Development of a fully coupled approach for evaluation of wellbore stability in hydrocarbon reservoirs.

Problem statement: When a well is drilled in a hydrocarbon reservoir, the original thermodynamic conditions are altered, the natural stresses are redistributed and a stress concentration occurs around the hole. The alteration of the original equilibrium can lead to wellbore damage, sometimes to its complete collapse. Loss of time associated with stability problems is estimated to account for 12-15% of drilling costs world-wide. Approach: The adoption of a reliable modeling approach to predict instability due to time-dependent alteration of natural equilibrium is fundamental for the optimization of drilling plans, completion design and production activities. Results: In this study the possibility of investigating instability phenomena in terms of both stress-strain and thermodynamic formation behavior through a fully coupled thermo-porous-elastic-plastic approach is demonstrated. According to the fully coupled approach, porous flow, temperature development and stress-strain calculations are performed together: the whole system is discretised on one grid domain and solved simultaneously for both the thermodynamic and the geomechanical variables. For the plastic analysis implementation, an iteratively coupled approach was adopted inside the fully coupled routine: the model basic equations (porous flow and rock deformation) and the plastic behavior equations were solved separately and sequentially at each non-linear iteration. The iterative coupling approach corresponds to an implicit treatment of the plastic variables, essential to preserve the stability of the elasto-plastic solution. The key points of the model analytical formulation, of the numerical formalization as well as of the implementation of the adopted solutions to make the thermo-porouselastic-plastic model applicable to assess wellbore stability are presented. Conclusion: The proposed model was first validated and then applied to several synthetic and real cases. In this study the effectiveness of the developed model to investigate the potential impact of instability phenomena on the well drilling design is demonstrated also by discussing the results from a case history.