Minimizing Drill-String-Induced Wellbore Instability

Objective/Scope: Every year the petroleum industry spends more than an estimated $6 billion in mitigating wellbore instabilities that account for nearly half of the drilling-related NPT (non-productive time). Researchers believe wellbore instability problems occur primarily due to physical and chemical interactions between rocks and drilling fluid and mostly neglect the impact of drill string vibrations on wellbore stability. However, such vibrations can cause significant damage to the formation, which then degrades the formation's mechanical integrity and compound wellbore instabilities. An appreciable body of evidence exists documenting that higher RPM (revolutions per minute), higher WOB (weight on bit), and pendulum BHA (bottom hole assembly) can cause enhanced agitation of the wellbore wall rock which may result in formation damage - often accompanied with an increased ROP (rate of penetration). The primary objective of this work is to review the state of modelling vibrations as documented in the literature and then advance the development and impact of vibrations and rock failure due to the aforementioned drilling parameters using numerical methods. Methods, Procedures, Process: The proposed research will analyze the complex dynamic interaction between drill-string and borehole using a commercially available finite element package (e.g., COMSOL or ABAQUS etc.). These packages have the capability to create multiphysics-based models and simulate engineering and industry applications. A drill-string borehole assembly will be modelled with geometric and associated environmental and material boundary conditions (e.g., stress state, pore-fluid pressure, mud pressure etc.). The results will then be calibrated with experimental and field data. Results, Observations, Conclusions: Subsurface drilling parameters are often undetermined or not measurable during drilling. The proposed model will aim at developing a drill-string-dynamics model to simulate the interactions between the wellbore and the drill string. This includes (i) estimation of drill string vibrations for different drill string and BHA designs, (ii) the resulting impact forces from drill string vibrations for different wellbore designs, and (iii) the influence of the estimated impact forces on the wellbore wall of particular (chosen) rock types at its current stress or yielding state. Novel/Additive information: The research project proposed herein will equip the well-planning engineers with advanced and robust tools to predict and mitigate wellbore instabilities resulting from drill-string vibrations. We develop new models that incorporate the dynamics mentioned above in order to evaluate quantitatively the effects of drill string vibrations on wellbore instabilities. We anticipate contributions along the following themes: 1. Improved predictive capability of wellbore instabilities for specific drill string and well designs, leading to minimal wellbore rock failure trends for different trajectories, lithology, geologic structure, mud weights (i.e., overbalances) and more. 2. Establish recommendations for optimal drilling parameters (WOB, RPM, and ROP) for specific drill strings and well designs as a function of input parameter uncertainties and variabilities.