Abstract:
A component for attachment to a drillpipe which is part of a drillstring carrying a drillbit, said drillstring rotatably driven in a working direction, which drillpipe contains a standard box tool joint at one end and a standard pin tool joint at the other end, which tool joints are of a diameter greater than the section of drillpipe between the two joints, and which drillpipe component is comprised of two elongated cylindrical half sections for clamping over at least a portion of the narrower section of drillpipe and which component, on its outer surface, contains at least one helical pumping chamber having a twist, when viewed in axial elevation, opposite to that in which said drillstring is rotatably driven in said working direction, said pumping chamber, when view is traverse section, having an undercut portion relative to the surface of the drillpipe component, said undercut portion defining a lip.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. Ser. No. 595,550 which was filed on Oct. 11, 1990, now U.S. Pat. No. 5,040,620. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus which can be attached to a drillstring to improve volumetric and drilling efficiencies, reduces time and energy costs of drilling, and increases drill bit life. The apparatus is a sleeve which can be attached to the outside of a drillpipe, which sleeve contains one or more helical pumping chambers for enhancing the movement of drilling mud and cuttings from bit/formation interface to borehole and thence to the surface. The full spectrum of boreholes from true vertical through &#34;high angle&#34; and including horizontal is encompassed. However, it is understood that the vertical portion of the borehole through unconsolidated formation and gumbo has been cased prior to running this apparatus. 
     BACKGROUND OF THE INVENTION 
     During the drilling of a borehole, or well, through a subterranean formation, drilling mud-a rheolitic slurry of fluid and buoyant suspension agent, e.g. bentonite-is pumped through a passageway in the drillstring to the bit, where it is injected at high velocity and pressure against the formation through jets located in the bit. The particular consistency of the drilling mud captures the cuttings generated by the bit, while its buoyant character assists the cuttings to rise out of the path of the bit. Because the diameter of the drill bit exceeds that of the other drillstring components, the cutting-laden drilling mud rises to the surface in the annulus defined by the drillstring and the wall of the borehole. Because the cutting-laden drilling mud can interfere with the drilling process, it is desirable to move it to the surface at a faster rate than conventional drilling presently allows. 
     Reduction in volumetric efficiency attributable to reduced effectiveness of the drilling mud hole-cleaning ability impacts a number of parameters. Because some of the cuttings are not removed from the path of the drill bit quickly enough, drilling efficiency (the rate of penetration or ROP) is reduced, leading to increased drilling time and energy requirements to achieve a specified borehole depth. Additionally, energy is lost by grinding the cuttings remaining in the path of the drill bit. The effect increases the difficulty of removing the cuttings and decreases the useful life of the bit--a substantial consideration in costly diamond drilling bit applications. Moreover, frequent removals of the drillstring to replace worn bits is a time consuming and expensive process, while concomitantly increasing the risk of a blow-out endangering personnel. 
     Yet another important problem encountered in drilling oil and gas wells is the phenomena of &#34;differential sticking.&#34; Differential sticking occurs when the fluid in the drilling mud, located in the drillstring-borehole annulus, is absorbed unevenly around the periphery of the drillstring through the porous media of the borehole wall. This fluid loss induces a pressure differential across the drillstring diameter which causes the drillstring to be deflected against the borehole wall on the side experiencing the fluid loss, and can lead to halting engagement of the drillstring against the borehole wall. Once so engaged, the unbalanced fluid pressure acts to keep the drillstring in engagement with the borehole wall. The torque required to free the drillstring may exceed the capacity of the rotary table or the top drive used to drive the drillstring, or may exceed the yield strength of a drillstring component, leading to &#34;twistoff&#34; (torsion induced fracture). Differential sticking may result in the loss of the drill bit and a portion of the drillstring, thereby necessitating time consuming and extremely expensive procedures to recover the detached drillstring portion. In some cases, where the detached portion cannot be retrieved, the drill operator may have to abandon the borehole and begin anew. 
     A final phenomenon observed with conventional drillstrings is that of &#34;key seating&#34; at &#34;doglegs&#34; (borehole direction changes) and &#34;kick-off-points&#34;, i.e., locations at which the angle of attack of the drill bit and drillstring is altered as the inclination from the vertical is increased. The phenomena of key-seating arises when there is sufficient bend in the borehole path to cause a portion of the drillstring to come into contact with one side of the borehole wall. This contact, if not substantial enough to cause differential sticking, can result in the drillstring forming a groove approximately the diameter of the drillstring in the borehole wall. If viewed in cross-section perpendicular to the borehole longitudinal axis, the borehole and groove would resemble a keyhole, with a large lower portion and a narrower upper portion. When key-seating occurs, it may no longer be possible to withdraw the drillstring from the borehole, since the larger diameter elements of the drillstring assembly (drill collars, stabilizers, etc.) will be unable to pass through the narrow groove. The phenomena of key-seating is due in large part to the rigidity of conventional drillstring components, which are unable to provide enough flexure to accommodate borehole directional changes and changes in the angle of attack. As with differential sticking, key-seating can lead to twistoff, necessitating time consuming retrieval procedures or abandonment of the borehole. 
