Abstract:
Apparatus and method for drilling a drain hole from a well are provided. A flexible tubing used for conveying fluid to a jet bit is confined radially by a reduced-diameter tubing piece or a liner in production tubing near the diverter used to direct the flexible tubing. Concentric tubing pieces allow location of the bit in a well by measuring weight of a work string.

Description:
This application claims priority to U.S. Provisional Application Ser. No. 61/853,615, filed Apr. 9, 2013. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to jet drilling drain holes from well bores, primarily in oil and gas wells. 
     2. Description of Related Art 
     Oil and gas wells are usually drilled vertically and cased with steel pipe. Typical casing pipes are from 4.5 to 8 inches in diameter. In a typical short-radius jet drilling technique, a flexible tubing or hose attached to the bottom of small rigid tubing (work string) turns 90 degrees within a channel in a diverter attached to a larger (production) tubing inside the casing. Fluid is pumped through the work string, flexible tubing and a bit on the flexible tubing to drill drain holes that may extend 15 to 100 ft. or more from the casing into the rock formation. The drain holes allow more contact area with the rock formation, increasing the flow capacity of the well. Buckman (U.S. Pat. No. 6,668,948), Landers (U.S. Pat. No. 5,413,184) and others have developed short-radius drilling systems that have a radius of 4 inches or less, in which a jet bit (nozzle) and hose pass down through a tubing string in a vertical well to a diverter, which contains a path to deviate the jet bit and flexible hose to enable drilling deviated or horizontal laterals or drain holes in oil and gas wells. 
     There are limiting factors that can prevent a flexible hose from passing through a tight 90-degree turn in a 4-inch radius. Like coiled tubing, a flexible hose can sinusoidally, helically buckle, causing extra friction or drag. Reduction of friction between a flexible hose and surrounding pipe can allow more force to be applied at a bit. Excess friction may lead to “lockup.” When lockup occurs, no matter how much force is applied the tubing can no longer move. If excessive force is continually applied from above in a larger tubular (well tubing) having sufficient diameter, the work string and the flex hose can “pass by itself,” meaning that the flexible tubing turns enough to pass alongside the work string and inside the larger (production) tubing. In this condition, an observation at the surface of the work string rapidly going down the production tubing creates the illusion of jet drilling of the formation while the jet bit is not moving. 
     Another problem in conventional short-radius drilling is that a jet bit may “catch” inside threaded connections of jointed production tubing. If this occurs during the deployment of the jet bit and flex hose downhole, it has been observed that it is near impossible to complete the trip of the bit to the diverter. 
     A further problem is knowing when the jet bit is at the diverter and then in a position to be engaged at the formation. Without simple and precise knowledge of formation engagement one can falsely claim the drilling of a formation. 
     Method and apparatus are needed to eliminate the jet bit catching on tubing connections as it is inserted through the tubing down the well. A signal or indication at the surface is also needed when the jet bit encounters the diverter and the formation, and a technique to transmit greater axial force to the jet bit as it passes through the diverter and jet drills is needed. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, a tubular system having an inner and outer pipe, the outer pipe enclosing an inner pipe and a flexible hose with a jet bit, is provided. The inner pipe is allowed to move freely a desired distance as the flexible hose and jet bit drill out into a formation. The tubular system also assures that the jet bit will not catch on the gaps in connections of the production tubing as the tubular system is placed in a well. A work string (coiled or jointed tubing) is used to place the tubular system in a well. A decrease in the work string weight at the surface will signal delivery of the outer tube to the diverter and then the jet bit can be lowered through the diverter. Because of a smaller-diameter confining tubular around the flexible hose, i.e., a “close-fitting tubular system,” the system assures minimum buckling of the flexible hose as the jet bit passes through the diverter and jet drills a lateral into a reservoir. Fluids may be used that are selected to reduce metal-to-metal frictional drag of the flexible tubing and other tubulars in the wellbore. 
