Discrete Vibration Stability Analysis With Hydromechanical Specific Energy

Drill-bit vibrations and bit wear have been identified as the two major causes for premature polycrystalline diamond-compact (PDC) bit failure and difficulty in accurately predicting PDC bit performance. The objective of this paper is to present a new approach to drilling optimization by developing an algorithm that defines and generates a constrained stable rotary speed (RPM)–weight-on-bit (WOB) working domain for a given system as opposed to the traditional RPM–WOB charts. The algorithm integrates the dynamic-stability model for bit vibrations with the bit-performance model for degraded bits. This study addresses the issues of dynamic-bit stability under torsional and lateral vibrations coupled with bit wear. The approach presented in this paper involves performing two separate analyses: vibration stability and bit-wear performance analysis. The optimum operating conditions are estimated at each depth of the drilling interval, taking into consideration the effect of bit wear and bit vibrations. Because the bit wears continuously while penetrating the rocks, discretization of depth is necessary for effective simulation. Discretization is done by dividing the drilling interval into subintervals of the desired length. Vibration-stability analysis and bit-wear performance analysis are preformed separately at every subinterval and then integrated over the discrete interval. For every subinterval, a WOB–RPM domain is determined within which the given system is dynamically stable (for vibrations), and the bit wear does not exceed the maximum allowable wear (MAW) for the section of the drilling interval selected. A unique concept to relate the fractional change in hydromechanical specific energy (HMSE) to the fractional change in bit wear has also been put forward that further constraints the WOB–RPM stable working domain. The new coupled vibration-stability chart, including the maximum rate of penetration (ROP), narrows down the conventional chart and provides different regions of operational stability. It has also been found that as the compressive strength of the rock increases, the bit-gauge friction factor also increases, which results in a compressed or reduced allowable working domain, both from the vibration-stability analysis and bit-performance analysis. Simple guidelines have been provided using the new stability domain chart to estimate the operating range for real-time optimization.