Conventional Drilling Straight Well Drill String Combinations:

  • BIT: Drill Bit
  • DC: Drill Collar
  • SDC: Helical Drill Collar
  • LZ: Motor Drill String
  • SJ: Bi-directional Shock Absorber
  • DP: Drill Pipe
  • HWOP: Heavyweight Drill Pipe
  • STB or LF: Stabilizer (Drill String)
  • LB: Fishing Basket While Drilling
  • DJ: Jar
  1. Tower-type Drill String Combinations:
    • Φ444.5mm BIT × 0.50m + Φ229mm DC × 27.24m + Φ203mm DC × 54.94m + Φ165mm DC × 54.51m + Φ139.7mm DP
    • Φ311.1mm BIT × 0.40m + Φ229mm DC × 54.38m + Φ203mm DC × 82.23m + Φ165mm DC × 81.83m + Φ139.7mm DP
    • Ф311.1mm BIT × 0.32m + Ф244.5mm LZ × 9.50m + Ф229mm DC × 45.40m + Ф203mm DC × 73.13m + Ф165mm DC × 81.83m + Ф139.7mm DP
    • Φ311.1mm BIT × 0.30m + Φ229mm SJ × 6.62m + Ф229mm DC × 53.94m + Ф203mm DC × 81.75m + Ф165mm DC × 81.83m + Ф139.7mm DP
    • Drill bit FX1951X0.44 m Φ311.1mm + 6A10/630 × 0.61 m + 9″ drill collars × 52.17m (6 pieces) + 6A11/5A10 × 0.47 m + 8″ drill collars × 133.19m (9 pieces) + 410/5A11 × 0.49 m + 61/2″ drill collars × 79.88m (9 pieces) + 51/2″ HWOP × 141.88m (15 pieces) + 51/2″ drill pipe + top drive
    • Φ215.9mm BIT × 0.25m + 430/4A10 + Ф165mm SDC × 161.56m + 4A11/410 + Ф165mm DJ × 8.81m + 411/4A10 + 61/2″ drill collars × 79.88m (9 pieces) + 51/2″ HWOP × 141.88m (15 pieces) + 51/2″ drill pipe + top drive
  2. Pendulum-type Drill String Combinations:
    • Φ660.4mm P2 × 0.50m + 730/NC61 female + Φ229mm SJ × 9.24m + Φ229mm SDC × 18.24m + 730/NC61 male + 26″ LF + 731/NC61 female + Φ229mm SDC × 9.24m + 730/NC61 male + 26″ LF + 731/NC56 female + Φ203mm DC × 94.94m + 410/NC56 male + Φ139.7mm DP + top drive
    • Φ444.5mm GA114 × 0.50m + 730/NC61 female + Φ229mm SJ × 9.24m + Φ229mm SDC × 18.24m + 171/2″ LF + Φ229mm SDC × 9.24m + 171/2″ LF + NC61 male/NC56 female + Φ203mm DC × 121.94m + 8″ shock absorber + 8″ DC × 18.94m + 410/NC56 male + Φ127mm HWOP × 141.94m + Φ139.7mm DP + top drive
    • Φ311.1mm BIT × 0.46m + Φ229mm DC × 18.08m + Φ308mm LF × 1.82m + Φ203mm DC × 9.10m + Φ308mm LF × 1.51m + Φ229mm DC × 27.32m + 203mm DC × 73.13m + Φ178mm DC × 81.83m + Φ139.7mm DP + top drive
    • Φ311.1mm DB535Z × 0.50m + 630/NC61 female + Φ229mm SJ × 9.24m + Φ229mm SDC × 18.24m + NC61 male/NC56 female + 121/4″ LF + NC56 male/NC61 female + Φ229mm SDC × 9.24m + NC61 male/NC56 female + 121/4″ LF + Φ203mm DC × 121.94m + 8″ shock absorber + 8″ SDC × 27.94m + 410/NC56 male + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive
    • Φ311.1mm DB535FG2 × 0.50m + 630/731 + 95/8″ LZ + Φ229mm SJ × 18.64m + 121/4″ LF + Φ229mm SDC × 9.24m + 121/4″ LF + Φ203mm DC × 148.94m + 410/NC56 male + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive
    • Φ215.9mm BIT × 0.33m + Φ172mm LZ × 8.55m + Φ165mm SDC × 1.39m + Φ165mm SDC × 1.39m + Φ214mm STB × 1.38m + Φ165mm DC × 236.14m + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive
  3. Full-bore Drill String Combinations:
    • Φ311.1mm H136 × 0.30m + 121/4″ LF + NC56 male/NC61 female + Φ229mm SJ × 9.24m + NC61 male/NC56 female + 121/4″ LF + NC56 male/NC61 female + Φ229mm SDC × 18.24m + NC61 male/NC56 female + 121/4″ LF + Φ203mm DC × 121.94m + 8″ shock absorber + 8″ SDC × 18.94m + 410/NC56 male + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive
    • Φ215.9mm roller cone bit BIT × 0.24m + Φ190mm LB × 1.10m + Φ214mm STB × 1.39m + Ф165mm SDC × 1.39m + Φ214mm STB × 1.40m + Ф165mm DC × 8.53m + Φ214mm STB × 1.39m + Φ165mm SJ × 5.08m + Ф165mm DC × 244.63m + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive
    • Φ215.9mm roller cone bit BIT × 0.24m + Φ214mm LF × 1.49m + Ф165mm SDC × 1.39m + Φ214mm LF × 1.40m + Ф165mm DC × 8.53m + Φ214mm LF × 1.39m + Φ165mm SJ × 5.08m + Ф165mm DC × 244.63m + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive
    • Φ215.9mm roller cone bit BIT × 0.25m + Φ214mm STB × 1.50m + Ф165mm SDC × 1.38m + Φ214mm STB × 1.40m + Ф165mm DC × 8.81m + Φ214mm STB × 1.40m + Ф165mm SJ × 6.11m + Ф165mm DC × 229.22m + Φ139.7mm HWOP × 141.94m + Φ139.7mm DP + top drive

