Solving Pre-Salt Underream While Drilling Challenge: Dynamic Simulations Optimize BHA/Cutter Block Configuration Offshore Brazil
Authors: Breton Mario Andres , Barreto Miguel , Hosein Fyaed , Palomino R Dario A. , Mayol Julio , Stewart Michael 
Published: September 2015

Abstract
Challenges while drilling pre-salt wells offshore Brazil have led to planed hole enlargement operations as part of the drilling solution of this type of imviroments in vertical sections from 18-1/8″ x 22-in. However, this application proved to be challenging for the hydraulic underreamer cutting structures as high vibration levels together with increased cutter block matrix exposure to the formation led to premature wear on the hydraulic hole enlarger cutter blocks.
Formation to be enlarged is composed of 40% disaggregated sandstone, 40% shale and 20% mudstones, the objective was to enlarge intervals that ranged from 850 mts to 1000 mts in one run. The BHA was experiencing high levels of shocks and vibrations that forced the operator to make additional trips to finish enlarging the section. These additional trips proved to be costly high with rig rates between $750,000-$1million USD.
Efficiently undereaming while drilling the entire section depth in one trip as planned would be challenging and present a significant risk because of high daily rig-rates. To achieve objectives, a Finite Element Analysis (FEA)-based simulation was performed aimed at improving BHA design and to evaluate a new desing of underreamer cutter block with the objective of improving cutter durability as well as reducing stresses experienced while enlarging the wellbore.
The study used a well records database and numerous offsets runs with a hydraulic expandable underreamer used in offshore applications. A rock-strength identification program, that uses log data including GR and sonic, was applied to determine formation type and foot-by-foot UCS. A hydraulic analysis was also performed to determine efficient hole cleaning as well as ensuring adequate flow distribution to ensure correct functionality of different BHA components, finally a FEA-based modeling system identified the following key points:
The BHA used in previous application had room for improvement by modifying the size of DCs used and by using a concentric expandable stabilizer to improve drilling efficiency and reduce vibration. A new cutter block re- arrangement was designed for the application with changes including: 1) introduction of a three row cutter-block with increased PDC density and optimized stresses distribution; 2) adjusted cutter block nose design for more efficient hole coverage; 3) reduced depth-of-cut without compromising ROP potential; 4) define an optimal operating parameters window with conclusions drawn from results of the FEA-simulations aimed at improving ROP and reduce levels of stresses at the hole enlargement tool.
The new-style BHA was run and underreamed while drilling 734 mts without requiring an additional trip to finish enlarging the section. The BHA also improved ROP during the UWD run. This new system saved the operator between 96-120 hrs of rig time and met the objectives set when compared to offset wells for similar applications.