Abstract:
Methods and apparatus to reduce flow in front of the drill bit while milling improve milling debris size. Reducing flow on the drill bit face while milling allows the drill cuttings to remain at the drilling surface longer and reduce the size of the cuttings. The smaller cuttings increase the ease and speed of cutting removal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 62/057,231 filed Sep. 30, 2014, entitled “REDUCED FLOW MILLING,” which is incorporated herein in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    None. 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to milling drill cuttings while drilling. More particularly, but not by way of limitation, embodiments of the present invention include drilling with reduced flow, methods of reducing flow while drilling, and drill bits designed to mill drill cuttings while drilling. 
       BACKGROUND OF THE INVENTION 
       [0004]    During hydrocarbon production from underground reservoirs, plugs are used for a variety of procedures. Once the procedure is complete, the plug may require removal. Frequently the plug is removed by drilling through the plug with a drill bit. The drill bit cuts the plug into smaller pieces. 
         [0005]    Many companies have commercially available plugs that are designed to be milled. Baker Hughes Quick Drill, Halliburton Fas Drill Fusion, Magnum&#39;s Mill EZ and Schlumberger&#39;s Diamondback gen 2 plugs are examples of commercially available millable plugs. Plugs are designed to adhere to the wellbore and prevent flow often under adverse conditions such as increased pressure, temperature, hydrocarbons, sour hydrocarbons, various solutions, and other tools and equipment interacting with the plug. 
         [0006]    Drill bits have been designed to drill well bores. Frequently these same drill bits, along with mills, are used to mill plugs once the plug is no longer required. Commercially available drill bits and mills include the JZ Tri-Cone drill bit, Torquato Plug Buster PDC mill, Weatherford Excalibur mill, ClearCut MP insert mill, and the Baker Hughes Glyphaloy insert mill. Other drill bit and mill designs are available. Plug and well conditions frequently dictate which drill bit or mill is used to mill out a plug. 
         [0007]    Drill bits are typically designed to drill as quickly as possible and exert maximum force on the drilling surface. To drill more quickly drill cuttings and debris are removed as rapidly as possible by flushing the face of the drill bit with drilling fluid. Drill bits are designed with ports and surface flow to improve cutting removal and increase drilling speed via maintaining a clean cutting face and cooling the drill bit. 
         [0008]    Unfortunately, plug milling provides unique challenges that aren&#39;t encountered during standard drilling. Milling is typically conducted in a completed well with plugs that may contain metals, plastics, ceramics, rubber, and other materials not encountered during normal drilling operations. These materials, if left in the wellbore may clog or damage pumps, interfere with additional operations, or fall back trapping the bit, or other equipment, in the hole. Additionally, milling a plug in the vertical orientation produces cuttings that are materially different from cuttings produced during horizontal milling. For example, slip debris was found to be significantly smaller within vertical cuttings as compared to the horizontal cuttings in surface tests using identical equipment. Gravity and the smaller typical gap between the bit and the casings inner wall found in vertical milling ensure cuttings do not leave the bit&#39;s cutting face until they are sufficiently small. Typically, horizontal milling has a larger gap at the high side of the casing that allows large cuttings to pass over the bit and land behind it where they will not fall back into the bit for additional size reduction. Mills with large water courses exacerbate the passing of large cutting debris away from the cutting face. A more effective milling technique is required that will allow drill cuttings to be milled to smaller pieces thus improving the removal of the cuttings during the milling process. It is essential to focus on finding a milling process that generates the smallest cuttings and delivers the easiest milling job, reducing costs, improving wellbore cleanliness, and increasing the effectiveness of milling. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0009]    The invention more particularly includes methods and apparatus to reduce flow in front of the drill bit while milling. Reducing flow at the drill bit face while milling allows the drill cuttings to remain longer at the drilling surface resulting in a greater reduction in the size of the cuttings. The smaller cuttings increase the ease and speed of cutting removal. Elimination, or reduction, of the forward jetting fluid, via the blind crossover or using a reduced flow drill bit design will prolong that duration of time which the plug debris remains between the bit/mill and the remnants of the plug during milling operations, which promotes smaller cutting sizes. 
         [0010]    In one embodiment, a process for milling a wellbore plug is described where a drillstring having a drilling motor, drillstring pipe, a crossover, and a drill bit, is rotated while circulating a drill fluid through said drillstring, and the drilling fluid is partially, or fully, diverted after the drilling motor before reaching the face of the drillbit. 
