Because of its brittleness, poor impact resistance, and strong thermal sensitivity, diamond is used under much stricter conditions than other drill bits.
1. Requirements for hole bottom cleaning
Before running the diamond bit, workover must be done with a carbide bit of similar shape to the diamond bit with a powder tube. The large rock debris, broken metal and other objects at the bottom of the hole are washed up by hydraulic power and settled in the powder tube.
2. First grind the new drill bit
The new diamond bit selected must be smaller in diameter than the non-diamond bit and be drilled in and out. The initial speed should be 40 ~ 50r/min, and the initial weight on bit should be 1/8 of the normal weight on bit. After drilling 0.15 ~ 0.2m, it should be switched to normal drilling.
3. Parameters selection for normal drilling
(1) Weight on bit is determined by considering formation lithology and lifting force of hole bottom caused by hydraulic washing. The maximum specific pressure on the bit lip in soft formations is 6.9MPa. In hard strata, the diamond cutting depth is small, the contact area with rocks is small, and the diamond is easy to fracture due to overload in hard strata, so it is recommended to use a small specific pressure, generally 5.1MPa. Figure 5-14 shows the weight on bit of diamond bits of different diameters used in different formations. The weight on bit for normal drilling should be at the lower end of the curve and gradually increase as the diamond blunts, but should not exceed the upper end of the curve. The actual weight on bit obtained by estimating or checking the curve should be added to the effect of buoyancy and lift force in the hole.
(2) Speed after the bit running-in, the ideal speed should be as high as possible, in order to give full play to the benefits of diamond drilling. It is considered safe to drill at 150r/min. If formation conditions, pipe quality, and equipment capabilities permit, it is better to drill at 300r/min. The diamond bit combined with turbine drilling has been successfully used for 600 ~ 1 100r/min with good results.
(3) Pump volume Pump volume should first meet the annular space minimum reflux speed requirements, but also to ensure that the bottom of the hole cleaning and cooling drill needs. Figure 5-15 shows the recommended ranges of pump rates for drill bits of different diameters, with lower limits for high density mud and upper limits for low density mud.
