Steerable underreaming bottom hole assembly and method

[0002] The present invention relates to steerable bottom hole assemblies when used for underreaming or enlarging a section of a wellbore.

[0003] Steerable bottom hole assemblies include a positive displacement motor (PDM) or “mud motor” which has a bend angle that allows a drilling operator to build at a desired rate when “sliding” the non-rotating motor housing. The bend angle of the bent sub or bent motor housing, in combination with the contact points of the bottom hole assembly (BHA) with the well bore wall, thus largely determines the build rate for drilling the curved borehole section. When the drill string and the motor housing are rotated, the drilling operator drills a straight or tangent section of the borehole. The rate of penetration (ROP) of the bit drilling through the formation is frequently significantly less when “sliding” to build rather than rotating the bottom hole assembly from the surface to drill the straight borehole sections. Since ROP translates to time and money, drilling operators prefer to maximize the time when the BHA is rotated for drilling straight, and minimize the time of building. For various reasons, high build rates are generally preferred, but the bend angle is preferably low to minimize problems when the BHA is rotated. A steerable bottom hole assembly is disclosed in U.S. Pat. No. 6,269,892. This patent discloses the benefits of a steerable bottom hole assembly with a long gauge bit, i.e., a bit having a gauge section with an axial length of at least 75% of the bit diameter.

[0004] Underreaming tools within the bottom hole assembly are used to enlarge a section of the well bore below a restriction. Advantages of underreaming are disclosed in an article “Simultaneous Drilling and Reaming Saves Rig Time”, Oil& Gas Journal,Nov. 27, 1989. Conventional underreaming tools use three or more cutting arms that are moved outward in response to fluid pressure within the tool. A reamer designed for hole opening while drilling is the NBR reamer offered by Security DBS, a Halliburton Company.

[0005] When reamers are used in a conventional steerable bottom hole assembly immediately above the bit and below the motor housing, the bottom hole assembly has very poor steerability. More particularly, it is difficult to accurately predict the build rate when sliding, regardless of the positioning of stabilizers above the reamer. Because of high vibration, the azimuth of the curved borehole formed while sliding may be inaccurate. Moreover, borehole quality is generally poor due to high vibration, which prohibits the motor housing from “holding steady” in the well.

[0006] Because of the disadvantages of a steerable bottom hole assembly and reamer combination discussed above, operators have used bi-center bits rather than reamers for performing the underreaming operation. Bi-center bits are discussed in “Application of Bi-Center Bits in Well-Deepening Operations, IADC/SPE 19921, page 131. While bi-center bits may be a better solution than a reamer in some applications, significant problems remain when using a bi-center bit. Vibration due to the bi-center bit is significant, and the bottom hole assembly is also difficult to steer. Vibrations are particularly high when drilling relatively hard formations. High vibration not only increases the likelihood of failure in the bottom hole assembly, but is generally indicative of poor borehole quality. High vibration typically results in excessive bit wear. Drilling operators are constantly seeking techniques which will result in better hole quality when drilling with a steerable downhole motor. The bottom hole assembly also should have good steerability and preferably be able to drill at a relatively high ROP.

[0007] The disadvantages of the prior art are overcome by the present invention, and an improved steerable bottom hole assembly and method are hereafter disclosed for underreaming a section of borehole with relatively low vibration and high borehole quality.

[0008] A steerable bottom hole assembly according to the present invention is used for drilling both a curved section and a straight section of the borehole. The bottom hole assembly includes a downhole positive displacement motor having a motor housing and a bend angle. The motor housing encloses a shaft or rotor offset at a selected bend angle from a central axis of an upper housing, which encloses the power section of the motor. The motor is powered by fluid to rotate the bit when sliding to drill the curved section of the borehole, and the bottom hole assembly including the motor is rotated from the surface to drill the straight section of the borehole.

[0009] A gauge section is provided directly above the bit and has a substantially uniform diameter cylindrical surface with an axial length of at least 75% of the bit diameter. A reamer is positioned directly above the bit and has a reamer diameter significantly greater than the bit diameter for drilling an enlarged section of the borehole compared to the restricted diameter through which the bottom hole assembly has passed.

[0010] It is an object of the present invention to provide a steerable bottom hole assembly which is useful for underreaming operations and results in relatively high borehole quality compared to prior art techniques.

