Evaluation Method for Tooth Wear of Roller Bits Based on the Fractal Dimension of the Rock Surface

During drilling operations, the drill bit is worn out while breaking the rock. The roller bit is an important rock-breaking tool, but our research on its wear mechanism is still inadequate. Therefore, in response to this problem, this paper establishes a fractal characterization method for surface roughness based on the morphological features of the rock and bit surface. Combining the worn bit surface morphology and energy spectrum analysis results, it is determined that the wear mechanism of roller bit teeth is mainly abrasive wear, fatigue wear and adhesive wear, which are mainly squeezing and spalling. The rock is broken by roller bit under the action of drilling pressure and tangential force, so the contact force model of the single tooth pressed into the rock is established, and the reacting force of the rock on the bit tooth is deduced based on the rock crush conditions. According to the rolling friction characteristics of roller cone bits, the friction coefficient in rolling conditions is calculated. Then, according to the wear mechanism of the cone bit, the actual contact force and real contact area between the cone bit and rock at the bottom of the hole, a roller bit teeth wear equation is established, consisting of rock properties, bit properties and working condition parameters. The bit parameters and drilling parameters are regarded as controllable parameters, and the rock properties are regarded as uncontrollable parameters. In order to highlight the basic parameters of rock, the controllable parameters in the equation are standardized. The calculated wear degree can better reflect the wear ability of rock to drill bit. This result is of great significance for the factor prediction of formation abrasiveness and bit selection.