Optimizing Journal Bearing Insert Bit Runs Using a Bearing Life Model

Abstract

ABSTRACT Advances in journal bearing insert bit technology have yielded very versatile bits which can often be run cost effectively at weight-on-bit (WOB) and RPM combinations which exceed manufacturers' catalogue recommendations. Insert breakage rather than tooth wear is the primary cutting structure concern at high mechanical horsepower levels. Field experimentation yields data on allowable WOB and RPM to avoid insert breakage. Below these WOB and RPM restrictions, insert wear is negligible, so the remaining unknown in WOB and RPM optimization is bearing life. This paper presents a semi-empirical bearing life model for journal bearing bits and a method for field implementation which has been useful in predicting bit life for insert bits. The model expresses expected bearing life within a given formation and depth range as a function of WOB and RPM. The model is also suitable for predicting bearing life for bits run on downhole motors. Data are presented to show statistical correlation of the bearing life model with actual field results. A method for using the model to select WOB and RPM to minimize cost-per-foot of bit runs is presented. A successful case history using the bearing life model to predict bit life on high WOB and RPM downhole motor runs is also presented.