Flexible drill string member and method of drilling deviated holes

Abstract: The invention relates to an improved flexible drill string member having at least one spiral groove (24) formed in its outside surface (30) and incorporating a combination of engineering considerations and criteria such that the invention member will have an extraordi­ narily high performance capacity in directional drilling with downhole motor driven bits. The said spiral groove (24) defining lands on said main body portion between the flights thereof and the depth of said spiral groove being at least 40% of the wall thickness of said body measured from the outside diameter (30) of said land to the inside diameter of said member.

Description: This invention relates to a drill string member for use in drilling deviated holes and to a method of drilling deviated holes using said drill string member. Whereas convention wellbores descend generally vertically from the surface to the oil or gas field it is becoming increasingly important to be able to drill holes which extend generally horizontally from the wellbore or are inclined at a substantial angle thereto. Such holes are generally referred to as "deviated holes". Because of the high stresses and strains imposed during deviated drilling special drill string members have been developed. However, none are suitable for drilling deviated holes with relatively small radii, for example 15m. US-A-4 460 202 (in common ownership herewith) discloses an intermediate weight drill string member which will operate in compression and in tension and is intended to be used between drill pipe and drill collars. The drill string member comprises tool joint means (a pin and a socket) which are separated by a main body portion which comprises a slip and elevator section and a spiral section. The grooves on the spiral section are cut so that the moment of inertia of the spiral section and the slip and elevator section are substantially equal. In one example in Table II the depth of the grooves is 71% of the wall thickness of the spiral section. According to the present invention there is provided a drill string member for use in drilling deviated holes and to be interposed in a drill string, said drill string member having tool joint means at each end thereof and a main body portion therebetween, said main body portion of said drill string member being formed with at least one spiral groove, said spiral groove defining lands on said main body portion between the flights thereof, characterised in that said spiral groove extends over the whole length of said main body portion continuously between said tool joint means, and the depth of said spiral groove is in the range of from about 55% to about 85% of the wall thickness of said body measured from the outside diameter of said land to the inside diameter of said member. It will be noted that drill string members in accordance do NOT have a slip and elevator section and that the spiral groove extends from tool joint to tool joint. Preferably, said grooves and/or lands are provided with a compressive prestress. Advantageously, said compressive prestress is provided by shot peering or carburization. Preferably, said drill string member includes a number of spiral grooves which is a multiple of three. The present invention also provides a drill string including a drill pipe and a drill string member in accordance with the present invention, said drill string having one or more of the following features:- (a) said drill string member has a weight per unit length only slightly more than that of said drill pipe; (b) said tool joint means each has an axial length substantially less than that of the tool joint means on said drill pipe; and (c) the difference in the maximum stress in said lands and in said groove of said drill string member is equal to or less than 43000 psi (296 N/mm²) at a bending radius of 80 feet (24.4m). The present invention also provides a method of drilling deviated holes using a drill string member in accordance with the invention, interposed in a drill string between a motor driven bit at the bit end of said drill string and drill pipe at the upper end of said drill string. Drill string members in accordance with the present invention are more particularly but not exclusively, adapted for use with downhole motors and with measurement while drilling (MWD) techniques. They can operate while containing substantial internal pressure from the drilling mud, in tension, in compression, in torsion, in bending, and in combinations of all of these forces as are demanded by the rigours of deviated drilling which are very demanding on the drill string. The spiral grooving helps ensure that the drill string does not tend to stick to the hole. Furthermore, if the spiralling is right-handed, clockwise rotation of the drill string member in effect "screws" the bit down into the extremity of the hole, whether this extremity is vertical or horizontal. This "screwing" effect is particularly important in the environment of the invention of deviated drilling, since the normal pressure on the bit created by the weight of the drill string is minimised when the drill string is deviated from vertical, and is especially so when the hole is horizontal. Yet another advantage of the spiral grooving is that it tends to churn the drilling mud which further enhances the efficiency of the overall drilling operation. Again, this is especially important in deviated drilling because gravity tends to force the mud and the entrained drill cuttings to collect at the bottom of the hole, which aggravates the problem even further unless the mud is kept churned and moving out of the hole. A related advantage is that the churning tends to keep a uniformly thick mud cake on the wall of the hole thereby preventing contaminants from extruding into the producing formations. Because of their robust construction, one can expect reuse of a conventional drill collar perhaps as many as 30 or 40 or more times. However, such conventional drill collars are also very stiff and lack the flexibility needed for deviated drilling. The approach of the invention is to design