     In another embodiment, a close-fitting tubular system is provided by installing a liner inside the production tubing before it is placed in a well with the diverter. In this embodiment, the bit is not enclosed as the flexible tubing and bit are placed in the well and the bit may catch in connections in the tubing. A soluble or degradable ball on the bit may be used to keep the bit from catching in the tubing gaps as it is being lowered. The close-fitting liner located above the diverter enables the hose to push the jet bit through the diverter and into the formation with significantly less buckling and frictional forces. The liner may be formed from a low-friction solid and fluids may be used that are selected to reduce metal-to-metal frictional drag of the flexible tubing and other tubulars in the wellbore. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
         FIG. 1  illustrates a cased well and drilling apparatus provided herein for drilling through a casing and drilling a drainhole in a reservoir. 
         FIG. 2  illustrates the concept of helical buckling of a hose and a jet bit being caught within gaps inside production tubing. 
         FIG. 3A and 3B  illustrate how a jet bit delivery system encloses a hose and jet bit and how it would travel as a jet bit and hose as a drainhole is drilled. 
         FIG. 4  illustrates fluid flow directed through a stinger and around the jet bit delivery system. 
         FIG. 5  illustrates an alternative design where a restriction (liner) is placed in the production tubing immediately above the diverter to provide a narrow path, which enables greater downward force transmission through a hose to a jet bit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , one embodiment of drilling apparatus, such as disclosed in U.S. Pat. No. 6,668,948, which is hereby incorporated by reference for all purposes, is illustrated. Jet bit  20  has been used to jet drill lateral or drain hole  36  into formation  38 . Diverter  28 , attached to production tubing  26 , is used as a kickoff point for jet bit  20  to turn 90 degrees or a selected angle from vertical well  14  into formation  38 . Diverter  28  may turn the jet bit from 20 to 130 degrees within the diverting path. Well  14  typically will have steel casing  16  that has a surrounding layer of cement  18  to hold it in place. Jet bit  20  is connected to the distal end of flex hose  22 . Flex hose  22  may range in size from ¼-inch to 1-inch in outside diameter. Flex hose  22  is connected to the distal end of work string  24  at connector  23 , which is usually coiled tubing, as illustrated, but may be jointed tubing. Flow can be conveyed from pump  34  at surface to jet bit  20  downhole to perform jet drilling operations. Diverter  28  is placed on the lower end of production tubing  26  at the depth where drilling is to be conducted. 
       FIG. 2  shows how jet bit  20  can catch in production tubing  26  at coupler or collar  30 , where two sections (joints) of production tubing  26  come together. There can be as much as a 2-inch gap across coupler gap  32  where jet bit  20  could catch and turn. Once the distal end of hose  22  is stopped, hose  22  would then begin to buckle and create excessive drag between flex hose  22  and production tubing  26 . If the axial force is further increased on flex hose  22  the buckling would then become helical buckling and eventually lead to lockup. Lockup is defined when the drag force exceeds the axial force applied to the flex hose  22 . This can prevent the bottom hole assembly from reaching the diverter. Continuing to apply force can damage flex hose  22 . 
     The theory of buckling of coiled tubing in a well casing or hose within another tubular is well known. A specific example through testing by the inventors is given below. Whereas a stainless steel braid hose of 0.40 inch outside diameter, that is 20 feet in length, with an internal pressure of 8,000 psi is enclosed in a stainless steel tubular with an inner diameter of 1.12 inch. Table 1 has the axial forces exerted on the upper end on the pressurized hose and the axial force produced at the bottom of the pressurized hose across the 20 foot length. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Upper Axial 
                 Lower Axial 
               
