Analysis of Rock Crushing Mechanism of PDC Rock Bit

1. Force analysis of the interaction between a single PDC rock bit and rock

In the formation process of the PDC rock bit, the PDC piece needs to be obliquely inserted into the bit matrix. Therefore, during the drilling process of the PDC rock bit, the force direction of the PDC rock bit is pressed in obliquely, thereby breaking the rock.

When a slanted cylindrical PDC is only subjected to tangential force, the pdc drill bit produces a horizontal displacement under the action of tangential force after it is normally pressed into a certain depth.

Because in the actual cutting process, the force on the rear side of the PDC is relatively small, so it is ignored. In the case of tangential force, the contact pressure between the rock and the PDC slice at the front and the friction between the PDC slice and the rock at the bottom are mainly considered.

2. What is the rock crushing mechanism of the PDC rock bit?

The analysis of rock crushing mechanism needs to combine formation conditions, external load conditions, crushing tool conditions and rock properties.

It is necessary to study not only the form and effect of rock crushing, but also the change trend and stress state of rock before and after crushing.

The uniaxial compressive strength of the rock refers to the ratio of the axial pressure to the area when the rock that is only subjected to the axial pressure is damaged without other surrounding pressure.

The hardness of rock refers to the ability of the rock to produce local deformation or crushing under the action of concentrated load, which is expressed by the ratio of contact pressure to contact area.

The brittle-plasticity of rock refers to the tendency of brittle-plastic failure of rock during failure, which has a significant impact on the drilling effect and crushing effect of rock.

In the actual drilling process, the stratum that can be broken by the PDC rock bit is soft-medium-hard strata. In general, the breaking method of the PDC rock bit is mainly cutting and shearing damage, supplemented by extrusion damage. The specific breaking mechanism will be varies with the stratum.

The rock forms a compacted rock mass under the action of pressure, and the axial pressure and the indentation depth are consistent with the theory, but the rock fracture trajectory is offset from the theoretical curve because brittle cracks are generated.

Because the broken debris on the rock surface is flying around, the axial load of the rock is reduced, and due to the further crushing of the rock, the larger cracks in the rock lead to large-scale crushing, which further increases the pressure depth.

With the further increase of the external load, the PDC rock bit repeats the process of fracturing and crushing to complete a complete crushing stage.

Therefore, in the process of breaking rock, the indentation depth of the PDC rock bit is not linear with the applied load, and the load will decrease and then increase.

When the PDC rock bit breaks the rock under vertical load, the limit state is generated at the bottom of the PDC at the beginning, so that it is pressed into a certain depth, and then with the increase of the contact area, the pressure needs to be increased, and then cracks are generated, resulting in large breakage.

When the PDC rock bit breaks the rock under the action of horizontal force, the pressure distribution in front of the cutting end is not uniform, and when the crack is subjected to the simultaneous action of pressure and tension, the crack will penetrate each other and form a large shear plane.

In the actual working process, PDC drilling into rock is not only related to the external load and rock properties, but also related to the number and spatial arrangement of PDC cutting pieces. Therefore, it is necessary to have a suitable distance when arranging PDC cutting pieces to achieve good cutting effect.