Patent Publication Number: US-6220372-B1

Title: Apparatus for drilling lateral drainholes from a wellbore

Description:
FIELD OF THE INVENTION 
     The present invention relates to apparatus for drilling lateral drainholes from a wellbore. More particularly, drainholes are drilled using a bit driven by a flexible shaft formed of two or more concentric coil springs, having opposite pitch, guided through a short radius turning elbow anchored within the wellbore. 
     BACKGROUND OF THE INVENTION 
     After a well, completed into a formation, has been producing oil or gas over an extended period of time, the rate of production generally diminishes, often due to depletion of the reservoir or due to near-wellbore effects. Methods of alleviating diminished production can include treating the near-wellbore effects and increasing the drainage area or wellbore access. Treatment of near-wellbore effects include hot oil flushing to melt paraffins, high pressure fracturing, chemical treatments, or re-perforation of the casing and acidizing to open up additional flow passages. Each of these treatments are subject to restrictive use or success of short duration. 
     A more progressive solution is to increase the drainage area. This is generally accomplished by drilling holes laterally outwardly from the wellbore so as to increase communication with the formation. These holes are known as drainholes. 
     Typically, the hydrocarbon bearing portion of the formation is rather shallow. This delimits where the lateral drainholes are placed, requiring significant precision in vertical placement. Additionally, the drainholes must first pass through the existing casing and then extend into the formation. 
     Whipstock diversion or horizontal drilling techniques using mud motors account for most of the re-entry drilling techniques. Generally a full drilling rig is required and is used in combination with a whipstock to deviate the drill string. A portion of the casing is milled out and a rotary drilling string or mud motor essentially drills a new wellbore. This requires a large radius of turn which complicates targetting of the payzone. The process is expensive and results in a single new hole. 
     Lance-type penetrators, such as that disclosed in U.S. Pat. No. 5,392,858 to Peters, introduce apparatus to first mill through the casing and then provide a flexible conduit which supplies high pressure fluid to a nozzle. The nozzle jets forward while advancing, hydraulically cutting into the formation. Small radii (12″) can successfully be achieved. Unfortunately, the high pressure fluid can erode the casing cement and re-establish undesirable cross-communication with vertically adjacent layers. 
     A lesser known technique is to provide a section of highly flexible drill shaft at the downhole end of a rotary shaft. These techniques use a single coiled spring as the power transmitting member with an internal or external elastomer sheath or hose to contain drilling fluids. These systems, as disclosed in U.S. Pat. Nos. 3,838,736 and 4,051,908 to Driver, have the following features in common: a tubing string is lowered into the casing, the string having a 90 degree elbow at its lower end; a flexible hollow shaft is connected to the lower end of drill pipe and is lowered down into the tubing string; the drill string is rotated, the flexible shaft is directed laterally by the elbow and proceeds to drill through the casing and into the formation. These and similar systems are limited to low drilling rotational speeds and low axial loading to avoid premature failure of the coil spring flexible shaft. 
     In the context of stabilizing the roofs of mines, a flexible drill shaft is used to drill holes upwardly into the roofs. By providing a flexible shaft, shaft lengths and thus hole depths greater than the height of the mine corridor can be achieved. As disclosed in U.S. Pat. No. 4,057,115 to Blanz, contra-wound bands or springs are used for the shaft. An outer band is helically wound about a coil spring having an opposite pitch. A drill bit is secured to the shaft&#39;s upper end. A rotary drive clamps onto the circumference of the outer band and applies torque. The drive and shaft are advanced axially upwardly, driving the bit into the mine&#39;s roof. When the rotary drive approaches the roof, it is unclamped, lowered axially and is re-clamped onto the shaft. During drilling, the outer band tends to contract, and the inner coil tends to expand, lending axial stability to the shaft. 
     This apparatus does not address the difficulties of downhole operation, including the ability and the need to introduce an axial load into the flexible shaft yet still make small radius turns, wherein the axial load originates before the turn is made. 
     SUMMARY OF THE INVENTION 
     Apparatus is provided or drilling drainholes laterally outwards from even very small diameter casings, enabling accurate and economical access to the hydrocarbon producing formation. 
