Three-dimensional elasto-plastic modeling of wellbore and perforation stability in poorly consolidated sands
A three-dimensional numerical model was developed to simulate the stability of wellbores and perforations in poorly consolidated sandstone formations. The model integrates the post-yield plastic behavior of granular materials in order to investigate the mechanical instabilities associated with wellbores completed in such formations. Fluid flow and poroelastic stresses are computationally coupled with mechanical calculations to generate pore pressure and stress distribution in the sand. The sand erosion model developed by Kim (2010) is adopted to predict the rate of sand production based on the proposed erosion criterion. It has been widely reported in the literature that sanding can be greatly influenced by in-situ stress anisotropy, completion geometry, wellbore placement, and perforation orientation. Through advanced modeling and meshing techniques, the model developed in this thesis is capable of simulating complex completion configurations and operational conditions for the purpose of researching the impact of these factors on the wellbore and perforation stability. Accordingly, the model can be utilized to design a completion that minimizes sand production and optimizes the mechanical stability of the wellbore for a specific in-situ state of stress. Results obtained from the model show that vertical wellbores produce less sand compared to horizontal wellbores in the case where the overburden stress is the maximum in-situ stress. In addition, orienting the perforation in the direction with the least plastic strain development results in a more stable perforation tunnel with less sand production. Therefore, in a horizontal wellbore, vertically oriented perforations are more stable than horizontally oriented perforations and can withstand higher drawdown pressure before sand is produced. The model was extended to simulate the impact of mechanical and hydraulic interference from adjacent perforations on the evolution of plastic strain. Results from simulation runs show that the perforation spacing has an influence on both the magnitude and the spatial spread of the plastic strain. The model combines the effect of the wellbore diameter, shot density, and the phasing angle to determine the completion configuration with the least sanding risk.