Tubular element for use in a rotary drilling assembly

Abstract: A tubular drill string element (1), such as a stabilizer or tool joint, comprises an outer surface (5) having in circumferential direction a ratchet profile. The ratchet profile is preferentially oriented such that it provides low resistance against right hand rotation but high resistance against left hand rotation of the drill string.

Description: The invention relates to a tubular element for use in a rotary drilling assembly. Rotary drilling assemblies used in underground well drilling operations generally comprise a drill bit connected at the lower end of an elongate drill string. The drilling assembly may comprise a downhole drilling motor which drives the bit while the drill string above the motor is not rotated or rotated slowly by the rotary table at the surface. As disclosed in EP-A-85444 and EP-A-109699 it may be desired that the drill string is not rotated during at least part of the drilling operations so as to maintain the toolface of the bit in a predetermined tilted orientation in the borehole in order to drill a deviated hole section. A difficulty encountered during such oriented drilling operations is that weight on bit fluctuations generate reactive torque fluctuations as a result of which the amount of twist in the elongated drill string varies and the orientation of the toolface becomes unstable. This unstable toolface orientation makes the steering process less effective and more difficult to control. Thus there is a need for a drilling assembly which can be prevented from making swinging motions in the borehole as a result of reactive torque fluctuations. US-A-3 194 331 discloses a tubular element for use in a rotary drilling assembly, the element having helical grooves along the outer surface thereof. EP-A-85 444 discloses a rotary drilling assembly comprising a tubular element having an outer surface facing the borehole wall during drilling, and a downhole motor for driving a drill bit arranged at a lower end of the drilling assembly. The invention as claimed is intended to provide a rotary drilling assembly which is able to suppress swinging motions of a drill string in response to such reactive torque fluctuations. The rotary drilling assembly according to the invention thereto comprises a drill string, a tubular element having an outer surface facing the borehole wall during drilling, and a downhole motor suitable to drive a drill bit arranged at a lower end of the drilling assembly, characterized in that said tubular element has a ratchet profile in a plane cross-axial to a longitudinal axis of the element, said ratchet profile being formed by blades of a bladed drill string stabilizer, which blades comprise each a smooth leading edge and a sharp following edge which penetrates into the borehole wall when the drill string rotates in a direction opposite to rotation of the drill bit during drilling therewith, whereby the radius of each blade gradually increases in said direction from the leading edge of the blade towards the following edge of the blade. In a preferred embodiment of the invention said ratchet profile is oriented such that it provides a high resistance against left hand rotation and low resistance against right hand rotation of the element about the longitudinal axis. In this manner during right hand rotation of the drill string, which is the normal rotation for most available drilling assemblies, only low friction forces are generated if the ratchet surface slides along the borehole wall. However, if the rotary table is held stationary and the drill string tends to swing back due to reactive torque fluctuations the sharp edge of the ratchet profile is the leading edge which penetrates into the borehole wall and generates resistance against any further left hand rotation. The ratchet profile may be mounted on any drill string tubular which faces the borehole wall during drilling, such as a stabilizer, tool joint, drill collar or housing of a downhole drilling motor. The ratchet profile may further be created by forming a sharp edge at one side of the blades of a bladed stabilizer, by mounting toothed inserts on said stabilizer blades or by forming longitudinal saw-tooth shaped ridges on the outer surface of a tool joint. The invention will now be explained in more detail with reference to the accompanying drawings, in which: Fig. 1A is a cross-sectional view of a stabilizer embodying the invention, Fig. 1B illustrates the low resistance of the toothed blades of the stabilizer of Fig. 1A against right hand rotation, Fig. 1C illustrates the high resistance of the toothed blades of the stabilizer of Fig. 1A against left hand rotation, Fig. 2A is a perspective view of a stabilizer comprising helical blades on which toothed inserts are mounted, Fig. 2B shows the encircled portion of one of the blades of the stabilizer shown in Fig. 2A, Fig. 2C shows a cross-section of the stabilizer blade of Fig. 2B taken along line A-A and seen in the direction of the arrows, Fig. 2D shows a longitudinal section of the stabilizer blade of Fig. 2B taken along line B-B and seen in the direction of the arrows, Fig. 3A is a side view of a tool joint embodying the invention, and Fig. 3B is a cross-section of the tool joint of Fig. 3A taken along line C-C and seen in the direction of the arrows. Fig. 1A shows a drill string stabilizer 1 comprising four helical or straight stabilizer blades 2A-D. Each blade 2A-D has a rounded leading edge 3 and a sharp following edge 4. The outer surface 5 of each blade is located at a radius R from the longitudinal axis I of the stabilizer, which radius increases in a direction from said leading edge 3 towards said following edge 4. In the situation shown the stabilizer lies on the low side of the boreholewall 6 so that the stabilizer blades 2A and 2B are in contact with the boreholewall 6 whereas there is some clearance between the other two stabilizers 2C and 2D and the boreholewall 6. Fig. 1B shows the movement of stabilizer blade 2A during right hand rotation of the stabilizer. During drilling operations right hand rotation is the usual direction of rotation of the drill string. As can be seen in Fig. 1B during such right hand rotation the rounded edge 3 of the stabilizer blade 2A is the leading edge. The rounded edge 3 has poor cutting characteristics because of the extremely large negative back rake angle and thus prevents the blade 2A from penetrating into the hole wall 6. In addition accumulation of filter cake 8 between the outer surface 5 of the blade 2A and the hole wall provides lubrication which assists in a low friction resistance of the blade against right hand rotation. As can be seen in Fig. 1C left hand rotation of the stabilizer causes the sharp edge 4 of the stabilizer blade 2A to penetrate into the borehole wall 6 and to build up resistance against further left hand rotation. In this manner it is avoided that when the rotary table is held stationary variations of reactive torque exerted by the bit to a downhole motor above the bit cause the drill string to swing back since such torque variations are transferred to the boreholewall via the stabilizer blades. The ratchet profile configuration according to the invention can be implemented in stabilizers with longitudinal stabilizer blades. In that case the stabilizer blades will under lateral pressure carve longitudinal grooves in the borehole wall while the string is lowered through the borehole, thereby creating resistance against left hand rotation without changing the angular orientation of the drill string. As illustrated in Fig. 2A-2D the ratchet profile configuration according to the invention may also be implemented in helical stabilizers. As can be seen in Fig. 2B and 2C each stabilizer blade 10 has a smooth leading edge 11 and a sharp following edge 12 formed by a toothed inserts 13. The outer surface 14 of each stabilizer is located at a varying distance from the longitudinal axis L of the drill string 15, which distance increases in a direction from the leading edge 11 towards the following edge 12. The outer surface 14 of each stabilizer blade 10 comprises a series of wear resistant tungsten carbide inserts 16 that are flush to said surface 14. Each blade 10 further comprises toothed inserts 13 which have in circumferential direction (see Fig. 2C) a saw-tooth profile and in longitudinal direction (see Fig. 2D) an elongate triangular shape. The orientation of the toothed inserts 13 is such that the cutting edge 12 has a longitudinal orientation thereby enabling said cutting edges 12 to carve longitudinal grooves in the boreholewall while the string 15 is lowered through the borehole and to create resistance against left hand rotation without changing the angular orientation of the drill string 15. The toothed inserts 13 provide low resistance against right hand rotation but high resistance against left hand rotation of the drill string 15. Fig. 3A and 3B show an embodiment of the present invention wherein a ratchet profile is created by carving longitudinal grooves 20 in the essentially cylindrical outer surface 21 of a tool joint of a heavy weight drill pipe section 22. The ratchet profile thus created comprises circumferentially distributed cutting edges 23 which provide low resistance against right hand rotation of the section 22 but high resistance against left hand rotation of the section 22. The high resistance against left hand rotation provided by the ratchet profile according to the invention is of particular importance in combination with the continuous bit steering concept using mudmotors in deviated wells as disclosed in European patent specifications No. 85444 and 109699. During drilling in the oriented drilling mode with these continuous steering concepts, which requires that the drill string does not rotate, utilization of stabilizers or tool joints with the ratchet profile according to the invention ensures that reactive torque fluctuations generated by weight-on-bit fluctuations are transferred to the borehole wall and do not induce variations in drill string twist. It will be understood that the average torque level in the drill string is transmitted to the surface and can be balanced by the rotary table. It will further be understood that instead of providing stabilizers or tooljoints with a ratchet profile any other tubular drill string element which faces the borehole wall during drilling may also incorporate the ratchet profile according to the invention.