the drill string member so that it will perform at the maximum limits of its mechanical properties. Durability is "traded" for flexibility. Thus it is anticipated that drill string members embodying the invention will be reused perhaps 6, 8, 10 or so times, and perhaps even fewer times for certain bend radii. However, drill string members will be able to perform much better as to making tighter radii holes. The absence of slip and elevator areas permits a gentle "feathering" of the spiral grooves into the tool joint areas, which prolongs the life of the drill string member by more evenly spreading the stresses and strains throughout the drill string member rather than concentrating them as would occur in a slip or elevator area. MWD techniques require that the first few lengths of the drill string next to the bit be non-ferrous so that formation data can be gathered, and so that the data produced by the MWD tool can be transmitted clearly back to the surface in order to control the downhole motor. For this purpose drill string members in accordance with the invention can be made of non-ferrous metal. While the invention was developed for and in the environment of a drill string member for use in drilling deviated holes, it is not limited in that fashion. For example, drill string members in accordance with the invention could be used in a shock sub - a device put at the end of the drill string between the drill string and the drilling bit used to dampen the shocks to which the bit is subject when drilling hard or irregular formations. This would be in the environment of conventional rotary drilling as well as the invention's primary environment of highly deviated or deviated drilling using downhole motor driven bits. For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a side elevational view through a section of the earth showing the kind of hole the invention is particularly well adapted to drill; Fig. 2 is a side elevational view of a drill string member which was built during the development of the present invention; Fig. 3 is a view similar to Fig. 2 showing one embodiment of a drill string member in accordance with the present invention; Fig. 4 is a cross-sectional view taken on line 4-4 of Fig. 3; Fig. 5 is an enlarged sectional view of the juncture between the main body portion of the drill string member of Fig. 3 and one of its tool joint ends showing the manner of feathering of the spiral grooves into the tool joint end; Fig. 6 is a cross-sectional view taken on line 6-6 of Fig. 5; Fig. 7 is an imaginary cross-sectional view generally similar to Fig. 4 and illustrating the depth of cut of various spiral grooves; and Figs. 8-11 are curves useful in understanding the engineering concepts and considerations utilised in drill string members in accordance with the invention. Referring now to Fig. 1, there is shown a well bore or hole 10 having a bend 12 therein at a right angle, and defined by a radius R′. Fig. 1 shows the most severe case of a deviated well, that is, where the well is made up of a vertical section having a length X′ and a horizontal section of the order of 500 feet (150m) or more. Of course, the invention could be used to drill wells wherein the bend 12 is more than 90 degrees, for example, 120 degrees. Using present technology the radius R′ is usually of the order of 500 feet (150m). It is anticipated that drill string members in accordance with the present invention will be able to make such bends on radii in the range of 50-100 feet (15-30m), and it is anticipated that with further development of the invention even sharper bends on radii of the order of 25-50 feet (7.5-15m) will be possible. For bends in the range of 100-500 feet (30-150m), present technology is in a "grey" area. Beyond 500 feet, present technology will be more economical to use than that of the present invention. Thus, the logical range and area of operation for the present invention is all radii less than 500 feet (150m) and radii as short as 25 feet (7.5m) are anticipated. Referring now to Figs. 2 and 3, there are shown two drill string members. The drill string member of Fig. 2 is generally identified by reference numeral 14 A whilst the drill string member of Fig. 3 is generally identified by reference numeral 14. Both drill string members 14 and 14 A were approximately 4.5m (15 feet) in length, and were turned down from identical drill collars. Each was provided with a box end tool joint 16 and 16 A and with a pin end tool joint 18 and 18 A . These tool joint ends 16 and 18 were conventional with the exception that they were shorter than ordinary new tool joints. The joints 16 and 18 will mate with similar tool joints on all other conventional drill string members. The version of Fig. 2 had a series of relatively short spiralled sections 20 separated by turned down, thinner sections 22. The sections 22 were turned down to the maximum thinness of the pipe which it was anticipated would still be usable. The invention version 14 of Fig. 3 was spiralled over its entire length. The drill string member 14 of Figs. 3 and 4 was provided with three spiral grooves 24 the ends of which were feathered into the tool joint ends 16 and 18 via transition zones 26 as more clearly shown in Figs. 5 and 6. The embodiment of Fig. 2 had an overall weight less than that of the drill string member 14 of Fig. 3. Nevertheless, despite this advantage of reduced weight, the drill string member 14, spiralled over its entire length, was found after testing to be more flexible than the embodiment of Fig. 2. It is not quite understood how or why that should have occurred. It is thought that perhaps with the alternating thicker and thinner sections 20 and 22 the plurality of transition zones which resulted had a detrimental effect on flexibility. Referring now to Fig. 4, it can be seen that the drill string member 14 is defined by an inside diameter 28 and an outside diameter 30. The raised areas surrounding the three spiral grooves 24 are called lands, and these are the parts of the drill string member 14 which contact the wall of bore hole 10 in use. Fig. 4 also shows the surface 32 defined by the bottoms of