Abstract:
A bypass sub which will automatically bypass fluid flow in excess of a selected optimal flow rate for a downhole mud motor. A spring biased mandrel within a housing is driven downwardly by increased fluid flow, and driven upwardly by spring force upon decreased fluid flow, to control the alignment of a port in the mandrel with a bypass port in the housing, thereby maintaining a desired rate of fluid flow to the downhole motor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/096,441, filed Aug. 13, 1998. 
    
    
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     TATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The primary use of this invention is in the field of equipment used in conjunction with downhole mud motors in the drilling of oil and gas wells. 
     2. Background Information 
     In many applications, an oil or gas well is drilled with a fluid driven motor, called a mud motor, which is lowered into the well bore as drilling progresses. The mud motor is affixed to the lower end of a drill pipe. Drilling fluid, or mud, is pumped down through the drill pipe by pumps situated at the surface of the earth, at the drill site. The drilling fluid pumped downhole through the drill pipe passes through the mud motor, turning a rotor within the mud motor. For a given mud motor, there is an optimum mud flow rate, and minimum and maximum allowable mud flow rates. The rotor turns a drive shaft which turns a drill bit, to drill through the downhole formations. Similarly, a milling tool can be affixed to the mud motor, instead of a drill bit, for milling away metal items which may be found downhole. After passing through the mud motor, the drilling fluid, or at least a portion of it, typically passes on through the drill bit or milling tool. After exiting the drill bit or milling tool, the drilling fluid passes back up the well bore, in the annular space around the drill string. 
     As the drill bit turns and drills through the formation, it grinds, tears, or gouges pieces of the formation loose. These pieces of the formation, called cuttings, can vary in size from powdery particles to large chunks, depending upon the type of formation, the type of drill bit, the weight on bit, and the speed of rotation of the drill bit. Similarly, as a milling tool turns, it removes metal cuttings from the metal item being milled away or milled through. As the drilling fluid exits the drill bit or milling tool, it entrains the cuttings, in order to carry the cuttings back up the annulus of the well bore to the surface of the well site. At the surface, the cuttings are removed from the drilling fluid, which is then recycled downhole. 
     Depending upon the type of formation, the drilling depth, and many other factors, the drilling fluid used at any given time is designed to satisfy various requirements relative to the well drilling operation. One of the prime requirements which the drilling fluid must satisfy is to keep the cuttings in suspension and carry them to the surface of the well site for disposal. If the cuttings are not efficiently removed from the well bore, the bit or milling tool can become clogged, limiting its effectiveness. Similarly, the well bore annulus can become clogged, preventing further circulation of drilling fluid, or even causing the drill pipe to become stuck. Therefore, the cuttings must flow with the drilling fluid uphole to the surface. Various features of the drilling fluid are chosen so that removal of the cuttings will be insured. The two main features which are selected to insure cutting removal are drilling fluid viscosity and flow rate. 
     Adequate viscosity can be insured by proper formulation of the drilling fluid. Adequate flow rate is insured by operating the pumps at a sufficiently high speed to circulate drilling fluid through the well at the required volumetric velocity and linear velocity to maintain cuttings in suspension. In some circumstances, the mud flow rate required for cutting removal is higher than the maximum allowed mud flow rate through the mud motor. This can be especially true when the mud motor moves into an enlarged bore hole, where the annulus is significantly enlarged. If the maximum allowed flow rate for the mud motor is exceeded, the mud motor can be damaged. On the other hand, if the mud flow rate falls below the minimum flow rate for the mud motor, drilling is inefficient, and the motor may stall. 
     In cases where keeping the cuttings in suspension in the bore hole annulus requires a mud flow rate greater than the maximum allowed mud flow rate through the motor, there must be a means for diverting some of the mud flow from the bore of the drill string to the annulus at a point near, but just above, the mud motor. This will prevent exceeding the maximum mud flow rate for the mud motor, while providing an adequate flow rate in the annulus to keep the cuttings in suspension. 
