(1) The stratigraphic depositional age is far away, the hardness of the stratum is high, inhomogeneous, and the abrasiveness of some layers is high.
The thickness of more than 500m of the Ziliujing Formation, mud shale, sandstone interbedded, sand mudstone dense, high hardness, Shujiahe section of the mudstone sandstone quartz sandstone, siliceous cementation, rock hardness up to grade 7; Permian Yangxin Tongzhi stratum rock drillability grade value of 4.7 to 7.5, rock hardness and softness changes, due to the role of the high peripheral pressure, the Permian greywacke hardness is higher than the triassic more than three times, and the central band flint nodules, coal and pyrite [ 7]. 7].

2) Borehole instability.
The upper part of Shaximiao Formation and Pearl Chong Formation in the high steep tectonic wells in east Sichuan (depth of the well is 0～2500m), the lithology is mainly sand, mud and shale interbedded. Due to the strong water sensitivity of the rocks and the combined effect of the geostress of the high steep tectonic structure, the borehole is easily destabilised and the wall of the well collapses severely. In order to balance the collapse and increase the drilling fluid density, resulting in a significant reduction in the mechanical drilling speed, such as Tiandong 26 wells in the Shaximiao Formation, Ziliujing Formation drilling in the drilling of the density of drilling fluid to 1.35 ~ 1.63g/cm3, the high column pressure generated by the "holding effect" of the mechanical drilling speed down to 0.96m/h; Permian Longtan Formation, Liangshan Formation is also very susceptible to the collapse of the wall. The Longtan Formation and Liangshan Formation of Permian are also very easy to collapse.

(3) The inclination angle of the formation is large, the inclination angle of the formation is 8～55°, and the highest is 85°, which is easy to occur the natural inclination of the formation.
In order to control the well inclination, light pressure drilling measures are often taken, which greatly reduces the drilling speed.

(4) The formation pressure system is complex, ranging from negative pressure to abnormally high pressure intertwined with each other, with the diversity of vertical pressure system and the relative independence of horizontal pressure system.
Carbonate gas reservoirs have three types of production layers: fracture, porosity and fracture and porosity. The pressure regularity of porous gas reservoirs is clearer and basically shows normal pressure, which can achieve balanced drilling, such as the Chuandong Carboniferous System, Changxing Bio-Reef and so on. On the other hand, the pressure of fractured gas reservoirs depends on the strength of geological movement and the strength of the rock itself - poor regularity and difficult to predict. When drilling wells according to the presence of cracks and the use of high-density drilling fluid drilling. It leads to overbalance affecting the mechanical drilling speed, but using low density drilling fluid for fast and strong drilling, there is a high risk of sudden encounter with fracture blowout. The two high-pressure producing formations of Yangxin Group and Changxing Group, due to the high pressure gradient, rock hardness, and deep depth of the formation, the drilling fluid density is usually 1.50g/cm3; (the highest pressure gradient of Yun'an 6 wells reaches 2.45MPa/hm, and the density of drilling fluid is 2.41g/cm3), which results in the drilling speed hovering around 1.00m/h for a long time.

(5) Long large-size borehole.
The section of boreholes larger than 311.Imm in the east Sichuan area accounts for about 1/2 to 3/4 of the total footage, and the drilling pressure can not be added due to the factor of well inclination. The drilling pressure of in bit can only be increased to 120～180kN, and the rotational speed is usually 60r/min. The rock breaking volume of the drill bit is large in large-size boreholes, for example, the rock breaking volume of in bit is 4.24 times of that of the 8Min bit, and the rock breaking volume of in bit is 2.08 times of that of the in drill bit. Obviously, the mechanical breaking energy of the drill bit at the bottom of the well is seriously insufficient, and it is difficult for the teeth to eat into the formation. The rock-breaking method is mainly based on grinding rather than volumetric crushing, resulting in a very low rock-breaking efficiency of the drill bit. Due to the large size and depth of the borehole, the actual annulus return speed is below 0.8m/s, and the hydraulic energy is also seriously insufficient, resulting in poor cleaning effect at the bottom of the well, and repeated crushing of the rock chips; at the same time, the experience of deep well drilling has shown that, with the deepening of the large-size well section . The crapping phenomenon of the tooth wheel bit is particularly prominent, and the effective drilling time of the bit at the bottom of the well is less than 50%, which also seriously affects the improvement of mechanical drilling speed.