Factors Affecting Drilling Speed of Bit

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1) The stratigraphic sedimentary age is far away, with high hardness and heterogeneity, and high abrasiveness in some intervals.
The artefact well group with a thickness of more than 500m has shale, sandstone interbedded, dense sandstone and mudstone with high hardness, the Xujiahe section is mudstone interbedded with quartz sandstone, siliceous cementation, and rock hardness reaches grade 7; the rock drillability grade value of the Permian New Formation is 4.7~7.5, and the rock hardness changes greatly. Due to high confining pressure, the hardness of Permian limestone is more than 3 times than Triassic system, coal and pyrite [7].

2) The borehole is unstable.
The upper Shaximiao Formation-Zhenzhechong Formation of the high-steep structural well in eastern Sichuan (well depth 0~2500m), with the lithology mainly of sand, mud and shale interbedded. Due to the strong water sensitivity of the rock and the combined effect of the in-situ stress of the high and steep structure, the borehole is easy to lose stability and the borehole wall is seriously collapsed. In order to balance the collapse and increase the density of drilling fluid, the ROP is greatly reduced. For example, the drilling fluid density of Well Tiandong 26 in Shaximiao Formation and Ziliujing Formation is increased to 1.35~1.63g/cm3, and the "pressure holding effect" generated by high liquid column pressure reduces the ROP to 0.96 m/h. Longtan Formation and Liangshan Formation of Permian are also prone to collapse.

3) The formation dip angle is large, with the formation dip angle of 8~55 °, up to 85 °, which is prone to natural formation.
In order to control the well deviation, the measures of light pressure hanging are often taken, which greatly reduces the drilling speed.

4) The formation pressure system is complex, from negative pressure to abnormal high pressure, with the diversity of the pressure system in the longitudinal direction and the relative independence of the pressure system in the transverse direction.
Carbonate gas reservoirs have three types of production layers: fracture, porosity and fracture-porosity. The pressure regularity of porous gas reservoirs is relatively clear, which basically shows normal pressure and can realize balanced drilling, such as Carboniferous system in east Sichuan and Changxing reef. The pressure of fractured gas layer depends on the strength of geological movement and the strength and regularity of the rock itself, which is difficult to predict. High-density drilling fluid shall be used for drilling due to the existence of cracks during drilling. The mechanical drilling speed is affected by over-balance, but the rapid and strong drilling with low-density drilling fluid is at great risk of sudden fracture blowout. For the two high-pressure production formations of Yangxin Formation and Changxing Formation, due to high pressure gradient, high rock hardness and deep formation depth, the drilling fluid density is usually 1.50g/cm3. (The highest pressure gradient of Well Yunan 6 is 2.45MPa/hm and the drilling fluid density is 2.41g/cm3), resulting in the drilling speed hovering around 1.00 m/h for a long time.

5) Large size borehole length.
The well interval larger than 311.Imm in east Sichuan area accounts for about 1/2~3/4 of the total footage, and the WOB cannot be added due to well deviation. Generally, the in-head drilling pressure can only be increased to 120 ~ 180kN, and the rotating speed is generally 60r/min. The large-size borehole bit has a large rock breaking volume, for example, the rock breaking volume of the in bit is 4.24 times that of the 8Min bit, and the rock breaking volume of the in bit is 2.08 times that of the in bit. Obviously, the drill bit at the bottom of the hole is seriously lacking in mechanical rock breaking energy, and it is difficult for the teeth to eat into the formation. The rock breaking method is mainly grinding, rather than volume crushing, resulting in low rock breaking efficiency of the drill bit. Due to the large borehole size and depth, the actual annular return speed is below 0.8 m/s, and the hydraulic energy is also seriously insufficient, resulting in poor bottom hole cleaning effect and repeated crushing of cuttings. At the same time, deep well drilling experience shows that with the deepening of large-size intervals. The bouncing of the cone 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 the mechanical drilling speed.