     The aforementioned problems have provided a fertile ground for invention, and a number of prior art drillstring component designs are directed toward resolving one or more of these problems. One solution adopted by a number of prior art drillstring components, including the present invention, is the use of a helical flat or groove around the periphery of the drillstring component. Prior art drillstring components using such a solution may generally be grouped into two categories, each characterized by a disadvantage that the present invention is designed to overcome. 
     A first category of prior art helical groove drillstring component employs screw-like threads or broad V-shaped notches. Fitch U.S. Pat. No. 3,085,639 discloses a drill collar having screw-like threads on its periphery for drilling straight boreholes, wherein the flights of the screw coact with the borehole as a screw conveyor in removing cuttings from the vicinity of the drill bit. Arnold U.S. Pat. No. 3,194,331 and Massey U.S. Pat. No. 3,360,960 disclose, respectively, drillstring components having a single and multistep V-shaped helical groove on the circumference designed to reduce differential sticking, increase drilling mud flow through the borehole-drillingstring annulus, and to act as a broach to reduce key-seating. 
     In operation, the configuration of the helical groove in all three of these patents is such that the sharp edges of the grooves may strip the drilling mud lining the borehole wall (referred to as mud wallcake), leading to instability of the borehole wall and concomitant loss of fluid from the borehole. The drillstring component of the present invention is designed to leave intact the desired wallcake thickness, generally 3/32&#34;, while still providing superior performance by increasing drilling mud flow up the annulus, plus reducing differential sticking and key-seating. 
     A second category of helical groove drillstring component employs a spiral groove wherein the groove constitutes essentially a chord intersecting two points on the circumference of the drillstring component. Fox U.S. Pat. No. 2,999,552, Chance et al. U.S. Pat. No. 4,460,202, and Hill et al. U.S. Pat. No. 4,811,800 all disclose spiral groove drillstring components wherein he groove forms a chord on the component, when viewed in traverse section. The purpose of the groove is to reduce differential sticking, improve flow of drilling mud up the borehole-drillstring annulus and to increase the load on the drill bit in directional drilling applications. Hill et al. U.S. Pat. No. 4,811,800 discloses trading-off drillstring component service life in favor of increased drillstring flexibility by employing a relatively deep spiral chord-style groove. The drillstring component of the present invention is designed to provide the benefits attributed to these prior art chord-style spiral groove drillstring components, plus superior service life and flexibility in short radius directional drilling applications. 
     In view of the foregoing, it is an object of this invention to provide a drillstring component for drilling high angle and short radius directional and horizontal boreholes which experiences reduced mechanical fatique duty relative to previously known drillstring components, and which is readilly integrable with existing drilling systems, including downhole drilling mud-driven turbine style motors (&#34;mudmotors&#34;). 
     It is a further object of this invention to provide a drillstring component for drilling high angle, directional and horizontal boreholes which improves volumetric and drilling efficiencies, reduces time and energy costs of drilling, and increases drill bit life relative to that achieved with previously known drillstring components. 
     It is another object of this invention to provide a drillstring component for a drilling high angle, directional and horizontal boreholes which substantially reduces the incidence of differential sticking, thereby reducing the major costs associated with retrieval of detached drillstring portions or abandonment of a partially drilled well. 
     It is yet another object of this invention to provide a drillstring component for drilling high angle and horizontal boreholes which has adequate flexibility to reduce the costs and additional effort required by incidents of key-seating and possible twistoff of the lower portion of the drillstring. 
     It is still another object of this invention to utilize the rotary motion of the drillstring to induce a turbine-style pumping (&#34;turbo-pumping&#34;) action of the cutting-laden drilling mud away from the drill bit and subterranean formation interface toward the drilling mud treatment equipment at the borehole entrance. 
