The primary benefit of a double centralizer is stabilizing the azimuth. In practice, we’ve found that in most cases, dual centralizers stabilize and slightly increase azimuth. Some drill string configurations often exhibit a strong azimuth increase, but proper use prevents azimuth loss. Therefore, dual centralizers should always be used when the azimuth is just right or slightly off. 

In other words, if the azimuth is trending too strongly and further reduction would result in a smaller azimuth, the dual centralizer should be deployed immediately to stabilize the azimuth. Delaying even one or two measurement points will miss the opportunity. Regarding stabilization, generally speaking, the closer the lower centralizer is to the drill bit, the greater its azimuth-increasing capability. Above L1 of 5 meters, the azimuth-stabilizing capability is lost. Furthermore, the closer the distance between the two centralizers, the greater their azimuth-stabilizing capability. 

Furthermore, regarding the outer diameter of the centralizers, the larger the outer diameter of the upper centralizer, the greater its azimuth-stabilizing capability. Generally speaking, the outer diameter of both centralizers should be no less than 210 mm. The specific placement of the centralizers should be determined based on the required well inclination. The following is an introduction. Please feel free to criticize and correct any errors.

1. Strong Inclination Increase Combination

This is a classic dual-centralizer inclination increase combination used by various teams, especially in Drilling Lines 2 and 3. Its structure is:

8 1/2-bit + dual female centralizers (over ¢213) + one NDC + one DC + male and female centralizers (¢210-213).

This drill string combination boasts a highly effective inclination increase, generally ensuring an inclination increase rate of over 5°/100m, with an average of 7°/100m. It is the best choice when the outer diameter of the NDC is too large, preventing wellbore inclination from increasing. The key to its use is that the outer diameter of the lower centralizer must be larger than that of the upper one; otherwise, the drill string will behave like a pendulum, reducing inclination.

Generally, there’s no need to add a connector below the lower centralizer. If you truly need to reduce the inclination increase rate, other dual-centralizer configurations can be used. Strictly speaking, due to the distance between the two centralizers, this configuration’s azimuth stabilization capability is not particularly strong. Adding a long joint below the lower centralizer would further weaken the configuration’s azimuth stabilization capability, negating the point of using two centralizers.

1. Weak Inclination Increase Combination

This combination sacrifices some of the inclination increase capability of the strong inclination increase combination in exchange for stronger azimuth stabilization capability. Its structure is: 8 1/2 Bit + dual female centralizers (over ¢213) + one NDC + one NDC + one short drill collar (2-3m) + male and female centralizers (¢210-213).

This combination is characterized by a strong inclination increase and azimuth stabilization. Even when running the combination under a strong downward trend, it can significantly curb the momentum. Its inclination increase rate is 2-4°/100m, making it an excellent combination for adjusting the inclination increase rate and forcing azimuth stabilization. The key to its use is to ensure that the outer diameters of the two centralizers are larger at the bottom and smaller at the top. Never reverse this! Furthermore, the larger L2 is, the higher the rate of increase in deflection, which must be at least 12 meters. Therefore, it’s not easy to omit the short drill collar. If you do need to increase deflection, you can adjust the rate of increase by adding a short sub below the centralizer in this combination, depending on actual needs. The longer the sub, the lower the rate of increase in deflection. The adjustment range of the sub should be within 0-1 meter. Any longer and the tool will become inertia-stabilizing or even lowering. Adding a short sub has little impact on the tool’s azimuth stability.

2. Deflection Stabilization Combination

The deflection stabilization combination further sacrifices deflection-increasing capability in exchange for greater azimuth stability. While its structural forms vary, they all share the same core principle: they are all evolutions of the deflection-increasing combination discussed above.

8 1/2 Bit + Dual Female Centralizer (¢213 and above) + One Short Drill Collar (2-5m) + Male and Female Centralizer (¢210-213). This configuration is described in the Drilling Manual (Party A) and also demonstrated in a Drilling 3 paper that it offers excellent stabilization. I believe it offers the strongest stabilization among all dual-centralizer configurations. The key to its use lies in ensuring the outer diameters of the two centralizers are larger at the bottom and smaller at the top. Similarly, connectors can be added to adjust the distance between the two centralizers to alter the rate of increase in azimuth.