     More particularly, apparatus is lowered down within the well&#39;s casing. The apparatus comprises a flexible shaft having a bit at the lower end and a shaft guide conduit. The “L” shaped guide conduit re-directs the shaft from a path parallel to the casing, to one substantially perpendicular to the casing. The shaft guide conduit is rigidly anchored within the casing. Accordingly, the bit is directed towards the casing, enabling cutting through the casing and into the formation. The flexible shaft has upper and lower ends and is formed of a helically wound outer coil spring and one or more helically wound and smaller inner coil springs residing concentrically therein. Each successively smaller inner coil spring has an outer diameter substantially the same as the inner diameter of the adjacent larger coil spring. Each coil spring is wound opposite in direction to that of the next adjacent coil spring. Each coil spring is held in rigid relation to each other coil spring at the shaft&#39;s upper and lower ends. The direction of the bit&#39;s rotation is coordinated with the direction of winding of the outer coil spring so that the diameter of the outer coil spring tightens upon the expanding diameter of the next adjacent inner coil spring. The “L” shaped shaft guide conduit has an upper straight portion and a lower elbow portion, the combined length of which is at least as long as the length of the shaft. Bushings are located within the lower elbow portion of the shaft guide conduit for causing the shaft to flex and turn while permitting rotation and axial movement therethrough. A motor is drivably connected to the top of the shaft and is movable up and down within the casing. Accordingly, when the motor is rotating the bit, and the shaft is lowered, the shaft guide conduit supports the shaft, guides it through the elbow portion and directs the bit against the casing for cutting through the casing and then into the formation. 
     Preferably, a driveshaft positioned between the motor and the shaft permits the shaft to pass through the shaft guide conduit without interference between the shaft guide conduit and the motor. 
    
    
     BRIEF DESCRPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a vertical wellbore with an embodiment of the present invention installed therein; 
     FIG. 2 is a side view of the flexible shaft of FIG. 1, with portions of the outer and inner coil springs cut away to illustrate both left and right windings of the coils; and 
     FIG. 3 is a partial side view of a flexible shaft having two inner coil springs installed concentrically within the outer coil spring. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a vertical well  1  is completed into a hydrocarbon-bearing formation, or producing zone  2 . The well  1  comprises a casing  3  forming a wellbore  4 . Cement  5  is placed about the casing  3 . The cement  5 , among other objects, prevents contaminating fluids from travelling along the casing  3 , between the producing zone  2  and from other zones  6 . 
     Whether the well  1  is new or existing, drainhole drilling apparatus  7  is lowered down the wellbore  4  to a location above the producing zone  2 . The apparatus  7  enables drilling of one or more holes  8 , each being substantially perpendicular to the wellbore, and each extending into the producing zone  2 . The holes form laterally extending drainholes  9  which increase the drainage area of the well  1 . 
     Generally, the apparatus  7  comprises a flexible shaft  10  extending through a shaft guide conduit  11 . The shaft  10  has an upper end  12  and a lower end  13 . A bit  14  is fitted to the shaft&#39;s lower end  13 . The shaft guide conduit  11  acts to turn or guide the shaft  10  from a path which is parallel to the axis of the casing  3  to a path which is rotated substantially 90°, or is perpendicular to the casing&#39;s axis. A rotary drive means 15 such as an electric, hydraulic or mud motor enables rotation of the shaft  10  and the bit  14 . Accordingly, for a vertical oriented casing  3 , rotation of the shaft  10  about the vertical axis and turning of the shaft through the shaft guide conduit  11 , will result in bit rotation and drilling about a horizontal axis. 
     The shaft  10  is movable up and down within the casing  3 ; down to enable drilling of the drainhole  9 , and up to recover the bit  14  and shaft  10  from the drainhole  9 . 
     More particularly, the shaft guide conduit  11  comprises a straight supporting portion or support conduit  16  at its upper end, and an elbow turning portion or turning elbow  17  at its lower end. The shaft  10  passes through the support conduit  16  and through the turning elbow  17 . The turning elbow  17  turns the shaft 90 degrees. An anchor  18  is secured to the shaft guide conduit  11  and, when actuated, engages the casing  3  to hold the guide conduit  11  stationary within in the wellbore  4 . 