[0011] It is a feature of the present invention that the bend-to-bit distance is less than 15 times the bit diameter, preferably less than the 12 times the bit diameter, thereby obtaining a relatively high build rate for a low angle bend in the motor. To provide this low bend-to-bit distance, the lower end of the motor includes a pin connection at the end of the shaft or motor for mating engagement with a box connection at the upper end of the reamer.

[0012] It is a feature of the present invention that the positive displacement motor is preferably run slick, i.e., with no stabilizers for engaging the wall of the well bore.

[0013] It is another feature of the present invention to provide a bottom hole assembly with a second reamer repositioned above the bend in the downhole motor.

[0014] In a preferred embodiment, the bend in the bottom hole assembly has a bend angle of less than 3°, and the gauge section has an axial length of at least 90% of the bit diameter.

[0015] A related feature of the present invention is to provide a method of forming a subterranean borehole utilizing a steerable bottom hole assembly as discussed above, including a reamer below the downhole motor for enlarging the borehole diameter substantially beyond the bit diameter. A gauge section is provided between the bit and the reamer to add stability to the BHA. The motor is powered with fluid to rotate the bit while the motor housing is slid to drill a curved section of the borehole, and the bottom hole assembly is rotated from the surface to build a straight section of the borehole.

[0016] A significant advantage of the present invention is that the bottom hole assembly does not require specially made components. Instead, each of the components of the bottom hole assembly may be selected by the operator as desired to achieve the objectives of the invention.

[0017] These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.

[0018] FIG. 1 is a simplified view of the bottom hole assembly according to the present invention for performing underreaming operations.

[0019] FIG. 2 is a cross-sectional view illustrating the mechanical interconnection between the lower end of the motor rotor and the box connection on the reamer.

[0020] Steerable drilling systems conventionally utilize a downhole motor (mud motor) powered by drilling fluid (mud) pumped from the surface to rotate the bit. The motor rotates the bit using a drive section, with the rotor output from the drive section extending through a bent sub or bent housing to rotate the bit. The bent sub may actually comprise more than one bend to obtain a net effect which is referred to as the “bend angle” of the bottom hole assembly. The downhole motor which utilizes a lobed rotor is referred to as a positive displacement motor (PDM).

[0021] FIG. 1 depicts a steerable bottom hole assembly (BHA) for drilling both a curved section and a straight section of the borehole. The BHA includes a PDM 12 which is conventionally suspended in the well from a tubular string 14, which is conventionally threaded drill pipe. PDM 12 includes a motor housing 16 with a substantially cylindrical outer surface and a conventional lobed rotor 18 within the power section of the motor for rotating the bit. The lower housing 22 includes a bearing package assembly 24 which conventionally comprises both thrust and radial bearings. The upper housing 16, has a central axis 27 which is offset at a selected angle from the central axis of the lower housing 28, thereby forming the bend 13. A reamer 38 is positioned below the motor 12, and is discussed further below.

[0022] The bottom hole assembly includes a rotary bit 30 having a bit end face 31 which defines a bit diameter. The bit includes a long gauge section 32 with a cylindrical outer surface. The gauge section is fixed to and may be integral with the bit. The axial length of the gauge section is at least 75% of the bit diameter, and preferably the axial length of the gauge section is at least 90% of the bit diameter. In many applications, the gauge section axial length may be from 1 to 1½ times the bit diameter. The diameter of the gauge section may be slightly undergauged, e.g., {fraction (1/32)} of an inch less than the bit diameter.

[0023] The gauge length is from the top of the gauge section to the bottom of the gauge section, and at least 50% of this gauge length has a substantially uniform diameter cylindrical surface. One or more short gaps or undergauge portions may thus be provided between the top of the gauge section and the bottom of the gauge section. The spacing from the top to the bottom of the gauge section will be the total gauge length, and that the portion which has the substantially uniform diameter cylindrical surface is at least 50% of the total gauge length.

[0024] It is a feature of the present invention to maintain a relatively short distance between the bit face and the bend 13. According to the present invention, the bend-to-bit face spacing may be less than 15 times the bit diameter, and preferably is less than 12 times the bit diameter. In order to reduce the distance between the bend and the bit face, the PDM motor is preferably is provided with a pin connection 34, as shown in FIG. 2, at the lowermost end of the shaft 36, while the reamer 38 is provided with a box connection 40 at its uppermost end. The combination of the pin down motor and the box up reamer allows for shortening the bend to bit face distance.

