Factors affecting the drilling speed of the drill bit

1) The strata are deposited a long time ago, with high hardness and heterogeneity, and some sections are highly abrasive.

The Ziliujing Formation with a thickness of more than 500m is composed of interlayers of mudstone and sandstone. The sandstone and mudstone are dense and hard. The Xujiahe section is composed of mudstone interbedded with quartz sandstone, siliceous cementation, and the rock hardness reaches level 7; the drillability level of the rock in the Yangxin Formation of the Permian System is 4.7-7.5, and the hardness of the rock varies greatly. Due to the high confining pressure, the hardness of the Permian limestone is more than 3 times higher than that of the Triassic System, and there are flint nodules, coal and pyrite in the central belt.

 

2) Borehole instability.

The upper Shaxi Temple Formation-Zhenzhuchong Formation of the steep structure well in eastern Sichuan (well depth 0-2500m) is mainly sand, mud and shale interlayers. Due to the strong water sensitivity of the rock and the combined effect of the geostress of the steep structure, the wellbore is easily unstable and the wellbore wall collapses seriously. In order to balance the collapse, the density of drilling fluid is increased, resulting in a significant reduction in the mechanical drilling speed. For example, the drilling fluid density of Tiandong 26 well was increased to 1.35-1.63g/cm3 in the Shaximiao Formation and Ziliujing Formation formations. The "pressure holding effect" generated by the high liquid column pressure reduced the mechanical drilling speed to 0.96m/h; the Longtan Formation and Liangshan Formation of the Permian System are also very prone to collapse.

 

3) The formation dip is large, ranging from 8 to 55°, up to 85°, and it is easy for the formation to naturally deflect.

In order to control the well deviation, light pressure and lifting measures are often taken, which greatly reduces the drilling speed.

 

4) The formation pressure system is complex, with interlacing from negative pressure to abnormally high pressure, with diversity of vertical pressure systems and relative independence of lateral pressure systems.

Carbonate gas reservoirs have three types of production layers: fractured, porous, and fracture-porous. The regularity of the pressure of porous gas reservoirs is relatively clear, and it is basically normal pressure, which can achieve balanced drilling, such as the Carboniferous System in eastern Sichuan and the Changxing bioherm. The pressure of fractured gas layers depends on the strength of geological movement and the strength of the rock itself. The regularity is poor and it is difficult to predict. When drilling, high-density drilling fluid is used as if there are fractures. Overbalance affects the mechanical drilling speed, but the use of low-density drilling fluid for rapid and strong drilling has a great risk of sudden fracture blowout. The two high-pressure production layers of Yangxin Formation and Changxing Formation have high pressure gradients, high rock hardness, and deep formation depths. The drilling fluid density is usually 1.50g/cm3; (the highest pressure gradient of Yun'an 6 well is 2.45MPa/hm, and the drilling fluid density is 2.41g/cm3), causing the drilling speed to hover around 1.00m/h for a long time.

 

5) Large-size wellbore length.

In the eastern Sichuan region, the well sections with boreholes larger than 311.1mm account for about 1/2 to 3/4 of the total footage. Due to the well inclination factor, the drilling pressure cannot be increased. The drilling pressure of the 1.25mm bit can generally only be increased to 120 to 180kN, and the rotation speed is generally 60r/min. The rock breaking volume of the drill bit of a large-sized wellbore is large. For example, the rock breaking volume of the 1.25mm bit is 4.24 times that of the 8Min bit, and the rock breaking volume of the 1.25mm bit is 2.08 times that of the 1.25mm 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. The rock breaking method is mainly grinding, rather than volume crushing, resulting in a very low rock breaking efficiency of the drill bit. Due to the large size and depth of the wellbore, the actual annular return velocity is below 0.8m/s, and the hydraulic energy is seriously insufficient, resulting in poor bottom hole cleaning effect and repeated crushing of cuttings. At the same time, deep well drilling experience shows that as the large-size well section deepens, the cone drill bit's jumping phenomenon is particularly prominent, and the effective drilling time of the drill bit at the bottom of the well is less than 50%, which also seriously affects the improvement of the mechanical drilling speed.