Downhole measurement systems have reached an impressive level of accuracy and reliability. However, the predictive behavior aspect of complete drilling systems, whether it be vibrational, or directional until recently has not received the same focus. The two key components affecting the overall dynamics of a drilling system are the effect of the cutting structures of drilling tools, (drill bits, underreamers etc.) and the behavior of the BHA and drill string. Typically, these aspects have been modeled in isolation, or rudimentary assumptions are made to approximate the effect one on the other. To more accurately predict the performance of the drilling system, it is essential to consider the sum of all forces affecting the system. This must include accurate modeling of the drill bit cutting structure, based on actual bit design, rock properties and drilling parameters. The effect of these forces must be coupled with the forces generated by the BHA and drilling string, and the interaction of these forces should be modeled and understood. These issues have significant cost implications for the operator but in both cases are complex to model. A system approach is required to achieve true optimization of these two historically separate areas of drillstring design.
Typically, drillstring analysis programs currently available to the industry are run off personal computers with limited random access memory capabilities and are therefore restricted to modelling only the BHA portion of the drillstring. For the same reason bit effect factors that significantly influence drillstring behavior are at best assumed.
Utilizing a suite of Finite Element Analysis, (FEA), based programs running on a Unix platform; four dimensional, time based simulations of the complete drilling process are now possible. The exact interaction between the bit and the formation drilled are modelled using lab derived rock mechanics. The bit and and every string component are modelled individually.
The system generates the behavior of the entire drillstring, from the cutting edge of the bit to surface. The modelling accurately predicts the vibrations and accelerations often seen to have detrimental effects on directional control, tool reliability, drill string integrity and drilling performance.
The ability to pinpoint the source and effects of torsional, axial and lateral oscillations enables users to qualify design changes to the drillstring configuration and optimize parameters, prior to drilling the well, so eliminating the traditional "trial" and "error" approach and operator risk.
Introduction
The use of Finite Element Analysis, (FEA), has its beginnings as far back as 1943 when it was employed by Richard Courant who used the Ritz Method of variational calculus to analyse vibration systems. Many papers and articles were published in the mid 1950s on the subject, including the widely regarded classic Stiffness and Deflection Analysis of Complex Structures published in 1956 by Turner, Clough, Martin and Topp.
By the early 1970s, FEA was gaining ground but really only in the domain of the supercomputer users, principally the aerospace, nuclear and defense industries. This was a time of algorithm refinement and improvement. The rapid advance of computing power and the even more spectacular reduction in computing cost would subsequently lead to more widespread use of FEA.
More recently, non-linear and time domain modelling solutions have become possible. This is critical to achieving the required level of accuracy to make meaningful contributions to drilling system behavioural prediction. While the Finite Element Method has been used for a number of years in the oilfield, it has been limited to sections of the drillstring or discrete components. Solutions for the entire drilling system are exceptionally time consuming and complex both in setup and computational time. These factors are considered when planning the length of drilling intervals to be analysed.