the spiral grooves 24. As discussed above, during deviated drilling the drill string member is subject to enormous stresses made up of combinations of torsion, compression, bending, tension, and bursting pressure from the pressure of the drilling fluid carried inside the drill string. Combination cyclic stresses have been found to be highest at the bottom of the groove, and since the thinnest part of the drill string member is at the bottom of the spiral grooves 24, it is anticipated that this is the zone which is most critical. In order to prolong the useful like of the drill string member, an optional feature is the application of a compressive prestress at the bottom of the groove in the surfaces 32. The surfaces 32, but not necessarily the lands may be treated by shot peening or other equivalent means such as heat treatment. When the prestress is applied to the grooves it is possible the lands will also become at least partially prestressed. Such a prestress in the lands, it is thought, will neither hurt nor help although so applying the prestress to the drill string member in both the lands and the grooves may facilitate the prestressing operation. Referring again to Figs. 5 and 6, it is most desirable to provide as smooth a transition as possible of the ends of the spiralled grooves 24 "feathering" into the tool joints 16 and 18. The purpose of this is to provide a uniform area for the change of stress from the thicker tool joint portion to the thinner main body portion 100. It has been found that this smooth transition through the transition portions 26 results in improved flexibility and overall performance for the drill string member. It is believed that three spiral grooves 24 are optimal. As will be clear from the discussion of Figs. 8-11 below, the number of flights of the spiral groove has an effect on performance. If only one groove were provided, it would have to have such a small pitch that the desirable stress relationships would be thrown off. Two spirals would produce irregular cross-sections or cross-sections with excessively large lands. Four spirals would produce a square cross-section, and would require cutting down land width to get a sufficiently thin wall at the bottom of the spirals. For reasons discussed immediately below, a square cross-section is not desirable. It is necessary to have a certain minimum dimension across the lands for proper drilling. Beyond four spirals, five or six or even more might be workable, and especially multiples of three spirals might be workable, although this might produce problems of insufficient land width. Therefore, for all of these reasons, three spirals presently appears to be the optimum for the drill string member 14. Fig. 7 shows another aspect of the drill string member, namely the depth of the spiral grooves 24. Basically, following the engineering concept of the invention to design the drill string member 14 to perform at the maximum limits of its properties. the spiral groove 24 is to be as deep as possible while at the same time producing a drill string member having an acceptable minimum number of endurance cycles. This depth will be in the range of about 55% to about 85% of the wall thickness of the body measured from the land O.D. to the member's I.D. In connection with Fig. 7, the average spiral depth as a percentage of wall thickness is somewhat as illustrated and is: The 0% figure associated with drill pipe is provided because it is unusual to spiral drill pipe since it is already a relatively thin walled member. Drill pipe is used as the example in order to more dramatically shown the different depths of grooving of the different members in Fig. 7. Considerable development work and testing preceded bringing the embodiment of Fig. 3 to its present status. Table I below correlates a great deal of this test data and other data extrapolated therefrom. Columns 1 and 2 are variable parameters selected by the user. Column 3 shows actual measured values based on tests performed in development. Columns 4, 5 and 6 are calculated based on the first three columns. Particular attention is invited to the fifth column, "endurance cycles", as this is the measure of life with which those skilled in the art are most familiar. The sixth column, "number of wells", has been calculated as a convenience and correlates directly numerically to "endurance cycles", the endurance cycle data simply being divided by a constant to product "number of wells". It is assumed in Table 1 that a well is being drilled in the configuration of Fig. 1, that is from vertical to horizontal, around the particular radius as stated, out some relatively short distance after the bend, and then drilling is stopped. As a result of this, the total feet of well drilled will be longer for the longer radii than for the shorter radii. However, this is deemed to be insignificant because the stress produced when rotating in the bend and not when drilling in a straight direction is deemed the destructive factor for the drill string member. The total length drilled has an effect on the bit itself, but that is not a primary consideration here. The second column, "rate of penetration", accommodates different formations, different bit weights, and other factors well known to those skilled in the art. In summary, on occasions one drills faster and another time one drills relatively slower, and this column accommodates that reality. As shown in Table 1, all of the data for the entire Table is based on 40 rpm which is the slowest speed which conventional surface equipment can rotate the drill string. The speed of drilling is a function of both the actual feet of penetration per hour as well as of rpm. Likewise, the number of cycles of stress is a function of both rpm as well as rate of penetration. For purposes of the invention, a high speed is desirable. The faster the well is drilled, the faster the bit rotates and the faster each drill string member according to the invention progresses through the bend, and thus the fewer cycles of stress on each such drill string member. The reduced stress increases the productive life of the drill string members. The rpm of the