     Some tools are known for this and similar purposes. Some of the known tools require the pumping of a ball downhole to block a passage in the mud flow path, usually resulting in the shifting of some flow control device downhole to divert drilling fluid to the annulus. Such tools usually suffer from the disadvantage of not being returnable to full flow through the mud motor, in the event that reduced mud flow becomes possible thereafter. Other such tools might employ a fracture disk or other release means, with these release means suffering from the same disadvantage of not being reversible. At least one known tool uses mud pump cycling to move a sleeve up and down through a continuous J-slot to reach a portion of the J-slot which will allow increased longitudinal movement of the sleeve, ultimately resulting in the opening of a bypass outlet to the annulus. This tool suffers from the disadvantage that the operator must have a means of knowing exactly the position of the J-slot pin, in order to initiate bypass flow at the right time. Initiating increased flow when bypass has not been established can damage the mud motor, while operating at low flow when bypass has been established will lead to poor performance or stalling. 
     Therefore, it is an object of the present invention to provide a tool which will reliably bypass a portion of the drilling fluid to the annulus when a predetermined flow rate is exceeded, and which will close the bypass path when the flow rate falls back below a predetermined level. This will allow the operator to have complete control of the bypass flow by operation of the drilling fluid pumps at selected levels. 
     BRIEF SUMMARY OF THE INVENTION 
     The tool of the present invention includes a housing, within which is installed a slidable hollow mandrel. A bypass port is provided in the housing, between the inner bore of the housing and the annular space around the housing. A mandrel port is provided in the mandrel, between the inner bore of the mandrel and its outer surface. The hollow mandrel is biased toward the uphole direction by two springs stacked one upon the other. The uppermost spring has a lower spring constant than the lowermost spring. A nozzle is fixedly mounted in the bore of the hollow mandrel. The tool is affixed to the lower end of a drill string just above a mud motor. Compressible or incompressible fluid pumped down the drill string flows through the tool to the mud motor. As it passes through the tool, the fluid passes through the nozzle and through the hollow mandrel, and then on to the mud motor. The fluid used with the present invention can be either a liquid or a gas. 
     When the mandrel is in its upwardly biased position, all of the fluid flow passes through the mandrel and on to the mud motor. As the flow rate of the fluid is increased, the force on the nozzle increases, moving the hollow mandrel downwardly in the flow direction, against the bias of the two springs. After the upper spring is compressed, the mandrel acts against the increased resistance of the lower spring. At this time, the mandrel port begins to align with the bypass port in the housing, allowing a portion of the fluid flow to begin flowing into the annulus, bypassing the mud motor. As the flow rate is further increased by speeding up the pumps, the lower spring is further depressed by downward movement of the mandrel, which causes the mandrel port to allow more bypass flow through the bypass port. This maintains the flow rate through the mud motor below the maximum allowed level. If the flow rate is decreased, the mandrel moves upwardly, reducing the amount of the bypass flow and maintaining the mud motor flow rate in the optimal range. 
     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a longitudinal section view of the bypass sub of the present invention, showing the tool in the non-bypass configuration; and 
     FIG. 2 is a longitudinal section view of the bypass sub of the present invention, showing the tool in the full bypass configuration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, the bypass sub  10  of the present invention includes a top sub  12 , which is threaded to an upper housing  14 , which is in turn threaded to a lower housing  16 . The upper end of the top sub  12  is adapted to be affixed to the lower end of a drill string (not shown), such as by threading. The lower end of the lower housing  16  is adapted to be affixed to the upper end of a mud motor housing (not shown), such as by threading. Fluid which passes through the bypass sub  10  passes through a nozzle  18  which is located in the inner bore of the top sub  12 . The nozzle  18  is fixedly mounted within the inner bore of a hollow mandrel  20 , held in place by a nozzle retainer ring  52 . The hollow mandrel  20  is in turn slidably mounted for reciprocal longitudinal movement within the inner bore of the top sub  12  and the inner bore of the upper housing  14 . 
     The outer surface of the lower portion of the top sub  12  is sealed against the inner bore of the upper portion of the upper housing  14  by an O-ring seal  40 . Similarly, the outer surface of the lower portion of the upper housing  14  is sealed against the inner bore of the upper portion of the lower housing  16  by an O-ring seal  44 . Further, the outer surface of the upper portion of the hollow mandrel  20  is sealed against the inner bore of the lower portion of the top sub  12  by an O-ring seal  38 . Still further, the outer surface of the lower portion of the hollow mandrel  20  is sealed against the inner bore of the upper housing  14  by an O-ring seal  42 . 
