Utilizing Downhole Sampled High-Frequency Torsional Oscillation Measurements for Identifying Stringers and Minimizing Operational Invisible Lost Time ILT
Authors: Hohl Andreas , MacFarlane Danial , Larsen David Selvaag , Olsnes Kjetil , Grymalyuk Sergiy , Gatzen Matthias , Hovda Sigve 
Published: September 2021

Abstract
Horizontal drilling has been the industry standard for oil production wells in the North Sea for decades. Significant improvements have been made in the precision of directional drilling by rotary steerable systems (RSS), nevertheless there remain opportunities to mitigate operational challenges in complex drilling environments.
One such challenge is the occurrence of hard stringers interbedded between soft sandstone and limestone formations within the reservoirs. The interaction between the bit and hard stringers at the interfaces can lead to a deflection of the bit, resulting in high local doglegs (HLDs), and excessive static loads unless mitigation actions are triggered in a timely fashion. Operational parameters have to be adjusted during hard-stringer drilling, but are also constrained in the underlying formation to avoid HLDs and guarantee bit and BHA integrity.
The key to efficient stringer drilling presented here is a consistent, timely and reliable method of detecting stringers. This is enabled by a fit for purpose stringer detection algorithm embedded in a measurement-while-drilling (MWD) tool for vibration and load measurements, combined in a systems approach with an automated surface system.
Different indicators such as vibrations, loads and ROP that are traditionally used for stringer detection have been analyzed in the development phase of the algorithm. High-frequency torsional oscillations (HFTO) have been found to be a leading indicator for stringer drilling: HFTO is a torsional vibration phenomenon with high frequencies (50Hz-450Hz) and is only excited by the bit-rock interaction in hard formations. The HFTO amplitudes in sand/lime stones and calcite stringers show well separated distributions.
Finally, HFTO is unique in that it is not directly affected by the driller, or due to other downhole dysfunctions, e.g. compared to a change in weight on bit (WOB) which may be caused by a surface parameter change or a stabilizer.
The physics-based algorithm embedded in the MWD tool combines tangential acceleration and dynamic torque measurements to calculate the maximum HFTO load in the BHA. A stringer is identified if an HFTO maximum amplitude threshold is exceeded and the energy is localized in one frequency. The downhole indicator is aggregated to a 1-bit value (stringer/no stringer) that enables a high telemetry update rate and thereby a timely reaction at surface.
The stringer indicator and advice are displayed to the driller and are actively used for stringer drilling. The paper describes the technology as well as the operational setup, and experience from the first field deployments.
By using the new technology, the driller can react faster to any stringer and use appropriate parameters to avoid costly HLDs. First field deployments demonstrate a significant improvement in invisible lost time (ILT) caused by deflections of the bit, resulting in a considerable reduction in well delivery costs.