Autodrillers, Rig-Control Systems, Driller Procedures Reduce BHA Failures

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 214997, “The Role of Autodrillers, Rig-Control Systems, and Driller Procedures in Reduction of Bottomhole Assembly Failures,” by Pablo E. Barajas, SPE, Paul Pastusek, SPE, and Aaron Lacey, ExxonMobil, et al. The paper has not been peer reviewed. _ Reducing bottomhole-assembly (BHA) failures has been this operator’s focus in US land intermediate and lateral sections to reduce cost per foot. BHA failures can be mitigated with better autodriller processes, improved rig-control-system technology, and drilling-mechanics-based procedures. The operator, rig contractor, and BHA service providers engaged in an intensive continuous improvement process for 18 months to help two rigs in Oklahoma reduce the number of BHAs used per well. The goal was to determine how to best tune the autodriller and standardize set-point practices. Introduction The operator has found that rig-control systems, particularly the autodriller, can be unstable in many drilling situations. The operator and contractor laid out the framework to measure autodriller dysfunction suspected to affect rate-of-penetration (ROP) and BHA integrity. The team engaged in a structured and intensive continuous improvement process with the following objectives: - Understand if end-user tuning and set-point practices from the contractor were being applied consistently at the rig floor level - Evaluate if the tuning practices were appropriate or effective in mitigating dysfunction in different formations - Increase subject-matter experts’ support and involvement in proactively identifying when rig-control systems were not behaving as designed to reduce impact on ROP and the BHA - Evaluate whether the operator’s drilling roadmaps and BHA safety-limit protections were being properly integrated into autodriller control loops - Use downhole sensors to understand how surface controls induced dysfunctions and confirm when they were mitigated - Propagate learnings across all rigs and crews in the basin to reduce the number of BHAs required to drill each well Observations Autodriller Dysfunction Identification. Standardizing the identification of dysfunction patterns to be avoided was a step based on physics of the system, bit-run performance and dulls (Fig. 1), directional tool damage, high-frequency (1,024 samples per second) downhole data, and high-frequency (six samples per second) surface data. Currently, the focus is to quickly identify and mitigate autodriller dysfunction by the rig teams, including the types of dysfunction described in this section. Stick/Slip. The operator developed a test to run the autodriller in different control loops to determine when stick/slip was initiated or intensified by the autodriller. Restarting drilling in ROP mode with the rig’s torsional dampening system active eliminated the autodriller as the source of stick/slip. It was determined that excessive weight on bit (WOB) used to pass the tool joint through the rotating head in the managed-pressure-drilling’s rotating control device (RCD), combined with low rotational speed, were key factors in initiating stick/slip. WOB Dysfunction. Any significant WOB variation can drive enough torque variation to drive stick/slip. This is particularly true if the WOB variation has a frequency close to the natural torsional frequency of the system.