Using and Troubleshooting of Roller Cone Drill Bit

Roller Cone Drill Bit Usage

Roller Cone Drill Bit selection is certainly important, but the usage of the bit should not be overlooked. Even if the appropriate bit model is selected, improper use can result in not achieving the expected performance.

Roller Cone Drill Bit Inspection and Acceptance

This mainly refers to the appearance inspection of the Roller Cone Drill Bit, the matching issues of the bit handler, nozzles, and nozzle wrenches, etc. Before going down the well, manually check the rotation condition of the cones, and record information about the bit, including bit model, manufacturer, steel grade, number and size of water holes, and the type of connection thread. Confirm it is matched with the crossover sub.

Bit Handling

Use a bit handler that matches the Roller Cone Drill Bit, protect the bit well, apply thread lubricant, tighten with the torque recommended by the bit manufacturer, and check if it is tightened in place (confirm the installation sequence or position of the wear sleeve).

Running Drill, Hole Making

Control the lowering speed through the BOP (Blowout Preventer) assembly, casing head, reduced diameter section, build section or high dogleg section, and casing shoe. Lower the drill smoothly to avoid sticking, and pump circulation section by section in long wellbore intervals to prevent sticking, leakage, and pump blockage. Ensure the pump is working normally before starting drilling.

Bottom Hole Detection

When approaching the bottom hole while running or making a hole, monitor the weight on bit and torque of the Roller Cone Drill Bit. If weight on bit and torque increase, it indicates contact with the bottom hole or sediment. Avoid blindly lowering or detecting the bottom to prevent blocking the bit's nozzles. If sediment or obstruction is encountered, lift the drill string, gradually start the pump, ensure the pump is working normally, then clean to the bottom hole. During single joint or stand connections, start the pump early and stop it late.

Drilling or Plugging

Before drilling after reaching the bottom hole, clean the bottom hole thoroughly based on downhole conditions. Use measures such as low weight on bit, low rotation speed, high flow rate, and low torque to shape the bottom hole. Move the drill string up and down while circulating and rotating to aid in cleaning the bit. Control the weight on bit to avoid damaging the bit. Monitor pump pressure changes to prevent nozzle blockage. If bouncing or severe torque changes occur, adjust the Roller Cone Drill Bit's rotation speed with low weight on bit and low rotation speed to stabilize the torque. If using a drive tool, reduce the flow rate; if not using a drive tool, increase the flow rate. The matching of weight on bit and rotation speed differs; use high rotation speed and low weight on bit in soft formations, and high weight on bit and low rotation speed in hard formations to achieve better economic results.

Roller Cone Drill Bit Fault Diagnosis

Bit Diameter Reduction

Roller Cone Drill Bits are prone to diameter reduction when drilling in abrasive formations. Characteristics of bit diameter reduction include increased torque on the rotary table or top drive, downhole sticking, unstable rotation speed (showing wavelike changes over time), and reduced torque without increasing despite not feeding the bit. These issues cannot be alleviated by drilling with low weight on bit and low rotation speed. When drilling in unaltered lithology, the issue increases exponentially. After stopping the pump, lifting and lowering the drill string shows no obstruction, and there are no large rock cuttings in the shale shaker. These signs indicate bit diameter reduction.

Cone Seizure

During drilling, a noticeable increase in torque (like increased load on the rotary table or top drive, periodic jumping of the drill string at the wellhead, large power fluctuation of the power system, fluctuating engine noise, or reverse spinning after disengaging the rotary table or top drive) indicates potential cone seizure of the Roller Cone Drill Bit. Comprehensive judgment based on the bit's usage time and formation conditions should prompt immediate circulation and tripping.

Cone Dropped

If large torque fluctuations occur, and after stopping the rotary table or top drive, the drill string shows severe jumping, weight indicator needle swings back and forth, penetration rate significantly decreases, or there's no footage advancement, lift the drill string and change the direction downwards. Height changes possibly indicate the height of a single cone (this phenomenon is less noticeable in deep wells).