1 Influence of geological factors
(1) In the deeper formation structure, the drillability is poor due to the greater hardness of the rock layer. With the increase of depth, the drillability level is higher, and the drillability also changes according to the different geological structure of each layer. In deeper well sections, due to high temperature and pressure, the level of drillability will increase. In the strata containing mudstone and argillaceous sandstone, due to the change of elasticity, plasticity and brittleness, the difficulty of crushing is increased. In the sand-mudstone interbedding and containing gravel layer, the phenomenon of drilling will also occur, which will reduce the service life of drilling tools and affect the speed of drilling machinery.
(2) The lithology of the deep formation is heterogeneous, and gravel block peeling often occurs, and the probability of occurrence of underground complex situations increases. The deep formation is not only macroscopically complex lithology, but also complex structure of the same lithology. During the drilling process, gravel blocks often peel off and collapse, causing early damage to the drill bit.
(3) the formation temperature is high. High temperature and high pressure have a great influence on the performance of drilling fluid, which also leads to the loss of the gauge teeth and cutting teeth of the drill bit, the failure of the seal of the downhole tool and the loss of the tungsten steel column, resulting in the increase of the complexity of the downhole situation and the difficulty of cementing.
(4) The formation dip angle is large. For the formation dip angle is about 20~60. In order to control the quality of the well, it is often necessary to lightly press and hang, which seriously affects the mechanical drilling speed of deep wells.

2 Drilling Engineering Factors
(1) Drilling mode.
The selection of drilling mode will directly affect the efficiency of drilling engineering, in general, after determining the completion depth structure, the corresponding drilling mode will be selected according to the structure, which is an important factor to improve the drilling efficiency of unit well section. In previous drilling projects, the following drilling methods are basically adopted. In the strata with relatively soft geology in the upper part, the jet type with hydraulic rock breaking as the main part and mechanical rock breaking as the auxiliary part is generally adopted; in the well section of the middle and deep strata, the mode with mechanical rock breaking as the main part and hydraulic rock breaking as the auxiliary part is adopted; in the hard strata with relatively deep strata, mechanical rock breaking is mainly adopted because it will cause relatively large pressure in the pipe and increase loss. This traditional mechanical drilling method, not only low speed, and long consumption cycle, in the period of large energy demand, can not meet the development needs of energy exploration.
(2) Drilling equipment.
Due to the limitation of capital and technical conditions, most of the existing drilling equipment in China is still relatively backward and cannot meet the development needs of energy exploration. At this stage, the drilling equipment mainly includes drilling rigs, mud pumps, solid control equipment for mud purification, drill pipe drill collars and other downhole drilling tools, and the existing performance of these equipment is a factor affecting drilling efficiency. In drilling engineering, due to the large diameter of the upper section, a large displacement is required, which requires the simultaneous drilling of dual pumps. Under the condition that the outer diameter of the drill pipe and drill collar is not enough, the mechanical energy in the well will be insufficient, which will affect the drilling speed. The existing mud purification and solid control equipment is of low level, which cannot effectively reduce the solid content in the mud. Under the condition of high mud density, cause the downhole pressure difference increases, prone to repeated crushing phenomenon; because the drill pipe drill collar diameter is small, so it will increase the lower section of the water energy loss, reduce the bit pressure drop, affect the effect of bottom hole cleaning.
(3) Drilling fluid.
In order to ensure the mechanical lifting speed and safety of deep drilling, drilling fluid is generally used to prevent bottom hole collapse, diameter expansion and temperature reduction. However, in deep drilling, with the decrease of depth, the pressure difference at the bottom of the well will become larger and larger, and the mechanical lifting speed will be affected. Therefore, the density of drilling fluid should be reduced as much as possible to reduce the impact on the pressure difference at the bottom of the well. In addition, the selection of drill type, the optimization of various parameters in the drilling process and the combination of drilling tools are all important factors affecting the drilling speed, so it is necessary to choose scientifically and reasonably.