segments of drill pipe (not shown separately) threadedly coupled together. The drill bit  10  may be coupled to the drill string  4  directly or through various drilling tools such as a drill collar  6 , and rotary steerable drilling system  7 . It should be understood that the drill string configuration shown in FIG. 1 is only one example of a drilling tool assembly which may be used with a drill bit according to the invention, and therefore, the drill string configuration of FIG. 1 is not intended to limit the invention. The drill string  4  may be rotated by a rotary table (not shown in FIG. 1) or a top drive system  2  which is itself hoisted and lowered by a drilling rig  1 . Drilling fluid (“drilling mud”) is circulated through the drill string  4  by mud pumps  3  of any type known in the art. The drilling mud is pumped through the interior of the drill string  4  where it is ultimately discharged through jets (not shown in FIG. 1) on the drill bit  10 . After being discharged through the jets, the drilling mud returns to the earth&#39;s surface through an annular space between the wellbore  5  and the exterior of the drill string  4 . As is known in the art, the number of, placement of and sizes of the jest (not shown in FIG. 1) are selected to provide a desired amount of fluid pressure drop in the drill string, among other factors. 
     A typical drill bit which may include any one or more of a number of various embodiments of the invention is shown in oblique view in FIG.  2 . The drill bit  10  includes a bit body  12  made from steel or matrix material. The bit body  12  typically has a coupling  14 , usually a threaded pin or box, to attach it to the drill string ( 4  in FIG.  1 ). This particular bit body  12  includes a plurality of blades  16  onto which are affixed cutting elements  18 , such as polycrystalline diamond compact (“PDC”) inserts, for example. Referring to FIG. 3, the drill bit  10  includes jets  20  which, as previously explained, provide a path for discharging the drilling fluid from the interior of the drill string ( 4  in FIG. 1) into the wellbore ( 5  in FIG.  1 ). One or more of the ports (not shown separately in FIG. 3) for the jets  20  may alternatively be filed with a solid plug instead of a jet. This example drill bit  10  also includes one or more adjustable ports  22 , some of which may include a solid plug therein, or a fixed orifice, depending on the total flow area (TFA) required for the particular drill bit and earth formations being drilled. As is known in the art, the flow area of each of the jets  20  and any orifices in the adjustable ports  22  are selected to provide the desired amount of TFA for the drill bit  10 . In prior art drill bits, as explained in the Background section herein, changing the TFA includes changing any one of more of the jets  20  and/or plugs or orifices in any of the adjustable ports  22 . 
     Although a bit according to the invention is shown in FIGS. 2 and 3 as being included in a PDC (fixed cutter) drill bit, it should be clearly understood that the invention is equally applicable to roller cone bits. Accordingly the type of bit is not intended to limit the scope of the invention. Irrespective of the type of bit, for purposes of defining the invention, the bit body can be thought of as having at least one cutting element operatively coupled to the bit body. In the case of PDC or similar fixed cutter bits, such as shown in FIGS. 2 and 3, the cutting element is affixed to the bit body. Roller cone bits have at least one cutting element in the form of a milled tooth or insert affixed to at least one roller cone, which is itself rotatably mounted to the bit body. 
     Generally speaking, a drill bit according to the invention includes at least one flow relief disposed in the bit to make an hydraulic connection between the interior of the bit body and the exterior of the bit body upon application of a selected drilling fluid flow characteristic to the interior of the drill bit. The selected fluid flow characteristic may include application of a selected differential pressure, or application of a selected fluid flow rate and/or total mud flow volume to the drill bit. The at least one flow relief is adapted to provide an increase in TFA when the at least one flow relief is actuated. 
