1) The sedimentary age of the strata is long, the stratum is hard and heterogeneous, and some strata have high abrasiveness.
The artesian well group with a thickness of more than 500 m has shale and sandstone interbedded, the sandstone and mudstone are dense and have high hardness, and the Xujia River section is mudstone and quartz sandstone, siliceous cementation, and the rock hardness is up to 7, and the drillability grade of the Permian Yangocene strata is 4.7~7.5, and the hardness of the Permian limestone is more than 3 times higher than that of the Triassic due to the high confining pressure, and the central belt has flint nodules, coal and pyrite.
2) The borehole is unstable.
The upper part of the high-steep structural well in eastern Sichuan is the stratum of the Pearl Chong Formation of the Shaximiao Formation (well depth 0~2500m), and the lithology is mainly sand, mud and shale interbedded. Due to the strong water sensitivity of the rock and the combined in-situ stress of the high and steep structure, the borehole is easy to be unstable and the borehole wall collapses seriously. For example, the drilling fluid density of Well Tiandong 26 was increased to 1.35~1.63g/cm³ in the formation drilling of Shaximiao Formation and Ziliujing Formation, and the "pressure effect" caused by the high liquid column pressure reduced the ROP to 0.96m/h, and the Permian Longtan Formation and Liangshan Formation were also very prone to collapse.
3) The stratum dip angle is large, the stratum dip angle is 8~55°, and the highest is 85°, which is prone to natural stratum inclination.
In order to control the deviation of the well, the measures of light pressure and hanging are often taken, which greatly reduces the drilling speed.
4) The formation pressure system is complex, from negative pressure to abnormal high pressure, which is intertwined, and has the diversity of longitudinal upper pressure system and the relative independence of transverse upper pressure system.
There are three types of carbonate gas reservoirs: fractured, porosity and fracture-porosity. The pressure regularity of pore gas reservoirs is clear, and the basic performance is normal pressure, which can achieve balanced drilling, such as the Carboniferous in eastern Sichuan and Changxing biological reef. However, the pressure of fractured gas reservoirs depends on the strength of geological movements and the strength and regularity of the rock itself, which is difficult to predict. When drilling, high-density drilling fluid is used to drill according to the existence of cracks. As a result, overbalance affects the ROP, but the use of low-density drilling fluid for rapid and strong drilling has a great risk of sudden fracture blowout. Due to the high pressure gradient, high rock hardness and deep formation depth, the density of drilling fluid is usually 1.50 g/cm³ in the two high-pressure production layers of Yangxin Formation and Changxing Formation (the highest pressure gradient of Yun'an 6 well is 2.45MPa/hm and the density of drilling fluid is 2.41 g/cm³), resulting in a long-term ROP of about 1.00 m/h.
5) Large-size borehole length.
The section of the borehole greater than 311.Imm in the eastern Sichuan area accounts for about 1/2~3/4 of the total footage, and the drilling pressure cannot be increased due to the deviation factor. The drilling pressure of the inhead can only be increased to 120~180kN, and the speed is generally 60r/min. The rock breaking volume of large-size borehole bits is large, such as the rock breaking volume of the in drill bit is 4.24 times that of the 8min drill bit, and the rock breaking volume of the in drill bit is 2.08 times that of the in drill bit. Obviously, the mechanical rock 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. Due to the large size of the wellbore and the depth of the well, the actual annular return velocity is below 0.8m/s, and the hydraulic energy is also seriously insufficient, resulting in poor bottomhole cleaning effect and repeated crushing of cuttings
