How Do Small and Medium-Sized Rotary Drilling Rigs Enter the Rock?

The rock entry of rotary drilling rigs has always been a major problem that has plagued our rotary drilling rig construction, especially the rock entry of small and medium-sized rotary drilling rigs.

1. Technical difficulties of small and medium-sized rotary drilling rigs entering the rock

There have always been several technical difficulties in the entry of small and medium-sized rotary drilling rigs into the rock: (1) small axial pressure;

(2) light weight;

(3) large impact of the rock on the equipment itself. Due to the lack of axial pressure of the rotary drilling rig, it is difficult to achieve efficient rock crushing in the form of "leap-forward crushing" for two main reasons:

○1 Rock is anisotropic, heterogeneous, and discontinuous tissue, and the rock is located deep underground with complex geological conditions;

○2 The weight of the small and medium-sized rotary drilling rigs is small. It is necessary to ensure the mobility and flexibility of the small and medium-sized rotary drilling rigs, and to ensure that their own weight can withstand the reaction impact of the rock, while ensuring their own safety and stability. This seems to be a challenge for small and medium-sized drilling rigs. The design of the drilling rig and the appropriate construction method have put forward a new requirement for the designers of rotary drilling rigs.

Small and medium-sized rotary drilling rigs have made a major breakthrough in this regard. In the construction of weathered limestone in Sichuan, the hole diameter was φ900mm, and the rock was successfully penetrated 1000mm. The uniaxial compressive strength of the rock reached 30MPa, and it took 1.2 hours. The construction method is to use a barrel drill, which uses the grinding or fatigue damage of the pick teeth, and then equipped with special tools to break the core, and then use the barrel drill to remove the core completely.

2. Rock entry construction

In April 2013, the rotary drilling rig was constructed in Chongqing, Sichuan. The geological conditions were medium-weathered limestone layers, the uniaxial compressive strength was 30MPa, the hole diameter was φ900mm, and the rock penetration was required to be 1000mm. A 4×9.5m machine lock rod was used, and the drilling tool used was a barrel drill with pick teeth. The main principle was to use the rotation and revolution of the barrel drill teeth to grind the rock. As shown in the figure below.

The pressurization construction method of rotary drilling rig mainly adopts "constant pressurization combined with inching pressurization". Constant pressurization mainly produces static load to provide constant pressurization for grinding rocks. Inching pressurization mainly forms a certain impact on rocks to achieve local crushing of rocks. These two pressurization methods are combined to achieve rapid rock cutting. Due to the uneven hardness of rocks, the rotation speed will change with the load, resulting in alternating fast and slow during the drilling process, and also causing a certain impact on the rocks, thereby accelerating rock cutting. In addition, water or mud must be continuously injected during the drilling process, mainly to play a role in lubrication and cooling the drill bucket, reducing equipment damage caused by heating of the drill bit.

3. Core extraction

When the drilling with the barrel drill reaches the specified requirements, the rock needs to be cut and removed. Because the small drill itself is light and has a small torque, it cannot directly cut the rock. This requires a special wedge drill bucket (as shown in the right figure). Using the principle of wedge pressure, the pressure F of the wedge drill bucket generates a radial force F1 on the core, and finally breaks the rock. As shown in the figure below, insert the wedge block into the gap between the core and the hole wall, and then apply the pressure F (using the impact force formed by the lowering of the drill rod) to successfully break the core. Then replace the barrel drill to remove the broken core.

4. Analysis of rock penetration mechanism

The reason why the rotary drilling rig can successfully and quickly enter the rock is mainly because the design of the drilling rig itself strictly complies with the EU EN791 safety design standard. On the basis of fully considering the safety and stability of the rotary drilling rig, the weight of the whole machine is reasonably distributed and its flexibility is achieved. The reliable design structure can withstand the reaction force of the small machine lock rod, thereby achieving greater pressure transmission and higher rock penetration efficiency, and achieving the best results. The rotary drilling rig fully considers the three stages of rock crushing mechanism: grinding crushing section, fatigue crushing section and leap crushing section.

(1) Grinding and crushing section: Under the action of small axial pressure, the contact pressure generated by the drill bucket and the rock is significantly smaller than the ultimate strength or indentation hardness of the rock. Crushing is done by friction cutting. The crushed particles at this stage are Small, the drill bit has a certain amount of wear.

(2) Fatigue crushing section: When the pressure load increases to a certain value but still does not reach the ultimate strength or indentation hardness of the rock, the rotating drill bit repeatedly impacts the rock, causing microcracks in the rock. The more microcracks there are, the more , the strength of the rock will be reduced a lot. When the strength of the rock is reduced to a certain extent, the rock will form large particles of rock debris after multiple impacts of the drill bit, thus forming fatigue crushing.

(3) Jump-forward crushing section: When the contact pressure between the drill bit and the rock is greater than or equal to the ultimate strength or indentation hardness of the rock under sufficient axial pressure, the rock will undergo jump-forward crushing. The particles crushed by Yuejin are relatively large. The rock crushing mechanism of rock penetration is to utilize the grinding crushing section and the fatigue crushing section. It solves the problem of light weight and small shaft pressure of small and medium-sized rotary drilling rigs, achieves reasonable values in pressurization and power head speed, and at the same time maintains the advantages of small and medium-sized drilling rigs in terms of mobility and flexibility. In order to extract the core, effective and reasonable methods and suitable drilling tools were used to successfully extract the complete core.

5. Conclusion

(1) By utilizing the rock crushing mechanism, adjusting the design of the rotary drilling rig and equipping it with appropriate drilling tools, it is completely feasible for the rotary drilling rig to enter the rock, and this has been fully demonstrated in practice. This solves the problem that small and medium-sized rotary drilling rigs cannot enter the rock.

(2) Due to the light weight of small drilling rigs, a machine lock rod must be used to transfer the weight of the entire machine to the working surface to achieve rock entry, but the larger reaction force places higher requirements on the reliability of the entire machine structure.

(3) When drilling holes in rocks, appropriate drilling tools must be equipped, especially for the problem of extracting rock cores. Therefore, research on drilling rig tools is also indispensable and will be a major direction for our future research on rotary drilling rigs entering the rock.

(4) Research on the working methods of rotary drilling rigs is even more indispensable, such as the reasonable distribution of the relationship between the pressure and the speed of the power head. When entering the rock, water or mud must be added to cool the drilling tools and increase lubrication.

(5) It is particularly important for small and medium-sized rotary drilling rigs to reasonably distribute the proportion of drill pipe, drilling tools, mast and total weight of the drilling rig while ensuring stability due to their own weight limitations, while giving full play to the flexibility and low fuel consumption of small and medium-sized rotary drilling rigs.

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