     This invention includes method steps carried out in sequence for obtaining the desired borehole-cleaning capability when drilling high angle, directional and horizontal boreholes. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a component for attachment to a drillpipe which is part of a drillstring carrying a drillbit, said drillstring rotatably driven in a working direction, which drillpipe contains a standard box tool joint at one end and a standard pin tool joint at the other end, which tool joints are of a diameter greater than the section of drillpipe between the two joints, and which drillpipe component is comprised of two elongated cylindrical half sections for clamping over at least a portion of the narrower section of drillpipe and which component, on its outer surface, contains at least one helical pumping chamber having a twist, when viewed in axial elevation, opposite to that in which said drillstring is rotatably driven in said working direction, said pumping chamber, when view is traverse section, having an undercut portion relative to the surface of the drillpipe component, said undercut portion defining a lip. 
     In a preferred embodiment of the present invention, the drillpipe component is manufactured from a polymeric material, such as a thermosetting plastic and is used in a cased borehole. 
     In another preferred embodiment of the present invention, the drillpipe component is manufactured from a metallic material and is used in an uncased borehole in a consolidated formation. 
     In both preferred embodiments of the present invention, the undercut defines a volute. The volute pumping chamber embodiment features a cross-section having at least two different radii of curvature, and has no sharp edges which could result in stress concentrations or which could damage the borehole mudcake. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view of the drillpipe component of the present invention clamped onto a standard drillpipe. 
     FIG. 2 is an elevation view of a drillstring, constructed in accordance with the principles of the present invention. 
     FIGS. 3-5 illustrate axial cross-sectional views of several preferred embodiments of a drillpipe component constructed in accordance with the principles of this invention. 
     FIG. 6 is a fragmentary view of a cross-section of the drillpipe component of the invention illustrating the volute pumping chamber dimensions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows drillpipe component 25 constructed in accordance with the principles of this invention. The drillpipe component is illustrated on a conventional drillpipe 12. Drillpipe component 10 has a left-handed helical pumping chamber 31. Standard American Petroleum Institute (&#34;A.P.I.&#34;) box tool joint 16 and pin tool joint 18 are attached, respectively, to the upper end and lower ends of drillpipe 12. A circular passageway 14 is concentrically located within drillpipe 12 for carrying drilling mud to the drillstring bit. Drilling mud is pumped downward through this passageway by a drilling mud pump located near the entrance to the borehole, as described heretofore. 
     Referring now to FIG. 2, an elevation view of an illustrative embodiment of a drillstring 20, practicing the principles of the present invention, is disposed in a directionally drilled borehole 21. As shown in FIG. 2, borehole 21 comprises a vertical leg leading from the borehole entrance (not shown), a transition zone, a substantially horizontal leg and an annular passage defined by the borehole wall and the exterior of the drillstring. Drillstring 20 is comprised of drill bit 22, downhole mudmotor 23, drill collar 24 and drill pipe 25 containing the component of the present invention. The drillstring may, in addition, employ stabilizer units, not shown. Full length drillstring components 25 are joined by mating their respective threaded box and pin tool joints. The drillstring is engaged by a rotary table near the entrance of the borehole in a manner per se known. Drill bit 34, downhole mudmotor 23 and the assorted joint sections and stabilizer units are conventional devices and form no part of this invention. Rather, the invention resides in use of the drillpipe component containing the uniquely designed helical pumping chamber 31 (of FIG. 1 hereof) which chamber is cascaded upwards at each end of drillstring component 25 near the tool joint connection. A single helical groove is illustrated in FIG. 2, but it is to be understood that any number of grooves can be used to accomplish the turbo-pumping objectives of the invention. Five or more chambers spaced apart in equal relation around the periphery of drillpipe component 25 are expected to provide the optimum cross-section for flexibility and fatigue resistance. 
     Drillpipes on which the component of the present invention are practiced may be any conventional drillpipe. Such drillpipe is typically manufactured from high strength steel meeting A.P.I. metallurgy specifications. Drillstring component 25, and the drillpipe to which the component of the instant invention is attached are of standard size (e.g., 71/4&#34; diameter for an 83/4&#34; borehole) and length for a given application. 
     FIG. 2 illustrates the flexibility of drillpipe component 25 at borehole kick-off point 26. 
     FIGS. 3, 4, and 5 show a number of drillpipe component axial cross-sectional plan views illustrating the uniquely designed pumping chamber constructed in accordance with the present invention. FIG. 3 provides an axial cross-sectional view of drillpipe component halves 30 and 30a, having five pear-shaped or finger-like continuously curving undercut pumping chambers 31. The pumping chambers are undercut with respect to the cylindrical surface of the drillstring components, thereby forming a lip 32 associated with each pumping chamber. In the preferred embodiments shown in FIGS. 3-6, the pumping chamber forms a volute having at least two portions of different radii of curvature. FIG. 4 shows six volute pumping chambers in a drillstring component cross-section, while FIG. 5 shows eight volute pumping chambers in a drillstring component cross-section. Each of these figures shows the two halves being held together by bolts 51. 
