# Review on the Influence of Complex Stratum on the Drilling Trajectory of the Drilling Robot
## 3 Impact of Typical Complex Coal Seams on the Deviation of Drilling Tool Track
### Reasons for Hole Bending  
Borehole bending is a common and widespread phenomenon in drilling production, and the formation of bending conditions is the result of the joint action of subjective and objective factors. The theoretical study of well deviation of conventional mud drilling formation shows that the geology is heterogeneous [69; 70; 71], the physical properties of each layer are different, the formation drillability and bedding dip angle are also different and the significant difference of well deviation degree indicates that the formation factor is often the dominant factor affecting the well deviation [72]. The influence of formation on well deviation mainly includes the following four aspects. (1) Influence of inclined layered strata  
Because the rock at the layered interface cannot support the drilling pressure for a long time and tends to be crushed along the vertical plane when drilling the inclined layered formation, a small, inclined step is formed on the plane at the downdip side of the borehole. This small, inclined step exerts a lateral force on the bit, pushing the bit towards the updip direction of the formation, resulting in the well deviation, as shown in Figure 12[73]. (2) Influence of rock anisotropy  
The physical and mechanical properties of strata with bedding and schistity are different in different directions. The rock stratum perpendicular to the bedding direction has the least hardness, the smallest breaking resistance and the fastest breaking speed; while the rock stratum parallel to the bedding direction has the greatest hardness, the highest breaking resistance and the slowest breaking speed; The hardness, breaking resistance and breaking speed of the rock stratum in the direction of oblique intersection with the bedding are between the above two. The natural trend will inevitably lead to well deviation when the local stratum is inclined. Figure 13 shows the hole breakage of the bit and the ground surface in different directions. Figure 12: Schematic diagram of bit working in layered formation. Figure 13a shows the hole breakage of the bit and the formation in different directions. In Figure 13b, the drill bit is oblique to the formation surface, and the drill bit is subject to a force from the dip direction of the formation--the formation deflecting force, which makes the drill bit axis force deflect to the direction perpendicular to the formation surface, and the drill hole is elliptical. In Figure 13c, due to the low hardness perpendicular to the layer, the rock on the hole wall is easily broken, the hole diameter formed is large, and the hole wall gap is large, which makes the bit stability poor, and the hole is also easily bent. (3) Influence of soft and hard interlaced strata [74]  
The influence of the drill bit on the bending of the hole when it passes through the soft and hard interbedding depends on the angle \(\delta\) of contact of the hole axis (\(\delta\) refers to the residual angle between the borehole axis and the normal of the rock layer) and the hardness difference of soft and hard rock layers. As shown in Figure 14a, WOB can be decomposed into component C perpendicular to the layer and component N parallel to the layer during drilling, as shown in Formula (2)  
\[\rm P=N+C \tag{2}\]  
Figure 14: Borehole bending when the drill bit passes through the soft and hard interbedding. (a)—Drill bit from soft rock to hard rock; (b)—Drill bit from hard rock to soft rock; (c)—The sliding condition of the drilling tool when the formation angle is small. 1—Soft rock; 2—Hard rock; 3—Contact surface of soft and hard interlayer. Figure 13: Hole breakage of bit and formation in different directions. (a)—The bit is perpendicular to the layer; (b)—The bit is oblique to the layer; (c)—The bit is parallel to the layer. 1—Crushing speed perpendicular to the layer; 2—Crushing speed at oblique intersection with the layer; 3—Crushing speed parallel to the layer. N is the sliding force of the drill bit on the rock layer, it is calculated as follows.