Improving Reliability and Performance of Under-Reaming Operations
Authors: Valbuena Franklin , Belloso Andres , Pessier Rolf , Oueslati Hatem 
Published: March 2017

Abstract
In reaming-while-drilling (RWD) operations it is critical to optimize the bottom hole assembly (BHA) design to reduce non productive time (NPT) due to a high risk of downhole tool failures or invisible lost time (ILT), which leads to inefficient drilling. Optimizing hole-opening BHA’s is a challenging task since conventional methods are often focused on partial solutions and incremental improvements. A new method is proposed on how to systematically review and analyze drilling and dynamics data from the surface and particularly from above and below the under-reamer. Done in real time or in preand post-well analysis it could provide insights into the root causes for failures and poor performance.
The study presented in this paper is a reevaluation of data from two test wells drilled in the past on a full scale experimental rig. The two wells were drilled using the same reaming-while-drilling BHA, but different pilot bits, one with and one without depth of cut (DOC) control. Sensors to measure weight, torque and vibration were placed above and below the under-reamer to measure the difference in the dynamics.
Another important factor is the mass of the pilot BHA below the under-reamer. The distribution of weight, torque and dynamic behavior were analyzed taking into account the difference between the weight applied to the pilot and the weight available below the under-reamer.
When the weight applied to the pilot is less than the weight below the under-reamer, the pilot BHA is not fully in compression, and a portion of the BHA is in tension. This tension in the pilot BHA (just below the under-reamer) acts as an additional weight applied to the cutting structure of the under-reamer which increases the torque and significantly changes the dynamics both above and below the under-reamer. Sensors located above the under-reamer do not measure this tension force; it must therefore be added to the weight on reamer values measured above the reamer to capture the true loads on the under-reamer.
Cross plots show a strong correlation between under-reamer torque oscillations and axial bounce at the pilot bit when lower weight is applied to the pilot bit. Axial vibrations are not common when drilling with polycrystalline diamond compact (PDC) bits; this indicates the dysfunction comes from the interaction of the under-reamer with the formation and the tension in the pilot BHA.
The same methodology was used to analyze field data from offshore wells where sensors were placed below the under-reamer. When the load on the pilot bit was dropped below the weight available below the under-reamer the torque on the pilot bit dropped proportionally while the torque on the under-reamer roughly tripled, this same phenomenon was observed in the tests on the experimental rig. At the same time the severity of the axial vibrations increases exponentially as the weight on the pilot bit drops further below the mass of the pilot BHA.
This paper outlines an analytical methodology that operators and service companies can use to prevent operating in an inefficient and high risk drilling mode. It can be used in the planning stage, real time drilling and post-well analysis. Using offset data, the information can assist in making better decisions in bit selection as well as drill string and BHA design. In real time with sensors below the under-reamer the drilling parameters can be optimized to keep the pilot BHA in compression and prevent destructive dynamics to lower NPT/ILT.