DrillingEngineering–PE311DrillBitOptimization.pptVIP

There is almost always some uncertainty about the best time to terminate a bit run and begin tripping operations. The use of the tooth-wear equation and the bearing-wear equation will provide, at best, a rough estimate of when the bit will be completely worn. In addition, it is helpful to monitor the rotary-table torque. In the case of a roller-cone bit, when the bearings become badly worn, one or more of the cones frequently will lock and cause a sudden increase or large fluctuation in the rotary torque needed to rotate the bit. With a PDC or fixed-cutter bit, when cutter elements are heavily worn or broken, or the bit becomes undergauge, the bit will exhibit much lower than expected ROP and cyclic or elevated torque values. Termination of a Bit Run Optimization of Economics When the ROP decreases rapidly as bit wear progresses, it may be advisable to pull the bit before it is completely worn. If the lithology of the formation is homogeneous, the total drilling cost can be reduced by minimizing the cost of each bit run. In this case, one way to determine when to terminate the bit run is by keeping a current running calculation of the cost per foot for the run, assuming that the bit would be pulled at the current depth. Even if significant bit life remains, the bit should be pulled when the computed cost per foot begins to increase. However, if the lithology of the formation is not uniform, this procedure will not always result in the minimum total cost. In this case, an effective criterion for determining optimum bit life can be better established after offset wells are drilled in the area, thus defining the lithological variations, and the contribution of the rock properties can be studied and understood better. Termination of a Bit Run Optimization of Economics Example: Determine the optimum bit life for the bit run described in the table below. The lithology of the formation is known to be essentially uniform in this area. The bit cost is $5000. The rig cost i