How to Design a BHA Rotary Speed Sweet Spot

Abstract

Abstract Correct placement of the rotary speed "sweet spot" of a bottomhole assembly (BHA) provides multiple benefits towards achieving the ultimate goal of drilling to section total depth in a single trip. Several field case studies will demonstrate the modeling approach and benefits achieved by selecting a preferred rotary speed range and designing the BHA contact points accordingly. BHA lateral dynamic modeling is now applied pre-drill in nearly every well the operator drills. In most applications, a proposed BHA design is solicited from the directional service provider with the required MWD/LWD tools for the hole section. This baseline configuration is entered into the model. Alternative configurations are then obtained by moving "contact points" through either stabilizer movement, tool rearrangement, or both. The model results for a suite of such designs are then generated and compared on a single chart, facilitating selection of the BHA that has the lowest vibration indices over the targeted rotary speed range, usually determined by hole size and drill string modeling. In most instances, BHA designs that have been evaluated and run may be successfully reused, provided that the governing assumptions are still correct. Redesign of the planned BHAs for a well is considered to be an important part of the well planning process, as early redesign provides the most flexibility to modify the tool configurations. Changes made to BHAs late in the process are constrained by tool availability and logistical considerations. The process of BHA redesign and "tuning" for the desired operating parameters provides greater control over the drilling process, with fewer surprises resulting from poor drilling dynamics. Gaining more control over drilling dynamics has helped to remove one of the most pernicious variables in the well construction process. In many runs, BHA designs with model-predicted vibrational sweet spots match field observations, and good drilling performance has helped the programs stay on target. Vibration minimization through proper design and operation of the BHA facilitates drilling objectives that include reaching section depth in fewer trips and providing acceptable borehole quality to run casing to depth.