         [0011]    In another embodiment a blind crossover is provided with a male thread for coupling drillstring pipe, a female thread for coupling a drill bit, having a fully blind or partially blocking flow path for blocking forward flow of the drilling fluid through the drillbit, and one or more ports for diverting forward flow of the drilling fluid. 
         [0012]    In an additional embodiment, a drill bit is provided having a male thread for coupling a drillstring, one or more ports located on the side of the drill bit, and a cutting surface for milling a wellbore plug where the bit diverts fully or partially the drilling fluid flow before reaching the drill bit face. 
         [0013]    The drilling fluid may be diverted with a blind crossover or within the drill bit thus reducing or preventing flow to the cutting face. 
         [0014]    The methods described herein may be used to mill a variety of plugs including composite plugs, cement plugs, cast iron plugs, alloy plugs, and combinations thereof. Additionally, pipe and equipment may be milled to be removed from the flow path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which: 
           [0016]      FIG. 1  is a blind crossover with a port. 
           [0017]      FIG. 2  shows drill bits A-K with reduced flow on the cutting surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. 
         [0019]    As used herein, ‘crossover’ is a connector designed to transition from a first (drillstring) size to a second (drill bit) size. A ‘blind crossover’ is a crossover which fully, or partially, prevents flow down the center of the drillstring and fully, or partially, diverts flow to the well bore before reaching the drill bit. 
         [0020]    A ‘drill bit’ is a tool used for scraping, crushing, or both, the material to be milled usually as part of a rotational motion. Two different types of drill bits exist: fixed cutter and roller cone. A fixed cutter bit is one where there are no moving parts, but drilling occurs due to percussion or rotation of the drill string. Fixed cutter bits can be either polycrystalline diamond compact (PDC) or grit hotpressed inserts (GHI). Roller cone bits can be either tungsten carbide inserts (TCI) or milled tooth (MT). The manufacturing process and composites used in each type of drill bit make them ideal for specific drilling situations. Additional enhancements can be made to any bit to increase the effectiveness for almost any drilling situation. 
         [0021]    The blind crossover,  FIG. 1 , is designed to couple the drill bit with the drill string. The blind crossover, as shown, may have different threading sizes, the male threads thread into the drill string and the drill bit threads into the female threads. Any threading configuration can be utilized in the construction of a blind crossover. Rather than promoting fluid flow through the drill string, the blind crossover fully, or partially, prevents flow from going through the drill bit and instead diverts all, or some, of the flow out to the side of the wellbore. By preventing, or reducing, flow through the drill bit, fluid and debris residence time in front of the bit increases and cuttings remain at the drilling surface longer where they are further milled to a smaller size. Allowing some portion of the pumped fluid to pass through the face of the bit or mill may be necessary for cooling for those types of bits or mills susceptible to overheating during milling operations. A balance will be reached where bit or mill cooling needs are met while pumped fluid diversion above the bit provides the maximum residence time for the debris between the bit or mill and the cutting face. 
         [0022]      FIG. 1A  represents a blind crossover with the side port angled toward the surface. By ejecting the drilling fluid from the side with the port angled toward the surface, flow is further reduced across the face of the drill bit.  FIG. 1B  shows the side port at a 90° angle with drilling fluid ejected directly into the side of the casing. The ejected fluid is represented in  FIG. 1D .  FIG. 1C  represents a crossover where fluid is partially blocked, by reducing the flow of the drilling fluid, debris remains on the mill face while part of the fluid is allowed to pass for cooling and/or to slowly remove the debris. By changing the size of the middle port to the mill face, the amount of drilling fluid passing across the mill face may be controlled. 
         [0023]      FIG. 2  represents a drill bit with reduced flow at the cutting surface.  FIGS. 2A-2K  represent a variety of port configurations. The cross-sectional dimension of the port is typically equal to or greater than the cross-sectional dimension of the drillstring inner diameter. Thus there will not be a dramatic increase in pressure loss as the fluid travels through the bit or mill. Additional smaller holes may be distributed around the bit or a single larger hole may be placed on one side. 
         [0024]    In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as a additional embodiments of the present invention. 
         [0025]    Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents. 
       REFERENCES 
       [0026]    All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication data after the priority date of this application. Incorporated references are listed again here for convenience:
   1. U.S. Pat. No. 5,984,007 (Yuan, et al.) “Chip resistant buttons for downhole tools having slip elements” (1999).