     The two halves of the drillpipe component of the present invention can be manufactured from any appropriate material. Non-limiting types of materials which can be used include plastics, such as thermoset plastics, which preferably contain a strengthening filler component. Filler components may include such things as carbon black and fibers and filaments comprised of spun glass or carbon. It is preferred that the drillpipe components of the instant invention be comprised of a thermosetting plastic. 
     The two halves may be joined by any appropriate means including the use of bolts and locknuts only, or in conjunction with a hinge along one side. 
     Each of the drillpipe component cross-sections in FIGS. 3-5 has a central bore 33 through which the drilling mud is pumped to drill bit 22. The direction of twist of pumping chambers 31, indicated by the arrow in FIGS. 3-5, is counterclockwise when viewed in axial elevation (i.e., a left-hand twist, see FIG. 1), based on the convention that the drill is rotated in a clockwise direction. The surface of pumping chamber 31, when view in axial cross section, may define a tear-shape, or pear-shape having a continuously curved perimeter so as to minimize the creation of stress concentration points that might otherwise result in fracture of lip 32 or destruction of the wallcake, or mudcake, lining the borehole. The pumping chamber is characterized by having an undercut portion, with respect to the surface of the drillstring component, so that lip 32 is formed to overhang the pumping chamber, as shown in FIG. 6. 
     In the preferred embodiment configuration, the pumping chamber, when viewed in axial cross-section, defines a continuously curved volute having at least two portions with different radii of curvature. Referring again to FIG. 6, pumping chamber 31 is comprised substantially of two portions having radii of curvature &#34;c&#34; and &#34;d&#34;. The precise configuration of the pumping chamber axial cross-section is not critical, provided that the radius of curvature of portion &#34;d&#34; of the volute is substantially smaller than that for portion &#34;c&#34;. In one preferred embodiment, the ratio c to d is 3.25:1. 
     In an alternate embodiment, the shape of the volute is a mirror image across the radius A--A shown in FIG. 6. This embodiment of the helical volute pumping chamber is contemplated to have the advantage of increasing turbidity in the drilling mud present in the borehole-drillstring annulus, while having lower pumping capacity. Creating turbidity in the drilling mud located in the borehole-drillstring annulus can have important advantages as described hereinafter. 
     The helical pitch of the pumping chambers 31 (i.e., the distance between portions of the same groove measured on a line parallel to the drillpipe component longitudinal axis) will vary depending upon the number of pumping chambers employed and the volume of the pumping chambers. It is contemplated that the pitch of the spiral should not be less than that necessary to encircle the circumference of the drillpipe component over a length equal to 12 times the outer diameter of the drillstring component, and not more than that necessary to encircle same over a length 3 times such diameter. However, the velocity in the drillstring longitudinal direction of any point on the interior of the pumping chamber must exceed that of the velocity of the drilling mud in the adjacent borehole-drillstring annulus, within the range of drillstring rotation speeds. 
     It is also contemplated that the cross-sectional area of the pumping chambers 31 may equal from 5 percent to 60 percent of the cross-sectional area of a smooth surface drillpipe component of the same inner and outer diameters. The minimum cross-sectional area within each pumping chamber must be such that a cutting of the maximum size likely to be encountered in drilling a given subterranean formation will pass cleanly through the pumping chamber, i.e., without becoming stuck in the pumping chamber. 
     The pumping chamber 31 in drillpipe component provides a number of advantages over prior art spiral groove drillstring components and conventional circular cylinder drill collars when used in high-angle, directional and horizontal drilling applications. The helical volute pumping chambers act partly in a manner analogous to an Archimedean screw by propelling the cutting-laden drilling mud generated at the drill bit backwards and upwards toward the top of the borehole. Furthermore, as the drilling mud is propelled upward by the pumping chamber, it induces a dynamic flow field in the annulus. Rotation of the drillpipe component creates a partial suction at the bottom of the borehole tending to draw up additional amounts of drilling mud due to the localized underbalanced condition at the drill bit/formation interface, thus increasing the rate of penetration. 