     At least one bypass port  46  is provided in the upper housing  14 , from the inner bore to the outer surface thereof. At least one mandrel port  50  is provided through the wall of the hollow mandrel  20 . A multi-element high pressure seal  48  is provided around the periphery of the hollow mandrel  20 , and within the inner bore of the upper housing  14 , between the longitudinal locations of the bypass port  46  and the mandrel port  50 , when the mandrel  20  is in the longitudinal position shown in FIG.  1 . The high pressure seal  48  prevents premature leakage from the mandrel port  46  to the bypass port  50 , along the outer surface of the mandrel  20 . 
     A tubular spring sleeve  22  is slidably positioned in the inner bore of the upper housing  14 , below the mandrel  20 . The spring sleeve  22  encompasses the upper end of a minor spring  24 , against which the lower end of the hollow mandrel  20  bears. A major spring  26  is positioned below the minor spring  24 , within the inner bore of the upper housing  14  and the inner bore of the lower housing  16 . The spring constant of the minor spring  24  is less than the spring constant of the major spring  26 . This ensures that the minor spring  24  will compress before compression of the major spring  26  commences. The length of the spring sleeve  22  is less than the length of the minor spring  24 , when the mandrel  20  is in its uppermost position as shown. 
     The spring constants of the minor and major springs  24 ,  26 , and the length of the spring sleeve  22  are designed to ensure that the minor spring  24  will compress until the spring sleeve  22  establishes a compressive connection between the mandrel  20  and the major spring  26 . During this compression of the minor spring  24 , the mandrel port  50  is moving downwardly toward the bypass port  46 . Thereafter, when the lower edge of the mandrel port  50  has reached the upper edge of the bypass port  46 , compression of the major spring regulates the relative positions of the ports  46 ,  50 , thereby regulating the amount of bypass flow of fluid to the annulus surrounding the upper housing  14 . A longitudinal alignment groove  34  is provided in the outer surface of the mandrel  20 , and a screw or alignment pin  36  protrudes from the upper housing  14  into the alignment groove  34 , to maintain longitudinal alignment of the mandrel port  50  with its respective bypass port  46 . 
     An upper spacer ring  28  is positioned between the lower end of the mandrel  20  and the upper ends of the spring sleeve  22  and the minor spring  24 . An intermediate spacer ring  30  is positioned between the lower end of the minor spring  24  and the upper end of the major spring  26 . One or more lower spacer rings  32  are positioned between the lower end of the major spring  26  and an abutting shoulder in the lower housing  16 . The thicknesses of the spacer rings  28 ,  30 ,  32  establish the desired preloading of the minor and major springs  24 ,  26 . These rings can be changed to control the desired amount of bypass flow for different total flow rates, thereby providing optimal fluid flow through the mud motor for all anticipated flow rates for a given application. 
     FIG. 1 shows the mandrel  20  in its uppermost position, where no bypass flow is provided. FIG. 2 shows the mandrel at or near its most downward position, where maximum bypass flow is being provided. It can be seen that pump speed has been increased to increase the total fluid flow rate. This has increased the resistance in the nozzle  18 , which has forced the mandrel  20  to compress the minor spring  24  until the spring sleeve  22  contacted the upper end of the major spring  26 . Thereafter, further increased flow has compressed the major spring  26 , until the mandrel port  50  has almost completely aligned with the bypass port  46 . In the most downward position, further downward movement of the mandrel  20  will not result in increased bypass flow. With proper selection of the nozzle  18 , the springs  24 ,  26 , and the spacer rings  28 ,  30 ,  32 , this maximum bypass flow rate will be sufficient to keep the cuttings in suspension. 
     It can be seen that, if total flow rate is decreased, the major spring  26  will push the mandrel  20  upwardly, partially closing the bypass port  46 , thereby maintaining the optimal amount of fluid flow through the mud motor. 
     While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention.