Based on years of experience in scientific research, development and production practice, the drilling speed and cost of water well drilling rig are slow and high due to improper bit selection.To choose drill bit correctly, on the one hand, it is necessary to have a clear understanding of the working principle and structural performance characteristics of existing drill bit.On the other hand, it is necessary to have a full understanding of the physical properties of rocks drilled, such as the hardness, plastic brittleness, abrasive property and drill ability of rocks, so as to reasonably determine the type of cone bit.The basic principle of selecting drill bits for wells is to minimize drilling costs.

1. General principles for cone bit selection

There is a great relationship between the selection of cone bit and the drilling cost.Therefore, it is very important for well drilling rig to select drill bit reasonably according to the construction situation.At present, the cone bits used in the water well drilling are all oil drilling specifications, among which the three cone bits are the most commonly used, but the self-made welded cone bits can only be used in the special water well construction.

2. Bit selection for abrasive formations

Rocks can be roughly divided into three types: one is brittle rocks, such as limestone, sandstone, granite, diorite, etc.One is plastic rock, such as mud-stone, argillaceous shale, etc.There are also plastic brittle rocks in between, such as argillaceous consolidated sandstone and so on.Cone bit is more suitable for drilling in hard brittle rock, which is broken by impact and crushing.

Abrasive formation will cause the bit teeth to wear out too quickly, and the rate of mechanical drilling will decrease very quickly.And the drill bit footage is less, especially will wear the drill bit track diameter teeth and cone back cone and claw, so that the drill bit diameter is smaller, causing bearing exposure, more speed up the damage of the drill bit, in this kind of formation drilling is best to choose  insert bit.

3. Bit selection for hard and soft staggered formations

In soft and hard cross-stratum, it is necessary to choose insert bit with high wedge-shaped teeth or high cone-spherical teeth, which can not only have higher penetration rate in soft stratum, but also ensure the drill-ability in hard stratum.However, there should be some differences between drilling pressure and rotating speed of water well drilling rig. In soft formation, drilling speed can be increased and drilling pressure reduced, while in hard formation, drilling pressure and rotating speed should be increased to achieve better geological and economic results.

4. Bit selection for inclined formation

The formation dip is the main cause of the formation dip, and the bending of the lower drill string and the improper selection of the bit type are the technical factors.

According to the long-term production practice, the shaft shifting bit is easy to cause the well deviation when drilling in inclined formation, so the cone bit with no shaft shifting or small amount of shaft shifting should be selected.At the same time, on the premise of small displacement axis, a bit with softer performance than formation rocks should also be selected, which can improve the mechanical drilling rate at a lower bit pressure, ensure the drilling speed and reduce the deviation.

5. Bit selection for shallow and deep well sections

In shallow well section, the bit with high speed should be selected, while in deep well section, the bit with long service life should be considered.