# Review on the Influence of Complex Stratum on the Drilling Trajectory of the Drilling Robot
## 1 Instruction  
A stratum is composed of various rock-forming minerals and is generally in a relatively stable state. However, due to internal dynamic geological processes (such as crustal movement, magmatism and metamorphism) and external dynamic processes (such as weathering, denudation, accumulation, diagenesis, etc. ), various voids, fractures and solution gaps are found in the stratum. Generally, there are fluids (water, oil, gas) and some soft solid deposits or chemical deposits filling these spaces [1; 2; 3]. Therefore, the original stable state of the stratum may be destroyed when it is drilled, leading to various complex problems such as borehole collapse, block falling, flushing fluid leakage, water gushing, blowout, expansion and shrinkage, which affect the normal drilling of the stratum. We refer to the stratum as a complex formation, as shown in Figure 1[4; 5]. A typical complex formation is characterized by uneven size, poor cementation, loose structure, frequent layer replacement, large differences in soft and hard layers, and large differences in particle grading due to geological tectonic movement or external geological action. This results in unstable stress during the drilling process and roll is easily generated with multiple cutting surfaces. This leads to low rock breaking efficiency, low core recovery rate, and a risk of accidents such as side hole, block falling and drill sticking on drilling robots (such as a crawler full hydraulic tunnel drilling rig for coal mines as shown in Figure 2, an oil drilling rig as shown in Figure 3, etc.) [6; 7; 8; 9]. At the same time, the loss of flushing fluid or water gushing and other accidents are easily caused due to the strong permeability of the broken formation. Figure 1: Schematic diagram of complex formation composition and structure. Figure 3: Oil drilling rig. Figure 2: Crawler full hydraulic tunnel drilling rig for coal mines. As the exploration and development of oil and gas resources are gradually expanded from shallow to deep areas, geological conditions become more complex and changeable, and the mining conditions are extremely complex, and may be seriously threatened by disasters such as coal and gas conflict, mine water inrush, and rock burst [10]. These arise from the impact of geological faults and water saturation formation on drilling. The impact of geological fault zones on the drilling trajectory is mainly reflected in the areas of well fields, system layout and safe production [11; 12; 13; 14; 15]. For example, faults affect the division of coal fields and mining areas, destroying the continuity and integrity of coal seams; large and medium-sized faults can make it difficult in a coal field or mining area to mine the different sides of coal seams containing faults. Faults affect the layout of the development system. When there are faults with large drops in the mine field, scientific and reasonable development methods should be selected according to the dislocation of the two panels of coal seams caused by the fault to ensure the technical rationality of the production and auxiliary systems and to meet the economic, technical and safety requirements of mine production. Faults can cause unstable footing for roadway excavation and problems for mining and excavation. When the driving roadway encounters a fault, the roadway is forced to pass through the fault, and the construction roadway may be scrapped due to fault judgment and processing errors, resulting in the instability of the driving roadway [16]. Faults also lead to loss of coal resource reserves. Protective coal pillars need to be reserved on both sides of large faults, resulting in the reduction in recoverable reserves. Local areas with densely developed faults have non-minable areas of coal seams, resulting in loss of resources. The existence of faults increases the production cost of the mine.