 

8 1/2 Bit + 430*410 (0.4-1.5m) + male and female centralizers (¢210-213) + one NDC + male and female centralizers (¢210-213).

 

This configuration also offers excellent azimuth stabilization. The desired rate of increase in azimuth can be achieved by adjusting the length of the connectors; longer connectors result in lower rates. The outer diameters of the two centralizers can also be adjusted to favor an increase in azimuth or a decrease in azimuth. A larger lower centralizer increases azimuth, while a larger upper centralizer decreases azimuth. Therefore, the inclination rate of this configuration can be flexibly controlled within a range of ±3.5°/100m. This also indicates that the inclination rate of this configuration is highly unstable, influenced by various factors, including the outer diameter of the non-magnetic outer diameter centralizer, joint length, formation, and drilling parameters. Therefore, if experience is lacking, sufficient leeway should be left for later well inclination control when using this configuration. This configuration is particularly unsuitable for the first inclination increase section. We have used this configuration many times, and while the well inclination adjustment effect has sometimes been unsatisfactory, its azimuth stabilization has always been very satisfactory (see the attached table for details).

8 1/2 Bit + Dual Female Centralizers (¢210-212) + One NDC + Male and Female Centralizers (¢213).

This configuration, with the centralizers connected in a “smaller at the bottom, larger at the top” configuration, also achieves stable inclination and azimuth.

3. Declination Reduction Combination

Adjusting the outer diameter of the centralizers in the previous inclination stabilization configuration to a “smaller at the bottom, larger at the top” configuration can achieve a slight declination reduction effect. Generally speaking, the outer diameter of the lower centralizer should be between 210 and 211.5 mm, and the outer diameter of the upper centralizer should be at least 213 mm.

 

A double centralizer configuration for strong deflection reduction: 8 1/2-bit drill + 430*410 mm drill + a short drill collar (over 2 m) + male and female centralizers (210 to 213 mm) + an NDC drill + male and female centralizers (210 to 213 mm).

 

This deflection reduction configuration also provides good azimuth stabilization, with a deflection reduction rate of 8 to 12°/100 m. If the two centralizers are moved further upward, this also creates a strong deflection reduction configuration, but the deflection reduction rate does not improve much and, in fact, significantly weakens the azimuth stabilization effect. Therefore, its use is unnecessary.

 

II. Proper Use of Double Centralizer

To properly use double centralizers, one must first fully understand the performance of the double centralizer configuration being used and must not interpret it out of context. This year, the process team issued a briefing on the use of dual centralizers. Some teams used a “dual female + NDC + male and female” system to increase inclination, but this resulted in stable or even reduced inclination. Others experienced unstable positioning of the dual centralizers due to the centralizer’s outer diameter being too small. Therefore, I believe the following:

  1. First, ensure proper mud purification and rock-carrying properties, clear the circulation system, and ensure normal mud flow rates to avoid downhole complications.
  2. Before drilling, accurately measure the outer diameters of the drill bit and centralizer to ensure they are no larger than the original wellbore and the outer diameter of the centralizer is smaller than the drill bit’s outer diameter. Some old or even new drill bits have an outer diameter that is too small, while the outer diameter of the new centralizer or reducer is too large, which can easily lead to drilling blockage or drilling resistance during pullout. 
  3. When drilling, ensure the precise positioning of both centralizers. Technicians should personally supervise the process from the rig floor to avoid inadvertent errors, such as reversing the positions of the centralizers or using the larger and smaller centralizers interchangeably, which could result in an unintended increase in inclination but ultimately lead to regret.
  4. During drilling, adhere to the practice of reaming twice after each hole drilled to correct the wellbore further.
  5. When increasing inclination in soft formations, minimize the impact of the jet flow. Otherwise, excessive wellbore impact will increase the clearance between the lower centralizer and the wellbore, significantly reducing the inclination increase effect.

Conclusion

The “strong inclination increase and azimuth stabilization” control method facilitates azimuth control and improves average ROP. The dual centralizer configuration provides excellent azimuth stabilization while meeting wellbore inclination control requirements.

Correctly understanding and using dual centralizers is crucial to achieving the desired results.