Significant performance improvement has been achieved by successfully managing drilling vibrations through bottomhole assembly (BHA) redesign. This effort has resulted in increased footage per day and reduced tool damage. Prior literature has described improvements in operating practices to manage vibrations(1,2) as a key component of this ROP (rate of penetration) management process. In a parallel work activity, the redesign efforts have provided additional performance improvements of approximately 36% in one drilling application. Dynamic modeling of the BHA has identified the key design changes leading to these improvements. The redesigned BHA has lower calculated vibration indices than the standard BHA.
The BHA design evaluation process uses a frequency-domain lateral dynamic model in both pre-drill forecast and post-drill hindcast modes. BHA lateral vibrations are characterized such that alternative BHA configurations may be developed and compared directly with a proposed baseline assembly. In the hindcast mode, the BHA model can be operated at the recorded WOB and RPM to generate corresponding model results in time or depth, and these values can be compared to the measured performance data.
In one case study, the redesign of a BHA with downhole motor and roller reamer is described, with corresponding field data for four original BHA's and four redesigned assemblies. In a second application, model and field drilling results for two rotary steerable assemblies are compared to evaluate the predictive ability of the model in smaller hole size and with different BHA types. Finally, the utility of the model to identify preferred rotary speed "sweet spots" is demonstrated in a motor BHA operating in larger hole.
Two prior publications describe the basic methodology that has been developed to model BHA lateral vibrations. The first paper(3) provides a general description of the model and presents case studies of four field applications of this model. The second reference(4) is a study of 13 BHA runs in the same field, for which slightly different BHA designs and operating parameters were used. The Appendix of the second paper comprises a detailed mathematical description of the basics of this frequency-domain lateral vibrations model, known as VybsTM. The present paper illustrates the application of these methods to a new set of BHA design problems in a joint study conducted by RasGas and ExxonMobil.
Briefly, the modeling process begins with an input panel that is populated with mechanical dimensions of the components of the BHA, usually up to the heavy-weight drillpipe (HWDP), with about the same level of detail as a fishing diagram. It is important that the positions of the contact point constraints are entered correctly, and that the stiffness and inertial properties of the assembly are a proper representation of the subject BHA. Then the desired operating parameters for drilling need to be provided, including the anticipated ranges of bit weight (WOB) and rotation rate (RPM).
The linear modeling process considers a dynamic perturbation about the static state. The model employs two vibration modes to compare and contrast the response of each candidate BHA design: lateral bending and twirl. In the lateral bending vibration mode, an identical reference bit side force input is applied to each design, and the magnitudes of the response at other locations along the BHA are compared. In the twirl mode, an identical mass eccentricity is applied to each model element to investigate the stability of the BHA to eccentric mass and centrifugal force effects.