NOV Downhole has developed a new approach to systematically model the potential for vibration in a given drilling application using computational and analytical methods. This approach utilizes a custom set of software tools to evaluate the drillstring natural frequencies.
The software allows the user to construct a finite element model of the Bottom Hole Assembly (BHA) and drillstring system using geometric and material definitions of the individual components that make up the entire drilling system. The software also permits the user to define a three-dimensional well path. Additionally, the user defines specific application variables that are used by the software as the calculation routines run, such as the drilling fluid densities and Weight-On-Bit (WOB) ranges for the drilling intervals evaluated.
In these calculation routines, the software identifies the operating RPM's at which the BHA and drillstring system will resonate in the axial, lateral, and torsional directions at a variety of depths along the well path and with the different WOB ranges specified by the user.
A case study will be presented where the software package was used to identify resonant vibration as the cause for a catastrophic drillstring failure. The drillstring model in the software was further validated on the following well by data from a downhole dynamics recorder. For the next well drilled by the Operator, vibration "roadmaps" were created using the software package to clearly represent high risk resonant vibration zones for use at the rigsite. Rig personnel were trained on the application of these roadmaps and drilling parameters for all sections on the well were selected based on this information.
As a result of this pre-drilling analysis and real-time application high risk vibration zones were avoided and the drillstring did not experience the large amplitude resonant vibration observed on previous wells, reducing the risk of drillstring failures.
Drillstring vibration has been identified as a leading cause of drilling inefficiency and Non-Productive Time (NPT). Excessive drilling vibration can result in such problems as downhole tool or electronics failures, increased fatigue rates, twist-offs or other mechanical failures1,2.
Historically, drilling vibration problems have been addressed with basic sets of rules associated with general drilling practices. These can include reduced bit and drilling parameter aggressiveness, changes to the drillstring such as adding complex tools to absorb the impact of vibrations, altering the design of individual components, or in extreme cases the removal of expensive components liable to failure. More recently, the use of real-time monitoring for reassurance has become commonplace. Most of these methods have negative consequences including reduced ROP and increased cost and do not often address the root cause of the problem.