Drill String Combinations for Directional and Horizontal Wells:

  1. Vertical Section Drill String Combinations: Use tower-type, pendulum-type, and full-bore drill string combinations.
  2. Directional Inclination Building Drill String Combinations:
    • 1 drill bit + motor drill string + bent sub + non-magnetic drill collar 1-2 pieces + drill collar depending on actual situation + Φ139.7mm HWOP + Φ139.7mm DP + bypass valve + Φ127mm DP + top drive when using wired MWD.
    • 2 drill bit + bent housing motor drill string + directional sub + non-magnetic drill collar 1-2 pieces + drill collar depending on actual situation + Φ139.7mm HWOP + Φ139.7mm DP + bypass valve + Φ127mm DP + top drive when using wired MWD.
  3. Inclination Increasing Drill String Combinations:
    • 1 Strong inclination increase drill string combination: drill bit + near-bit stabilizer + small diameter drill collars 2-3 pieces + stabilizer + drill collars 12-15 pieces + Φ139.7mm HWOP + Φ139.7mm DP + top drive.
    • 2 Regular inclination increase drill string combination: drill bit + near-bit stabilizer + drill collars 2-3 pieces + stabilizer + drill collars 12-15 pieces + Φ139.7mm HWOP + Φ139.7mm DP + top drive.
    • 3 Weak inclination increase drill string combination: drill bit + stabilizer + drill collar + stabilizer + drill collar + drill collars 12-15 pieces + Φ139.7mm HWOP + Φ139.7mm DP + top drive.
  4. Inclination Stabilizing Drill String Combinations: Conventional full-bore drill string combinations or weak inclination increase drill string combinations.
  5. Inclination Decreasing Drill String Combinations: Conventional pendulum-type drill string combinations; drill bit can be connected with straight motors at 0.75-degree single bend.

Special Construction Drill String Combinations:

  1. Milling Drill String Combinations: Reaming shell + fishing basket while drilling + drill collars 12-15 pieces + Φ139.7mm HWOP + Φ139.7mm DP + top drive.
  2. Casing Milling Drill String Combinations: Milling shoe + milling housing + safety joint + drill collars 12-15 pieces + Φ139.7mm HWOP + Φ139.7mm DP + top drive.
  3. Fishing Drill String Combinations: Fishing tool + safety joint + jars (lower jar, upper jar) can also add super jars + drill collars 3-6 pieces + accelerator + drill collars 3-6 pieces + Φ139.7mm HWOP + Φ139.7mm DP + top drive.
  4. Core Recovery Drill String Combinations: Φ215.9mm coring bit × 0.29m + Φ180mm CB × 11.09m coring barrel + Φ165mm DC × 235.86m + Φ139.7mm HWOP + Φ139.7mm DP + top drive.