     For drilling, the shaft&#39;s upper end  12  is subjected to downward load. The turning elbow  17  turns the downward load into a laterally directed load. To impart turning loads into the shaft, the turning elbow  17  has internal guide means or bearing surfaces  19  which act on the shaft and permit it to rotate under load while it advances therethrough. The bearing surfaces  19 , preferably three hardwood or other bushings, are placed at the 0.45 and 90 degree positions within the turning elbow  17 , thereby providing three points of contact between the turning elbow  17  and the shaft  10 . 
     The motor  15  is suspended from the surface via tubing  20 . The motor  15  and tubing  20  is movable up and down the casing  3 . Typically, the motor  15  is too large to fit within the support conduit  11  and impairs up and down movement. Thus, a driveshaft  21  is connected between the motor  15  and the upper end  12  of shaft  10 . The driveshaft  21  is smaller in diameter than is the motor  15 . When the motor  15  is moved up and down within the casing  3 , the driveshaft  21  moves up and down within the support conduit  16 . 
     The length of the shaft  21  is greater in length than is the length of the desired drainhole  9 . The support conduit  16  is at least the length of the shaft  10  for enabling lateral support of the flexible shaft throughout its drainhole drilling range. The driveshaft  21  is at least as long as the supporting conduit  16  so that the motor  15  does not contact the support conduit at the motor&#39;s lowest position. 
     Turning to the flexible shaft  10  in greater detail, and having reference to FIGS. 2 and 3, an assembly of concentric coil springs form a cylindrical, flexible shaft  10 . More particularly, a helically wound outer coil  30  is formed of spring material, such as spring steel. One or more helically wound inner coil springs  31  reside concentrically within the outer coil spring  30 . The inner coil spring or springs  31  are also formed of spring material. Use of a single inner coil spring is shown in FIG.  2  and the use of two inner coil springs  31   a ,  31   b  is shown in FIG.  3 . 
     Each inner coiled spring  31  has an outer diameter which is substantially the same as the inner diameter of the next radially adjacent and larger coil spring, be it to the next larger inner coil spring (spring  31   b  to  31   a ) or to the outermost coil spring (spring  31  to  30 ). As shown, the cross-section of each coil  32  of each coil spring  30 ,  31  is circular. The periphery of the cross-section of axially adjacent coils  32  of each coil spring  30 ,  31  are in contact (close-wound). 
     The outer coil spring  30  and the inner coil spring or springs  31  are wound in opposite directions. Each successively smaller inner coil spring ( 30  to  31   a  to  31   b  . . . ) is wound opposite to the adjacent larger coil spring. 
     It is essential that the direction of winding of the outer coil spring  30  be coordinated with the direction of rotation of the flexible shaft  10 . When subjected to torque, spring coils characteristically either shrink in diameter with a corresponding increase in length, or they expand in diameter and shorten in length. Accordingly, having chosen a direction of rotation of the bit  14 , say clockwise (“CW”) as viewed along the axis of the bit  14  towards the drilled subject (ie. the formation 2), the winding of the outer coil spring  30  is left handed (“LH”). In other words, when a coil spring is supported on a flat surface, with its axis lying parallel to the surface (ie. view FIGS. 2 and 3 rotated counterclockwise 90°), an individual coil of a LH coil spring angles downwardly to the left. 
     Accordingly, the outer coil spring  30  is formed with a LH winding and next adjacent inner coil spring  31  is formed with a right hand (“RH”) winding. Under a CW drilling torque applied to the flexible shaft, the diameter of the outer coil spring  30  shrinks onto the next adjacent inner coil spring  31 , whose diameter is correspondingly expanding. This action creates a strong and stable, yet flexible shaft  10 . 
     At the shaft&#39;s upper end  12 , the inner coil springs  31  and the outer coil spring  30  are held in rigid relation to each other. In other words the ends of inner and outer coil springs  31 , 30  are drivably interconnected to each other and are connected to the driveshaft  21  with means to prevent relative axial or rotary movement between first, the inner and outer springs  30 , 31  and secondly, to prevent rotary movement between the coil springs  30 , 31  and the driveshaft  21 . These means include mechanical means, welding or brazing. Similarly, at the lower end  13  of the shaft  10 , means drivebly interconnect the coil springs  30 , 31  and connect the coil springs  30 , 31  to the bit  14 . 
