The feasibility of pipeline directional drilling technology is generally considered technically achievable. The core of the process involves creating a stable underground tunnel to allow the pipeline to be smoothly pulled back, or to ensure that the surrounding soil becomes fluid enough for the pipeline to follow and be dragged through. This relies on the application of suitable drilling mud to maintain stability and reduce resistance.
However, ideal fluid conditions throughout the entire crossing section are rare. Localized debris or obstacles can damage the pipeline’s anti-corrosion coating, which makes pre-drilling preparation essential. Before pulling the pipeline back, it is necessary to form a clear tunnel and inject an appropriate type of mud to keep it stable. If the tunnel remains unobstructed during the pulling process, the maximum stress on the pipeline, drill pipe, and rig equipment should remain within acceptable limits, making directional drilling feasible.
Since each crossing project is unique, there is no universal standard to determine whether directional drilling is suitable. Instead, the decision largely depends on the technical capabilities and experience of the construction team. To simplify the feasibility analysis, engineers often compare the current project with previous ones based on key parameters such as formation conditions, crossing length, and pipe diameter. These three factors significantly influence the success of the directional drilling process.
The most challenging formation types for crossing are those with high gravel content or extremely hard and strong rock layers. High-gravel formations are difficult to drill because they lack stability, leading to potential collapse after hole creation. Additionally, the gravel may not be easily removed by the drilling mud, causing blockages during the reaming and pulling stages. Similarly, very hard rock formations increase drilling and reaming time, place higher demands on drilling tools, and make it harder to control the drilling direction. On the other hand, softer, easily breakable rock, like certain types of gravel, can also introduce instability into the process.
The geological conditions along the pipeline crossing route can be highly complex, with different strata present in various sections. Even if the crossing design optimizes the path, unexpected challenges may arise depending on the continuity and integrity of the strata. Changing the sequence of different rock types can dramatically affect the drilling outcome. Moreover, our understanding of the subsurface conditions is based on core samples, which are collected intermittently. This means that actual field conditions might differ from expectations, and careful consideration of all variables is crucial before starting the project.
The length of the crossing and the diameter of the pipeline are primarily limited by the capacity of the drilling rig and the tools used. As the crossing length increases, the flexibility of the drill pipe decreases, and the ability to control the drilling direction diminishes, making it more challenging to follow the planned curve. When pulling back the finished pipeline, the hole must be expanded to 1.2 to 1.5 times the pipe's diameter. For large-diameter pipelines, this expansion requires significant torque, especially in hard strata, where the drill pipe's strength may not be sufficient. In sandy or gravelly soils, larger tunnels are harder to stabilize, increasing the risk of collapse.
These limitations mean that directional drilling for large-diameter pipelines is more complex and risky. Table 2 provides data on the longest and largest diameter crossings completed so far, which can serve as a reference when assessing the feasibility of future projects.
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