[0025] According to the BHA of the present invention, the first point of contact between the BHA and the well bore is the bit face 31. The second point of contact between the BHA and the well bore is along the gauge section 32. The third point of contact between the BHA and the well bore is along the reamer 38. The motor housing including the bend in the BHA as shown in FIG. 1 does not contact the well bore, so that the fourth point of contact between the BHA and the well bore is spaced above the bend, and as shown in FIG. 1 is the reamer 42. Drill string stabilizers or other reamers may be provided above the reamer 42. The bottom hole assembly may also include an MWD system 44 positioned above the motor 12 for transmitting signals to the surface of the well in real time. The reamer 38 is used to enlarge the borehole to a diameter greater than the diameter of the bit, which approximates or is greater than the diameter of the casing string above the bottom hole assembly.

[0026] The PDM is preferably run “slick”, i.e., with no stabilizers on the motor for engagement with the wall of the borehole. The PDM motor may include a slide or wear pad. The BHA as disclosed herein has surprisingly low vibration, which results in good borehole quality. The benefits of improved borehole quality include reduced hole cleaning, improved logging operations and log quality, easier casing runs, and more reliable cementing operations.

[0027] U.S. Pat. No. 6,269,892 discloses a steerable mud motor which has low vibration and results in high borehole quality when a long gauge section is provided immediately above the bit, and when the next contact point between the bottom hole assembly and the borehole is above the bend, and is typically spaced considerably above the bend in the motor. The use of a reamer between the PDM motor and the bit inherently adds to the bit-to-bend distance. By providing another tool for contact with the formation below the bend, which is also a formation cutting tool, the assumption would be that vibration would significantly increase, that the build rate would significantly suffer, that the predictability of the build rate would be reduced, and that borehole quality would deteriorate. This result has not occurred, which is surprising.

[0028] Table 1 provides predicted build/drop rates for different gauge outer diameters on the reamer 42 above the bend. The BHA has a bent angle of 1.15°, a bit inclination of 14′, and a weight on bit of 17,000 pounds. The reamer 38 below the bend has a diameter of 14 inches, and the bit diameter has 12.25 inches. The build/drop rates when sliding with the tool face on the high side, the low side, and neutral are provided for different diameters of the stabilizer or upper reamer 42. Most importantly, Table 1 indicates that the build rate when in the rotary mode is in the acceptable range when the upper reamer is ¼ inch or less undergauged from a lower reamer. According to the present invention, the diameter of the upper reamer above the bend in the BHA should be less than about ½ inch from the diameter of the lower reamer.

[0029] Initial tests have indicated that the bottom hole assembly of the present invention with a reamer above the gauge section and below the mud motor has very good build rates and predictable steerability. Although the build rates are lower than a BHA without a reamer, build rates were substantially better than prior art bottom hole assemblies used for underreaming a section of a borehole, including systems which use bi-centered bits with stabilizers on the motor housing. Most importantly, vibration is significantly reduced so that borehole quality is much better than that typically obtained when using a BHA with either a bi-centered bit, or a conventional bit and a reamer. Due to better bit stability, the sliding ROP for the bottom hole assembly is surprisingly high, and was even better than the rotating ROP of conventional BHA's used for underreaming operations. The build rate when sliding was only about 20 to 30% less than the build rate when sliding using a steerable BHA without a reamer. Accordingly, the BHA may be operated in the oriented or steerable mode for relatively long periods of time to counteract the slightly lower build rate. High vibration associated with downhole motors with a high bend angle may thus be further reduced.

[0030] For the embodiment as shown in FIG. 1, a second reamer 42 is provided substantially above the bend. The addition of this second reamer improves directional tendency and stability while drilling, particularly in hard formations, such as salt formations. When drilling in the rotary mode, this additional second reamer acts a stabilizer to reduce vibration. The build rate may be reduced by adding this additional reamer, since the elimination of this contact point with the borehole wall increases the side force build tendency and reactive forces on the components below the motor. Additional drill collar stabilizers and/or additional reamers may be provided above the reamer 42.

[0031] While preferred embodiments of the present invention have been illustrated in detail, it is apparent that modifications and adaptations of the preferred embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention as set forth in the following claims.