     In operation, rotation imparted by the motor  15  is transmitted through the driveshaft  21  to the flexible shaft  10 , causing the bit  14  to rotate. The motor  15  is raised and lowered in the wellbore  4  using tubing  20 . To drill, the motor  15  and the driveshaft  21  are lowered in the casing  3 . The descending, flexible shaft is supported laterally by the support conduit  16 . The turning elbow  17  guides the shaft  10 , directing the bit  14  laterally to bear against the casing  3 . The bit  14  advances laterally, cutting materials encountered in its path including firstly the casing, and then the formation  2  itself to drill the drainhole  9 . 
     During drilling, the outer diameter of the outer coil spring  30  forms a helical augering surface  33 . During drilling, surface  33  augers drilled cuttings rearwardly along the outer coil spring and the drainhole until they fall into the bottom of the wellbore  4 . 
     Interaction of the coil springs  30 , 31  and their flexing around the turning elbow  17  involves reversing stresses and friction at the bearing surfaces  19 . For longest component life, lubrication is required. In a typical well, water or oil will be present at the bottom of the wellbore  4  and acts to lubricate and aid in heat dissipation. 
     After one drainhole  9  is drilled, the motor  15  is raised, retracting the flexible shaft  10  back into the turning elbow  17  and support conduit  16 . Anchor  18  is then released, the assembly  7  is vertically adjusted, the anchor  18  is reset and another drainhole  9  is drilled. 
     The present invention was tested to validate the ability to drilling through a short turning radius elbow. 
     EXAMPLES 
     In bench scale testing, a flexible shaft was assembled using an outer coil spring  30  and one inner coil spring  31 . The outer coil spring  30  had an outer diameter of 1{fraction (15/16)} inches. Each coil of the outer coil spring  30  utilized a circular cross-section having a diameter of 0.203 inches. The outer coil spring  30  had a right hand pitch and adjacent coils were in contact (closed). The coil was formed of a chrome silicon, oil tempered spring steel. 
     One inner coil spring  31  was snugly fitted within the outer coil spring  30 . One end was brazed to a shaft, the shaft being inserted into the chuck of an electric drill. The other end was brazed a conventional masonry bit having tungsten cutters. 
     Two tests were performed using the above flexible shaft, a ¾″ masonry bit and a 9″ turning radius elbow. In the first instance, using a ¼ HP motor and 500 rpm, a ¾ inch diameter hole was cut in 2000 psi concrete, about 2 inches deep in 120 seconds. In the second instance, using a ½ HP motor and 2000 rpm, a ¾ inch diameter hole was cut in 2000 psi concrete, about 2 inches deep in 30 seconds. 
     To demonstrate applicability to function within the bore of small diameter case, an elbow with a radius of less than 5 inches was prepared for installation within the bore of a 5 inch casing. The elbow was fitted with four hardwood bushings  21 . To enable installation of the inner bushings the elbow was formed of two 45° steel elbows. 
     To simulate the supporting structure about the bore of a drainhole which would be formed between the elbow and the subsequent drilling location of the bit, a five foot section of 2 inch ID PVC pipe was installed after the elbow. Using a custom 2″ diameter bit, having 4 tungsten carbide cutting faces, a 2 HP motor and 750 rpm, a 2 inch diameter hole was cut 2 inches deep in 60 seconds. 
     While the 2″ diameter bit was originally pinned through its shank and through the two concentric coil springs, brazing was also used with equal success. 
     The present invention provided several advantages including: 
     the ability to rework a well without a full rig and with a minimum of surface equipment; 
     the whole tool assembly (motor elbow and shafting) is lightweight, typically only about 500 pounds; and 
     fast workovers. 
     Various enhancements to the invention include: 
     use of helically coils having cross-sectional profiles other than circular, for increased shear strength; 
     coil material can be dictated to meet variable corrosion requirements, such as in sour wells; 
     use of in-the-wellbore mud motor and tubing string, hydraulic or electric-powered motors and connecting cables or conceivably, a tubing string extending from the surface could be used to impart rotation into the flexible shaft, albeit at lower rotational speeds and high torque; 
     vibratory or impact delivery means associated with the motor for enhanced drilling rates in the formation; or 
     Use of a flexible conduit extending within the inner coil spring for delivering lubrication and cutting fluids to the bit.