Small rotary drilling rigs often experience slippage when drilling into mudstone or sandstone, meaning they encounter no resistance and cannot apply pressure, making drilling impossible or difficult, thus affecting construction progress and production efficiency.#drill rig model#
I. Overview of Mudstone
Sedimentary rocks such as mudstone, sandy mudstone, argillaceous sandstone, sandstone, and shale are red, dark red, or brown due to their rich oxide content; these rocks are collectively called red sandstone. Red sandstone mainly exhibits two typical structural forms: granular clastic texture and muddy cemented texture. Most red sandstone can disintegrate and break down under atmospheric influence, even becoming muddy; therefore, the size and grain size of its rock blocks will change with the time of wet-dry cycles. Mudstone is a sedimentary rock formed from solidified mud and clay. Its composition and structure are similar to shale but it is less easily broken down. It is a clay rock with indistinct bedding or foliation. Mudstone has water absorption and binding properties.#drill rig model#
II. Causes of Slippage
During construction, when drilling dry holes into mudstone, using a spiral drill bit is more efficient. A double-bottom sand-scooping bucket can also be used, but with a higher load. Slippage occurs immediately upon encountering water in the hole. Therefore, slippage is directly related to water or mud. The main causes of mudstone slippage are: 1. Hardness of the mudstone; 2. Lubricating effect of water or mud; 3. Softening of the mudstone by water absorption.
III. Drilling Analysis with Different Drilling Tools
1. When drilling water-bearing mudstone holes with a spiral drill bit, the mudstone softens upon contact with water, filling the cone tip of the spiral drill bit with mudstone debris, forming a solid mass. The drill bit then loses its drilling ability and cannot continue drilling. Water or mud then enters between the drill bit and the mudstone for lubrication, resulting in slippage.
2. When drilling into mudstone water holes using a double-bottom cutting bit drill bit, the cutting method relies on the alloy tip at the front end. The alloy tip easily penetrates the mudstone, but because it's short, the cutting bit body, which holds the tip in place, must follow suit. However, the blunt cutting bit body creates resistance. Since the cutting bit penetrates too shallowly into the mudstone, which is cemented, the desired fracturing effect is not achieved, preventing continuous drilling. Water or mud enters between the cutting bit and the mudstone, providing lubrication and causing slippage.
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Based on the above analysis, mudstone's good cementation makes it difficult to break. Sharp cutting tools are needed to achieve a deeper cut with low resistance to achieve the desired fracturing effect.
IV. Drilling Through Mudstone#drill rig model#
Therefore, a double-bottom sand-scooping bucket is chosen, and the cutting-type drilling method aligns with the above analysis. However, slippage can still occur with the actual sand-scooping bucket, but this is influenced by numerous factors.
1. Defects in the drill bit, such as the central positioning tip being too long, wide, or blunt; the angle of the tooth tips and the gap between the tooth seats; the diameter of the side teeth; the type of bucket teeth; or tooth tip wear. Improvements can be made to reduce the contact area by varying the length of the tooth tips.
2. The diameter of the sand-collecting bucket. Larger diameter drill bits have a larger contact area, making it more difficult to overcome slippage. Therefore, for larger pile diameters, smaller diameter drill bits should be used initially.
3. Overcoming slippage: As mentioned earlier, the main cause of slippage is lubrication by water or mud. Therefore, overcoming the softened mudstone and the surface water or mud will overcome mudstone slippage! After overcoming this, continuous drilling is necessary. Water or mud will then be unable to enter the cutting surface, ensuring the tooth tips remain in contact with dry geological conditions, allowing for normal drilling.
4. Operational control is crucial. Due to the hardness and good cementation of mudstone, it is not easy to cut into it even with water lubrication. First, raise the power head high, then control the power head to rotate slowly, with slow and continuous pressurization. The power head and pressurization speeds must be well coordinated. If the pressurization stroke is completed without cutting into the mudstone, raise the power head again and continue drilling until the geological load increases. After that, continue drilling. When the load becomes too heavy, raise the power head to reduce the load and protect the drilling rig components.#drill rig model#