Commonly Used Drill String Combinations for Directional Wells:

  • Bent sub with powered drill string — Inclination Building Drill String Currently, the most commonly used inclination building drill string combination is one that uses a bent sub and a downhole powered drill string to perform directional inclination building or azimuth adjustment operations. This type of inclination building drill string uses the elastic torque generated by the bent sub to force the downhole powered drill string (motor or turbine-driven bit) to cut laterally, causing the new wellbore to deviate from the original wellbore axis, achieving the purpose of directional inclination building or azimuth adjustment.

    The inclination building capability of an inclination building drill string is related to the bend angle of the bent sub and the rigidity of the drill collars above the bent sub. The larger the bend angle of the bent sub and the stronger the rigidity of the drill collars above it, the greater the inclination building capability of the inclination building drill string and the higher the inclination rate.

    The bend angle of the bent sub should be selected based on the size of the borehole, the specifications of the downhole powered drill string, and the required inclination rate. Common angles used in the field are between 1° to 2.5°, generally not exceeding 3°. The inclination rate under different conditions of the bent sub is shown in Table 10-4.

    The model of the downhole powered drill string used in an inclination building drill string combination should be selected based on the depth of the inclined section or the azimuth adjustment section. For sections within 1000m, a turbine or motor drill string is usually used. For deep directional inclination building or azimuth adjustments, high-temperature resistant downhole motors should be used.

    The drilling parameters design and nozzle design for the drill bit should follow the manufacturer’s recommended drilling parameters.

    Due to the high rotational speed of the downhole powered drill string, requiring low drilling pressure (generally 3-8t), the use of sealed bearing drill bits is not recommended, especially in shallow formations with good drillability where milling bearing drill bits or suitable PDC bits should be used.

Hidden Content in Post: 2. Inclination Increasing Drill String Inclination increasing drill string combinations typically use a double stabilizer drill string configuration. The inclination increasing drill string is designed based on the principle of leverage. It has a near-bit full-sized stabilizer as the pivot point. The distance between the second stabilizer and the near-bit stabilizer should be determined according to the rigidity and dimensions of the drill collars between the two stabilizers and the required inclination rate, typically around 20-30m. Under the action of drilling pressure, the drill collars between the two stabilizers produce a downward bending deformation, generating a lateral force on the drill bit, which increases the wellbore inclination angle as the well deepens.

The drilling parameters for inclination increasing drill string combinations should be designed based on the specifications of the bottom hole assembly, the distance between the two stabilizers, and the required inclination rate. A weak inclination increasing drill string combination has similar force conditions underground as the inclination increasing drill string but reduces the inclination capability of the drill string by either decreasing the distance between the near-bit stabilizer and the second stabilizer or reducing the outer diameter of the near-bit stabilizer (worn stabilizer). Weak inclination increasing drill strings are used for drilling segments requiring low inclination rates such as catenary profiles or parabolic profiles. They can also be used when the inclination stabilization drill string fails to achieve the desired inclination stabilization effect due to surface factors, resulting in a trend towards inclination reduction. Using an appropriate weak inclination increasing drill string can yield the desired inclination stabilization effect.

  1. Inclination Stabilizing Drill String An inclination stabilizing drill string combination uses a rigid full-bore configuration to increase the rigidity of the bottom hole assembly, controlling the bending deformation of the lower part of the drill string under drilling pressure, achieving the effect of stabilizing the wellbore inclination and azimuth. Commonly used inclination stabilizing drill string combinations include:
    • Drill bit + near-bit stabilizer + short drill collars (2-3m) + stabilizer + single drill collar (9-10m) + stabilizer + drill pipe.

    For formations where the azimuth drift is severe due to formation factors, two stabilizers can be connected in series above the drill bit, which can provide better results for stabilizing both the azimuth and the inclination.

  2. Inclination Decreasing Drill String Inclination decreasing drill strings generally use pendulum-type drill string configurations, utilizing the pendulum force generated by the weight of the drill string itself to achieve the purpose of decreasing the inclination. Depending on the inclination decrease rate required by the design profile and the size of the inclination angle, designing the distance between the drill bit and the stabilizer can change the size of the pendulum force acting on the lower part of the drill string.

    The design of drilling parameters for the inclination decreasing section should limit the drilling pressure according to the borehole size to ensure the effectiveness of the inclination decrease, making the inclination rate meet the profile requirements.


Between the drill bit and drill collars — double female connections, connecting the drill bit and drill collars. For connecting drill collars and drill pipes of different sizes — transition joints, connecting drill tools with different thread forms. Between the drill pipe and square drill pipe are two connections, one is a transition joint, and the other is a protection joint because they need to be unscrewed and screwed frequently and replaced after a period of use.