A New Approach for Build Rate Estimation of Downhole Motors

Abstract

Abstract Build rate prediction is a critical element in downhole drilling and proper BHA design. This paper presents a simple build rate prediction method that demonstrates improved accuracy for short bit-to-bend (SBTB) motors compared to the industry standard method. The Three-Point Geometry method is the industry standard to calculate the predicted build rates of downhole motors. While this method provides reasonable results for conventional motor designs, it consistently fails to match with field data for SBTB motors. Field reports clearly demonstrate that SBTB motors can achieve significantly higher build rates than predicted by the three-point geometry calculations. A new analytical method has been developed to predict the build rate of conventional and SBTB motors. The new method assumes that as the bit advances, it will drill in the direction it is pointing so that the resulting curved path is tangent to the inclination of the bit. Based on this bit-tangency assumption and a defined upper contact point, the predicted hole curvature can be calculated for a given motor configuration. Results from the Bit-Tangency method provide a more accurate indication of the increased build rates that SBTB motors can achieve. Further, the results of the new method for conventional motors conform to field data and the three-point geometry results. Hence, the Bit-Tangency method can replace the standard Three-Point Geometry method of build rate prediction for both conventional and SBTB motors.