     One embodiment of a flow relief can be better understood by referring to FIG.  4 . In FIG. 4, the flow relief is a rupture disk  24 . The rupture disk  24  may be adapted to fit in any one or more of the adjustable ports ( 22  in FIG. 3) or may be adapted to replace any one or more of the jets ( 20  in FIG.  3 ). Rupture disks such as may be used in some embodiments of the invention are a type of plug which is adapted to fail (open to flow permanently) at a selected differential pressure. One type of rupture disk is described, for example, in a brochure entitled,  Pressure Activation Device,  published by Fike Corporation, Blue Springs, Mo. 64105 (1999). In a drill bit according to this embodiment of the invention, the TFA of the bit ( 10  in FIG. 3) may increased by momentarily increasing the flow rate from the mud pumps ( 3  in FIG. 1) to provide a pressure drop across the bit which exceeds the rated burst or failure pressure of the rupture disk  24 . When ruptured, the disk  24  provides an additional flow area through the bit, thereby increasing the TFA. The rated failure pressure of the rupture disk  24  can be selected to provide the increased TFA where, for example, a rotary steerable drilling system having a limitation on pressure drop is used, or where drilling progresses to a depth where it would be useful to increase the TFA of the bit to compensate for increases is fluid friction due to the length of the drill string ( 4  in FIG.  1 ). 
     In various embodiments of a drill bit according to the invention, any one or more of the adjustable ports ( 22  in FIG. 3) or any one or more of the jets ( 20  in FIG. 3) may be replaced with a rupture disk such as shown at  24  in FIG.  4 . 
     Another embodiment of a flow relief is shown in FIG.  5 . This embodiment of flow relief is a biased pressure relief valve  26 . Bias may be provided, for example, by a spring  30  which forces a valve ball  32  against a valve seat  28  to stop flow until the force of the spring  30  is overcome by fluid pressure acting against the ball  32 . The pressure relief valve  26  of FIG. 5 has the advantage, as compared to the rupture disk such as shown at  24  in FIG. 4, of being able to close again once the differential pressure across the pressure relief valve  26  drops below the rated differential pressure for the valve  26 . As in the previous embodiment of FIG. 4, the pressure relief valve  26  of FIG. 5 may be used in any one or more of the adjustable ports  22  or any one or more of the jet ports on the bit body ( 12  in FIG.  2 ). When opened, the pressure relief valve  26  provides increased TFA to the bit. 
     Another embodiment of a flow relief which can be used with a bit according to the invention is shown in FIG.  6 . The flow relief  34  shown in FIG. 6 is a type of relief valve which may be similar in principle to “gas lift” valves used in some oil production systems. This type of relief valve is adapted to be opened upon application of a selected range of differential pressure, and is adapted to be closed at all other values of differential pressure. This adaptation is enabled by having a port  42  in a biased valve body  36  that is aligned with a corresponding port  44  in the valve housing  40  upon movement of the valve body  36  a selected distance. The selected distance is related to the biasing force from, for example, a spring  38 , and the cross sectional area of the valve body  36 . As in the previous embodiment, the flow relief  34  of FIG. 6 may be used in any one or more of the adjustable ports  22  or jet ports on the bit body ( 12  in FIG.  2 ). When opened, the pressure relief valve  34  provide increased TFA to the bit. 
     Still another type of flow relief shown in FIG. 7 is adapted to provide an increase in TFA only by the flow of drilling mud through the bit for a selected time, and/or total flow volume. The flow relief  20 A in FIG. 7 can be similar in construction to a conventional jet or nozzle, but includes an erodible material  54  disposed on an interior surface of the orifice of the jet body  52 . The jet body  52  may be made from conventional jet body materials, such as tungsten carbide, while the erodible material  54  may be mild steel, or other substance that is adapted to wear away by the flow of mud through the relief  20 A. When the erodible material  54  is worn away, the relief  20 A presents a larger flow area to the bit than when the erodible material  54  is intact. A flow relief such as shown in FIG. 7 may be configured to provide the larger flow area of the jet body  52  after a selected volume of drilling mud has passed through the erodible material  54 . The total flow volume, as is known in the art, is related to the rate at which the mud pumps ( 3  in FIG. 1) discharge drilling mud, and the uneroded orifice flow area of the flow relief  20 A. As is the case for the other embodiments of flow relief according to the invention, the flow relief  20 A of FIG. 7 may be inserted into any one or more of the adjustable ports ( 22  in FIG. 3) or may substitute any one or more of the jets ( 20  in FIG.  3 ). 
     Various embodiments of the invention provide a drill bit which can have the total flow area changed during drilling without the need to remove the drill bit from the wellbore. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate that other embodiments of the invention can be readily devised which do not depart from the spirit of the invention as disclosed herein. Accordingly, the invention shall be limited in scope only by the attached claims.