Claims: A rotary drilling assembly comprising a drill string (15), a tubular element (1) having an outer surface (14) facing the borehole wall (6) during drilling, and a downhole motor suitable to drive a drill bit arranged at a lower end of the drilling assembly, characterized in that said tubular element (1) has a ratchet profile in a plane cross-axial to a longitudinal axis of the element, said ratchet profile being formed by blades (2A, 2B, 2C, 2D, 10) of a bladed drill string stabilizer (1), which blades (2A, 2B, 2C, 2D, 10) comprise each a smooth leading edge (3, 11) and a sharp following edge (4, 12) which penetrates into the borehole wall (6) when the drill string rotates in a direction opposite to rotation of the drill bit during drilling therewith, whereby the radius of each blade gradually increases in said direction from the leading edge (3, 11) of the blade towards the following edge (4, 12) of the blade. The rotary drilling assembly of claim 1, wherein said ratchet profile is formed by inserts (13) which are circumferentially distributed over said surface (14) and which have in circumferential direction a toothed shape. The rotary drilling assembly of claim 2, wherein each insert (13) has in longitudinal direction an elongate triangular shape. The rotary drilling assembly of claim 1, wherein said ratchet profile is oriented such that it provides high resistance against left hand rotation and low resistance against right hand rotation of the element about the longitudinal axis. The rotary drilling assembly of claim 1, wherein the tubular element is formed by a tool joint of a drill string section, the ratchet profile being formed by longitudinal saw-tooth shaped grooves (20) in the outer surface of the tool joint.

Inventors: ZIJSLING DJURRE HANS, ZIJSLING, DJURRE HANS
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1988
Main classification: E21B17
IPC codes: E21B7, E21B17