Rock drillability measurement and grading method for a polycrystalline diamond composite bit

The bit holds paramount importance as a rock-breaking tool in petroleum and natural gas industry drilling, rock drillability grading using a polycrystalline diamond composite (PDC) bit is the key basis for individualized design and selection of a bit and optimization of drilling parameters. This is especially true for formations that are above medium hard. However, the petroleum and natural gas industry standard “rock drillability measurement and gradation method” issued and implemented in China has obvious defects. For rocks above medium hard, the measurement is often invalid and drillability grade data cannot be obtained. Through many drillability measurements on rock samples of various lithologies, it has been found that the main reasons for the ineffectiveness of measurements of rock above medium hard are that the standard weight on the bit (WOB) is too low and that the structural parameters of PDC microbits are unreasonable. To solve these problems, we first studied the influence of the side rake of the microbit cutter on the interference state and proposed an optimal microbit side rake. This eliminates the possibility of interference between the microbit cutter and the rock. Second, we conducted rock drilling experiments on five kinds of rock under various WOB values. The experimental data show that there is a linear relation between the variable-weight drillability level and the WOB. We put forward a new method of drillability measurement and grading, which is based on the measurement of drilling pressure grading, equivalent conversion grading, or independent grading. This method not only effectively solves the problem of drillability measurement of medium-hard and harder rocks but is also compatible with past soft-rock measurement data.