               
                   
                 Force (LBS) 
                 Force (LBS) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 23 
                 6.4 
               
               
                   
                 42 
                 28 
               
               
                   
                 61 
                 40.4 
               
               
                   
                 81 
                 46 
               
               
                   
                 99 
                 45 
               
               
                   
                 120 
                 44.5 
               
               
                   
                 140 
                 44.3 
               
               
                   
                 159 
                 43.3 
               
               
                   
                 184 
                 43.5 
               
               
                   
                 200 
                 43 
               
               
                   
                 220 
                 43 
               
               
                   
                 240 
                 43 
               
               
                   
                   
               
             
          
         
       
     
     Note that with an upper axial force of 42 lbs. applied at the top yields a lower axial force of 28 lbs. at the bottom. Also, observe that once the applied upper axial force exceeds 99 lbs., the hose&#39;s buckling is such that lockup occurs in the tubular and no additional force is exerted at the lower end. Hence, if it takes a force above the buckling force for the jet bit and hose to pass through the diverter, the hose will just buckle and lock up in the tubing. A helically buckling segment will want to expand outwards adding to the frictional forces acting against the constraining outer tube, a normal force for the continuous length of the hose in contact. To decrease drag from buckling one can increase the hose bending stiffness and decrease radial clearance. Also, it is best that the inner surface of the pipe be smooth like stainless steel or other slick surfaces. 
     Further tests were conducted with different flex hoses that had varying diameters and bend-radius ratings. These variables all affect the buckling tendencies of flex hoses. Bend radius is one form of measurement of the flex hose&#39;s bending stiffness. Typically, in coiled tubing calculation a segment&#39;s bending stiffness is shown with the steel&#39;s Young&#39;s Modulus and the moment of inertia. Not being made of one continuous material, a flex hose&#39;s bending stiffness is hard to standardize, but for an example, a flex hose that has a 5-inch bend radius will have less tendency to buckle than a flex hose that has a 2.5 inch bend radius having the same diameter. The theory of buckling of tubing of hose within another tubular predicts that the normal force due to helical buckling is directly proportional to the radial clearance, r c  and inversely proportional to bending stiffness, EI. Therefore, reducing the diameter of larger tubing around the flexible tubing, forming a “close-fitting” tubular system, can be used to decrease resistance to movement of the flexible tubing through the larger tubing. 
     A typical jet drilling setup would use 2⅜″ production tubing, with about a 2-inch inner diameter and a flex hose of a similar size in the previous example. Since the radial clearance would be greater, the helical buckling of the flex hose would be created at a significantly lower force than the 99 lbs. in the example for lockup to occur. 
     Referring to  FIG. 3 , one embodiment of a close-fitting tubular system disclosed herein is shown. In  FIG. 3A , outer pipe piece  42  encapsulates flex hose  22 , preventing the catching of the hose in sharp transitions (not shown) in production tubing  26 . At the distal end of the outer pipe  42  perforated stinger  46  may be placed; this perforated stinger  46  is designed such that it engages with diverter  28  to give a smooth transition into the diverter path  29 . At the upper end of the outer pipe piece  42  is an outer pipe piece upper transition  50  Inner pipe piece  40  operates within outer pipe piece  42 . At the distal end of inner pipe piece  40  is flex hose or tubing  22 . The proximate end of inner pipe piece  40  is connected to the distal end of work string  24 ; this allows inner pipe piece  40  to convey pressure and flow from work string  24  to flex hose  22 . Inner pipe piece  40  has an inner pipe piece upper transition  48  and an inner pipe piece lower transition  44 . Inner pipe piece  40  is free to move downward until upper transition  48  reaches outer pipe piece upper transition  50 . Inner pipe piece  40  is free to move upward until inner pipe piece lower transition  44  reaches outer pipe piece upper transition  50 . Therefore, work string  24  is used to lower flexible tubing  22  and all other apparatus attached to work string  24  into a well. 
     During a jet drilling operation, during placement of the apparatus in a well, the close fitting tubular system illustrated in  FIG. 3A  will keep flex hose  22  contained until stinger  46  engages the top of diverter path  29 . Weight may be added to outer pipe piece  42  such that when it engages diverter  28  it can be more easily observed on a weight indicator at surface when the pipe piece contacts the diverter and there is a decrease in the string weight. This confirms the location of the bottom-hole assembly. Then pressure and flow can be applied to work string  24 , which would then be conveyed through inner pipe piece  40  and flex hose  22  to jet bit  20  for jet drilling. Inner pipe piece  40  and flex hose  22  will then continue to move within stationary outer pipe piece  42  until inner pipe piece upper transition  48  reaches outer pipe piece upper transition  50 . 
     Illustrated in  FIG. 3B , jet bit  20  has exited diverter  28  and drilled out into a rock formation, creating a lateral or drain hole. The length of the lateral will be limited by the travel of inner pipe piece  40 , restricted by the inner pipe piece upper transition  48  and the outer pipe piece upper transition  50  and the length of flex hose  22 . Outside pipe piece  42  remains stationary above the diverter while flexible hose  22  with bit  20  and inside pipe piece  40  move downward and jet drill a lateral. 
     Force can be transmitted from work string  24  through inner pipe piece  40  and flex hose  22  to overcome friction forces in diverter path  29 . Because of the smaller ID of outer pipe piece  42  than that of production tubing  26 , the radial clearance of flex hose  22  is less and therefore less drag will occur in outer pipe piece  42  than in previous tubing configurations. The surface of outer pipe piece  42 , of flexible hose  22  and of diverter path  29  may be formed from a low-friction material, which may be a solid liner or a coating applied to the surface. One low-friction material is TEFLON. 
     In  FIG. 4 , fluid flow is illustrated by arrows through perforated stinger  46  and continuing up production tubing  26 . Fluid containing rock cuttings from jetting has been known to circulate up and through diverter path  29  into production tubing  26 . The perforations in perforated stinger  46  allow this natural flow path to continue and also restricts fluid from flowing up into outer pipe piece  42  and inner pipe piece lower transition  44 . 
     In  FIG. 5 , another embodiment of a close-fitting tubular system restricting the buckling of flex hose  22  to allow greater force transfer by utilizing a tubing liner or smaller ID tubular  52  (hereinafter referred to as a liner) is illustrated. This enables greater force from work string  24  to be transferred through smaller pipe piece  40  to flex hose  22  and jet bit  20  to jet drill the lateral using diverter  28  attached to production tubing  26 . Smaller pipe piece  40  with transitions  44  and  48  may be omitted and work string  24  may be attached directly to flexible hose  22  if the diameter of work string  24  is small enough to pass through liner  52 . Tubing liner or smaller ID tubing  52  preferably has an internal diameter less than 1 inch greater than the external diameter of flexible hose  22 . “Soluble ball”  54  can be placed on the end of a jet bit  20  before the bit and flex hose  22  are lowered down the tubing. Ball  54  may be made of a material that is slowly soluble in water or a polymer material that degrades in water. Jet bit  20  will not catch on tubing connections with the rounded front of ball  54 . Once jet bit  20  is to the diverter or before drilling commences, pressure may be applied to blast off ball  54 , which then dissolves or degrades. 
     While the preferred embodiments directed in this invention have been discussed herein, further modifications to the preferred embodiments will occur to those skilled in the art and such modifications are included in the scope of this invention. Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.