bit, in downhole motor driven drilling, is different from the rpm of the drill string. For purposes of the invention, it is only the rotational cycles of the drill string in the bend which is important. This is also true in directional applications where downhole motors are not used. Drill string members built in accordance with the invention will be more expensive than comparable drill pipe. However, they do not need to be used during the vertical run of the well but only around the bend and in the horizontal run. Thus, in the example of Fig. 1, the hole could be any depth below the surface, but the length of drill string members in accordance with the invention will be equal to approximately 500 feet (152m) plus the distance required to go through the bend. Fig. 8 is a set of two curves based in part on the 80 foot (24.4m) radius test data. The curve defined by the data indicated by circles shows the stress measured in the grooves of the test sample, and the companion curve indicated by stars indicates the measured stress on the lands. The X-axis indicates different pitches that are used for the test samples. The double star indicates actual tested values; all other values are extrapolated. It is desirable to select manufacture points to the right of the crossover point of the two curves shown in Fig. 8 since the lands between the grooves wear in use. All of the tests were made on ordinary grades of steel as are used in oil field tubular goods. Because of the requirements of MWD techniques, non-magnetic stainless steel, copper, Monel, and other materials could also be used. It is currently believed that the difference between the maximum stress in the land and the maximum stress in the groove should be equal to or less than 43000 psi (about 296.5 N/mm²). By way of example, referring to Fig. 8, this would include all portions of the curve from the left extremity of the drawing to a point defined between the two pitches of 15 and 13 inches (5.9 and 5.1 cm). Table 2 illustrated the acceptable minimum size at the thinnest portion at the bottom of the spiral grooves for three different sizes of drill string member in accordance with the invention. Figure 9 is representative of the relatively large family of curves that could be drawn in a similar fashion from the data in Table 1. Fig. 9 shows that the useful life increases in a logarithmic manner as the penetration rate goes up. The same effect occurs in regard to different radii, for example considering the relatively high speed 20 feet per hour (6 m/hr) penetration rate at 70 foot (21m) radius, only about 8 standard wells would be expected. However, if the radius were increased to 90 feet (27m) a more than 10 fold increase in life would be achieved since the number of wells would then be equal to approximately 100. The drill string member is unique from the prior art in many ways as discussed above. Among these is the fact that it has a weight per unit length different from any other type of oil field tubular goods in use. This point is illustrated in the following Table. The following Table illustrated the advantages of the invention from a commercial acceptability point of view based on various performance characteristics. Of course, the first one "flexibility" is the important one as far as the drilling of deviated holes on short radii, as discussed above. Figure 10 is a graph showing the expected life cycles for different tubulars at different bend radii. Note the lines marked "useful life" for the drill string member in accordance with the invention and for the prior art members, and the difference in the acceptable life cycles. The presentation of Fig. 10 is on logarithmic scales in both the X- and Y-directions. Fig. 11 is a graph showing the % of yield stress as a function of bend radius for different tubulars. The value "100" on the Y-axis corresponding to failure.

Claims: A drill string member for use in drilling deviated holes and to be interposed in a drill string, said drill string member having tool joint means (16,18) at each end thereof and a main body portion (100) therebetween, said main body portion (100) of said drill string member being formed with at least one spiral groove, said spiral groove defining lands on said main body portion (100) between the flights thereof, characterised in that said spiral groove extends over the whole length of said main body portion (100) continuously between said tool joint means, and the depth of said spiral groove is in the range of from about 55% to about 85% of said wall thickness of said body measured from the outside diameter of said land to the inside diameter of said member. A drill string member as claimed in Claim 1, wherein said grooves and/or lands are provided with a compressive prestress. A drill string member as claimed in Claim 2, wherein said compressive prestress is provided by shot peening or carburization. A drill string member as claimed in Claim 1, 2 or 3, including a number of spiral grooves which is a multiple of three. A drill string including a drill pipe and a drill string member according to any preceding Claim, said drill string having one or more of the following features:- (a) said drill string member has a weight per unit length only slightly more than that of said drill pipe; (b) said tool joint means (16,18) each has an axial length substantially less than that of the tool joint means on said drill pipe; and (c) the difference in the maximum stress in said lands and in said groove of said drill string member is equal to or less than 43000 psi (296 N/mm²) at a bending radius of 80 feet (24.4m). A method of drilling deviated holes using a drill string member as claimed in any of Claims 1 to 4, interposed in a drill string between a motor driven bit at the bit end of said drill string and drill pipe at the upper end of said drill string.

Inventors: BEATTY GLENN ERIC, HILL GUY JAMES, SCHNITKER MARK WILLIAM, SMITH HOMER GRAFTON JR, BEATTY, GLENN ERIC, HILL, GUY JAMES, SCHNITKER, MARK WILLIAM, SMITH, HOMER GRAFTON, JR.
Applicants: WEATHERFORD US INC, WEATHERFORD U.S. INC.
Application year: 1988
Main classification: E21B17
IPC codes: E21B10, E21B17, E21B7