Proper Bit and Underreamer Synchronization Concept Delivered Excellent Performance in GoM Deepwater Well
Authors: Radford Steven Ralph , Hafle Mark Edwin , Ubaru Charles C. , Thomson Ian James , Morel Brian 
Published: July 2010

Abstract
Nowadays, one of the greatest deepwater drilling challenges is maximizing drilling efficiency while mitigating vibration dysfunctions when drilling and underreaming through salt and sub-salt formations. Historically, vibration has been responsible for considarable non-productive time (NPT) related to bottom hole assembly (BHA) twist-offs, downhole tool failures and premature bit and underreamer wear.
With the high operating costs associated with deepwater sub-salt wells, operators have increased their focus on improving drilling efficiency by mitigating harmful vibration. For years, bits and underreamers were designed and selected independently, not as part of a system. This tradition has contributed to drilling dysfunctions that cannot be controlled by drilling parameter management that can potentially lead to drilling failures. Through advances in technology and drilling practices over the past two years, operators and service companies have realized that optimum synchronization between bit and underreamer is critical and one very effective way to achieve operators drilling efficiency objectives.
This paper focuses on significant performance improvements achieved while drilling inter-bedded formations, salt and sub-salt sections in a deepwater well in the Gulf of Mexico Green Canyon area. In this case study, the operator deployed a concentric underreamer coupled with a fit-for-purpose bit, designed for this type of application. The proper selection of bit and underreamer and the implementation of drilling best practices allowed the operator to drill smoothly, limiting harmful vibration levels, even through lithology transition zones, including the salt exit. The drilling mechanical specific energy (MSE) was greatly reduced and the rate of penetration was significantly increased when compared to offset wells previously drilled in the area. This paper will identify, describe and discuss the factors leading to the creation of a smooth drilling environment, reduction in MSE, higher rate of penetration (ROP) and lower costs on this well.

Introduction
As the economics of drilling and completing wells in the deepwater Gulf of Mexico environment become more challenging, operators are seeking ways to maximize their reservoir recovery rates while minimizing non-productive time. This is even more critical with daily drilling spread costs in excess of $1 million dollars.
To reach the reservoir with the optimally sized production casing, as well as to address other drilling problems including equivalent circulating density (ECD) limitations and swelling shales, more operators' drilling programs recognize a growing need for reliable concentric expandable devices. These reamers allow users to enlarge the hole below the last casing shoe so that tighter tolerance casing strings can be run. A key to reducing cost is being able to concurrently drill and expand the hole section in a single run.
This paper describes the deployment of a newly introduced concentric underreamer coupled with a fit-for-purpose bit in the fourth of a series of exploration and appraisal wells in the Green Canyon area of the Gulf of Mexico with water depths over 4,000 feet. Drilling performance on this well is then compared to the previous two wells and clearly shows significant improvement in ROP, reduced MSE and lower vibration levels using this new concentric reamer and properly matched PDC pilot bit.