What Factors Affect Roller Cone Bit Failure

The influence of lithology on the roller cone bit

The lithology and softness of the formation are different, resulting in different rock fragmentation mechanisms and various forms of roller cone bit failure. The physical and mechanical properties of rocks in various oilfield drilling formations in China have been measured.

Based on the lithology collected on site and the recorded lithology per meter drilling, the hardness, plasticity, brittleness, grindability, and drillability of the formation rocks are analyzed, and the selection and use of roller cone bits including the tricone roller bit are compared with the failure forms to confirm its rationality.

The influence of well section position on the roller cone bit

The mechanical properties of rocks vary greatly depending on their position in the crust. Rocks buried deeper are in a state of multidirectional compression stress, which reduces the pores and increases the strength. The upper well sections generally have loose and soft rock bonding, high roller cone bit rotational speed, and low drilling pressure. The lower well sections generally have hard rocks, high grindability, and require low roller cone bit rotational speed, high drilling pressure, and long use time.

Based on the collected well section positions and lithology per meter drilling records, the hardness, plasticity, brittleness, grindability, and drillability characteristics of the formation rocks are analyzed. The failure form of the roller cone bit is compared to confirm the rationality of its selection and use.

The influence of wellbore structure on the roller cone bit

The different wellbore structures have special requirements for the size, model, and use of the roller cone bit. For example, the manufacture of a deflecting roller cone bit generally requires edge grinding teeth or a diameter-maintaining structure, and high rotational speed and low drilling pressure are required for use. Collected information on the wellbore structure, selection of roller cone bit types, and use parameters are evaluated and compared to the failure form of the roller cone bit to confirm its rationality.

The influence of drilling parameters on the roller cone bit

The determination of the drilling pressure and rotational speed not only affects the efficiency of the roller cone bit in breaking rocks, but also affects the wear of its teeth and bearings. For shallow wells and soft formations, the roller cone bit is mainly used for shearing, and high rotational speed and low drilling pressure are generally used. For medium-hard formations, the roller cone bit produces a comprehensive effect of shearing, impact, and crushing, and medium rotational speed and medium to high drilling pressure are generally used. For deep wells and hard formations, the roller cone bit is mainly used for crushing and impact, and higher drilling pressure and lower rotational speed are generally used.

The reasonable selection of drilling parameters largely determines the failure form of the roller cone drill bit. The drilling parameters used and their changes are analyzed based on the collected drilling records and weight on bit records to confirm the rationality of its use.

The influence of mud performance on the roller cone bit

The mud used for jet drilling must have specific gravity, viscosity, plastic viscosity, and dynamic shear force.

• Low water loss, low sand content, suitable shear force, and pH value can effectively protect the wellbore and suspend rock debris;

• Low specific gravity and low viscosity can reduce the pressure and power loss of the circulation system;
  

• Effectively carry rock debris at low return speeds;
  

•  Has good shear dilution characteristics. Different geological conditions of the formation require different types of mud and related performance, which affects the coordination of drilling pressure, rotational speed, hydraulic parameters, and the failure form of the drill bit. Mud performance is an important factor in the wear of the roller cone bit, such as sand-containing mud having a significant impact on the flow path erosion of the drill bit.