1. Formation Factors
Formation factors include the dip angle and direction of the strata, formation drillability, anisotropy of the formation, and formation stress. When the wellbore trajectory aligns with the direction of the strata, the orientation during rotary drilling tends to be stable. However, the heterogeneity and stress distribution of the formation can significantly affect the performance of the bottomhole assembly (BHA). Therefore, a comprehensive understanding of the formation is essential for ensuring drilling quality and efficiency.
2. Factors Affecting the Bottomhole Assembly Performance
2.1 Influence of Drill Bit Type, Structure, and Rock-Breaking Efficiency
The type and structure of the drill bit have a relatively minor impact on the bottomhole assembly, but in practical applications, such as in hard formations with low rock-breaking efficiency, it becomes difficult to build inclination while reducing inclination is easier. For instance, roller cone bits generally offer stable orientation, whereas PDC (Polycrystalline Diamond Compact) bits may cause a left drift tendency. In complex formations, the drill bit tends to drill more efficiently along the direction where the formation is more easily broken due to the effect of formation stress. Therefore, selecting the appropriate drill bit based on specific formation conditions is essential for enhancing the overall drilling performance.
2.2 Influence of Drill String Weight and Rigidity
A decrease in the rigidity of the drill string has a relatively small effect on the inclination force of the drill bit but can increase the directional force. Particularly during de-surfacing operations, if no drill collar is added to the second centralizer, increasing the weight on the bit will shift the point of force application to the weighted drill pipe above the second centralizer, which may induce an unintended increase in inclination. Therefore, when designing the bottomhole assembly, both the rigidity of the drill string and the applied weight need to be carefully considered in relation to the inclination force.
2.3 Influence of Drilling Parameters
Drilling parameters, particularly weight on bit (WOB), are the key factors that influence the performance of the bottomhole assembly. Once the bottomhole assembly structure is determined, controlling the drilling parameters, particularly the WOB, can adjust the inclination effect within a certain range. In contrast, the impact of rotational speed (RPM) is relatively small; however, excessive RPM can lead to instability in the movement of the BHA, which can reduce the drill bit’s inclination and orientation forces and even result in early fatigue failure of the downhole tools. Therefore, it is crucial to maintain optimal control over both RPM and WOB to ensure stable and efficient drilling performance.
2.4 Impact of Wellbore Geometry on Bottomhole Assembly Performance
The geometry of the wellbore, including the wellbore’s inclination and azimuth change rates, plays a significant role in the bottomhole assembly’s performance. Generally, the drill bit has an inertial tendency to maintain the current trend of the wellbore. For stabilizing assemblies, as the wellbore inclination increases, the inclination force exerted on the drill bit decreases. When the wellbore inclination is large, the directional force of the drill bit also decreases, and the lateral vibration of the bottomhole assembly’s various cross-sections is reduced, which makes the orientation more stable.
The effect of build-up angle on the performance of directional drilling tools is reflected in the following three aspects:
- As the wellbore inclination increases, the build-up force increases while the directional force decreases. This is one of the main reasons that the orientation becomes more stable when the inclination angle is large, particularly in medium-hard or hard formations. The side forces on the centralizer also increase with the wellbore inclination, which, in soft formations, can lead to the centralizer scraping the wellbore wall and reducing the ability of the drill bit to build inclination, or even preventing inclination.
- The wellbore inclination angle influences the motion state of the entire bottomhole assembly. As the inclination angle increases, the lateral vibration of the various cross-sections of the bottomhole assembly decreases, leading to improved stability and a tendency for the assembly to bend and vibrate in the vertical plane.
- With an increase in the wellbore inclination, the variation amplitude of both the inclination force and directional force decreases, while the frequency of the changes remains relatively constant. As a result, at higher wellbore inclinations, adding weight on the bit and using heavier drill pipe will not lead to premature fatigue failure of the BHA.
Additionally, the angle between the wellbore trajectory and the formation can impact the rock-breaking efficiency of the drill bit. When the angle between the wellbore trajectory and the formation is less than 30°, the horizontal component of the drill bit’s inclination force increases, making drilling easier. Thus, the design of the wellbore trajectory should take into account the formation characteristics to optimize the drilling process.
3. Wellbore Enlargement and Formation Characteristics
The expansion of the wellbore can significantly influence the wellbore trajectory. During directional drilling in soft formations, using high-quality drilling fluids and designing suitable hydraulic parameters can prevent excessive wellbore enlargement. Wellbore enlargement can affect the trajectory’s stability, and this characteristic can be utilized appropriately during construction. If the build-up rate is insufficient, the wellbore may fail to reach the target angle after completing a single section; conversely, if the build-up rate is too high, it can lead to excessive enlargement of the wellbore, impacting construction quality. Therefore, controlling the build-up rate is essential to maintaining the stability and accuracy of the wellbore trajectory.
Conclusion
From the above analysis, it is evident that a variety of factors—including formation conditions, the choice of bottomhole assembly, drilling parameters, and the geometry of the wellbore—affect the performance of the centralizer and the bottomhole assembly. To ensure wellbore stability and drilling efficiency, it is crucial to comprehensively consider and optimize all these factors during the drilling process.
Post time: Mar-07-2025
