Many casing failure incidents have been reported in oil and gas fields around the world. These casing failure events can occur not only within reservoirs but also in surrounding formations. Engineers must evaluate risks of casing failure when drilling and completing wells especially in highly compacting reservoirs. However, one of the challenges encountered during the evaluation of casing failure risks is that field-scale stress changes and displacements as a result of drilling wells and producing hydrocarbon from reservoirs must be properly taken into account for casing stability analysis. The objective of this study is to develop an efficient integration method for large-scale reservoir compaction and small-scale casing stability analyses for the evaluation of casing deformation and failure.
The numerical model developed in this work is based on 3D elasto-plastic finite element method (FEM). Reservoir compaction and subsidence are analyzed using a large-scale FEM model considering details of geological settings while casing stability is analyzed separately by a small-scale FEM model. The two FEM models are integrated by interpolating displacements calculated by the large-scale model and assigning resultant displacements for boundaries of the small-scale casing stability analysis model. The validation of the proposed integration method is also presented in the paper.
Our study results indicate that the integration method presented in this paper significantly improves computational efficiencies on an order of 5 times faster than the conventional simulation method that requires a large number of finite elements for reservoir, surrounding formations, cement, and casing. Also it is demonstrated that the integrated model can be applied to inclined wells completed in highly heterogeneous formations at sufficient accuracy. The field case study also indicates that the risk of casing deformation highly depends on its inclination and the position relative to the compacting formation.
The small and large scale coupling method developed in this work helps engineers evaluate casing deformation and failure in various locations in reservoir and surrounding formations in an efficient manner and also develop safe and efficient drilling and completion programs to reduce risk of casing mechanical problems.