*3.2. Test Results*

The compression test diagrams of the casing with different internal support structures are shown in Figure 18. All the internal support structures have obvious deformation during the compression process, and the deformation of the internal support structure reduces the deformation of the borehole protection tube so that it can provide good support to the borehole.

**Figure 18.** Three internal support structures of the borehole protection pipe.

The variation of the radial compressive force of the borehole protection tube of different internal support structures of 100 mm and 160 mm is shown in Figures 19 and 20. The peak values of the radial compressive force of 100 mm different support structures are 5.59 kN, 21.03 kN, 11.79 kN, and 29.43 kN respectively. The peak values of the radial compressive force of different 160 mm support structures are 7.29 kN, 18.31 kN, 13.89 kN, and 31.30 kN respectively. The radial compressive force of different internal support structures of the two types of openings are "cross-shaped" > "line-shaped" > "Y-shaped" > Conventional pipe, and the peak compressive force of 160 mm borehole protection pipe is slightly greater than 100 mm.

**Figure 19.** The radial compressive force of 100 mm borehole protection pipe with different internal support structures.

**Figure 20.** The radial compressive force of 160 mm borehole protection pipe with different internal support structures.

The displacement of different internal support structures in the radial compression test is shown in Figure 21. The displacement trends of 10 mm diameter and 160 mm diameter are basically the same, and the displacement of 160 mm of the same kind of internal support structure is slightly larger than 100 mm; smaller deformation of hole protection tubes are line-shaped and cross-shaped, and the cross-shaped is the smallest.

**Figure 21.** Displacement of borehole protection pipes with different internal support structures at peak compression force.

Therefore, according to the comprehensive analysis of the radial compressive force and deformation of the casing, it can be concluded that all three internal support structures can improve the compression resistance of the casing. Considering that the cross-shaped internal support structure can not only effectively support the vertical compression to resist compression, but also support the pipe wall in the horizontal direction to prevent deformation. Thus, the borehole protection effect of the cross-shaped internal support structure of the borehole protection pipe is the best.

#### **4. Discussion**

The directional long borehole on the roof has the characteristics of penetrating more layers and larger borehole diameter. Prior to drilling, the coal seam is in a state of equilibrium under the combined action of overburden pressure, horizontal pressure, and

formation pore pressure. However, the stress balance of the original coal seam is disturbed by the formation of the well, the stress of the coal seam is redistributed, and the pore and fracture structure of the surrounding rock is changed. When the stress in a certain part of the surrounding rock of the borehole exceeds the maximum load that the coal rock can bear, the fractures will spread and penetrate rapidly, and the coal body will be squeezed into the borehole, resulting in instability and collapse of the borehole. From the numerical simulation results, it can be seen that both lithology and borehole diameter significantly influence the stability of roof-directional boreholes. On the one hand, borehole stability is significantly lower in coal and mudstone than in coarse and fine sandstone. Because the mechanical strength of coal and mudstone is lower than that of sandstone, the stress on the rock surrounding the borehole is different, and the radial deformation around the borehole is greater. When the radial deformation reaches the limit, the inner ring coal near the borehole wall fractures and the coal in the fractured area collapses and falls into the borehole. On the other hand, borehole diameter is inversely proportional to borehole stability, and when the borehole diameter exceeds 160 mm, the plastic failure and stress concentration of the borehole are severe and tend to be unstable. Therefore, to avoid instability and collapse of the borehole in soft rock such as coal and mudstone, it is essential to fully consider the rock mass distribution and the selected borehole diameter when designing and constructing directional long boreholes on the roof. Effective hole protection measures must be taken to prevent the failure of critical holes due to the oversized hole diameter.

According to the compression test results, the internal support structure can effectively improve the compression resistance of the conventional borehole protection pipe; especially, the cross-shaped internal support pipe has the best compression resistance which can provide strong support from two directions perpendicular to each other, and the internal support can bear part of the pressure of the borehole protection pipe to keep it in a relatively stable state. The internal supporting structure casing is suitable for roof-directional long holes. Improving the stability of the roof-directional long borehole by the internal support structure borehole protection pipe is beneficial to the efficient gas extraction in the goaf and can promote the popularization and application of the roof-directional long borehole technology. However, the compressive effect of the internal supporting casing cannot be matched with the field effect. Therefore, it is necessary to conduct systematic field experiments in various mines to further investigate borehole protection effects.

### **5. Conclusions**

Borehole construction is the prerequisite for gas extraction in drilling. However, during the drilling construction, factors such as lateral pressure factor, the mechanical strength of rock, confining pressure, and so on are influenced, so that the drilling hole is easily deformed and collapsed, so that the drilling gas extraction performance is poor, and brings hidden hazards to the coal mine safety production. Research on the borehole collapsing law and corresponding borehole protection techniques has great practical significance for preventing borehole distortion and collapsing, increasing borehole stability, and improving gas drainage. Numerical simulations and laboratory experiments have been carried out to investigate the stability of directional long boreholes on the roof, and the main conclusions are as follows:


tubing does provide some support to the borehole. However, severe plastic damage and stress concentration still exist when applied to large-diameter borehole protection, so the borehole protection effect is weak.

(4) Compared with the conventional wellbore protection pipe, the peak value of the compressive force of the wellbore protection pipe with line-shaped, Y-shaped, and cross-shaped internal support structures is significantly increased and the displacement is reduced. In particular, the peak value of the compressive force of the crossshaped internal support structures is four times that of the conventional ones, and the displacement change is half that of the conventional ones. It indicates that the internal support structure can improve the borehole protection effect, and the cross-shaped has the best borehole protection effect.

Due to the limited computing power, the numerical simulation is mainly based on two-dimensional simulation, and many simplifications were made. A closer combination of drilling and mining should be considered for analysis in the future. Moreover, the production cost of the internal support hole protection pipe is higher than that of the common hole protection pipe. Further research should improve and reduce the cost, so as to facilitate the subsequent popularization and application.

**Author Contributions:** Methodology, Z.W.; conceptualization, X.Y.; formal analysis, X.Y., Z.W., and G.W.; investigation, X.Y. and H.G.; writing—original draft preparation, Z.W. and H.G.; writing review and editing, X.Y., and H.G.; translation, Z.W.; funding acquisition, X.Y. and G.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the key control factors and influence mechanism of coal seam nitrogen injection critical breakthrough pressure of the National Natural Science Foundation of China (Grant No. 52204196) and Study on Cyclically Variable Flow Nitrogen Flushing Coal Seam Gas Technology and its Key Controlling Parameters of the National Natural Science Foundation of China (Grant No. 51974161).

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