     In conventional drilling applications, only about one-half of the borehole depth is attributable to the mechanical cutting energy of the drill bit; the balance of the earth cutting power is supplied by the hydrodynamic impact forces created by injecting the drilling mud through the drill bit jets. Drillstring component 25 harnesses the rotational energy of drillstring 20, which would otherwise be lost, for example, as heat, and uses that energy to increase the volumetric efficiency of the drilling rig. The turbo-pumping action induced by spiral pumping chamber 31 (of FIG. 1) enhances cuttings removal and provides a clear path for the drill bit to contact uncut formation, rather than pulverizing previous cuttings which heretofore were not quickly removed from the drill bit path. Consequently, significant increases in the rate of penetration of the drill bit and a concomitant increase in drill bit life may be realized. 
     Referring again to FIG. 1, pumping chamber 31 of drillpipe component 25 significantly reduces the incidence of differential sticking because pumping chamber 31 acts to equalize fluid pressure around the periphery of the drillpipe component. Also, since the drilling mud is free to flow through pumping chamber 31 to equalize any gradients around the drillpipe periphery, there is no longer a problem of lateral fluid pressure imbalance maintaining the drillstring in halting engagement with the borehole wall. Finally, since drillpipes incorporating the components of the present invention are not subject to drag induced by lesser degrees of differential sticking (i.e., downhole torque reduction), the drillstring can achieve higher rotary speeds with less concern about twistoff. 
     Finally, the configuration of pumping chamber 31 is designed to permit increased flexion of the drillstring component relative to previously known devices. Whereas, for example, a drillstring component designed in accordance with Hill et al. U.S. Pat. No. 4,811,800, based on the data contained in FIG. 10 of that patent, would experience twistoff within six hours (assuming a conservatively low rotary speed of 35 r.p.m. and a bend radius of 50 feet), it is contemplated that a drillpipe component constructed in accordance with the present invention, and having five or more helical pumping chambers, would have a service life of several hundred hours. 
     It is to be understood that the number of spiral pumping chambers 31 employed at equally spaced locations around the circumference of the drillpipe component may vary from one to many, and that precise configuration of the pumping chambers is not critical, provided that the pumping chambers preferably have a twist oriented in the direction opposite that of the drillstring rotation. Furthermore, the range of cross-sectional area of the drillpipe component that can be dedicated to the pumping chamber is limited at the lower end only by the minimum needed to induce a pumping action (dependent in part also upon the helical pitch) and at the upper limit by the minimum amount of metal required to maintain the torsional strength of the drillstring component. 
     EXAMPLE 1 
     For the volute pumping chamber shown in FIG. 6, wherein the dimensions a-f are: a=3.25&#34;; b=1.50&#34;; c=0.5&#34;; e=0.19&#34; and f=0.25&#34;, the cross-sectional area of the pumping chamber is about 2.0 in 2 . 
     Calculated values of the pumping capacity for a 30 foot long drillpipe component embodying the present invention, with the foregoing pumping chamber dimensions, and having a pitch of 1/10 turns per foot, are presented in Table 1 as a function of the number of volutes present on the drillpipe component periphery. 
     
                       TABLE 1______________________________________               Pumping CapacityNumber of    % Reduction               GPM @ RPMVolutes  in Area    10 RPM    25 RPM 50 RPM______________________________________1         7.6        5.5      13.8   27.63        22.9       16.5      41.4   82.85        38.3       27.5      69.0   138.06        46.0       33.0      82.8   165.68        61.3       44.0      110.4  221.8______________________________________ 
    
     While the prior art helically grooved drillpipe components emphasize that the grooves serves to increase the load on the drill bit when used in directional and horizontal drilling applications, the counter-rotation twist of the drillstring of the present invention is particularly suitable for use with downhole mudmotors, since operation of invention drillstring component will not induce any &#34;screw down&#34; or other forces which might cause the mudmotor or bit to deviate from its intended path. Since the function of the mudmotor and assembly is to maintain a true course for the interpenetration of oilsand zones, extraneous forces introduced by the prior art drillstring components may be undesirable. In fact, such &#34;screwing down&#34; action may result in aggressive contact between these other prior art devices and the borehole wall, thereby destroying the wallcake and impeding progress. 
     Finally, the pumping capacity of the present invention, as represented in Table 1, gives a drillpipe component embodying the present invention the additional advantage of borehole cleaning in the event of a drilling mud pump shutdown or failure. With presently existing drillstring components, drilling mud pump shutdown can result in cuttings quickly settling out of suspension and packing in against the drillstring stabilizers, drill collars and bit, thereby impeding or preventing withdrawal of the drillstring. However, simply rotating a drillstring embodying the drillpipe component containing the pumping chambers of the present invention--using the rotary table or top drive--will keep the cuttings in suspension and pump cutting-laden drilling mud to the surface. Thus, a drillstring embodying the present invention features greatly enhanced retrievability, even in the event of drilling mud pump shutdown or failure.