Drilling and completing wells in deep salt stratum are technically challenging and costing, as when serving in an incomplete borehole in deep salt formation, well casing runs a high risk of collapse. To quantitatively calculate casing remaining strength under this harsh condition, a three-dimensional mechanical model is developed; then a computational model coupled with interbed salt rock-defective cement-casing and HPHT (high pressure and high temperature) is established and analyzed using multiphysics coupling FEM (finite element method); furthermore, open wellbore shrinkage and casing remaining strength under varying differential conditions in deep salt formation are discussed. The result demonstrates that the most serious shrinkage occurs at the middle of salt rock, and the combination action of salt rock creep, cement defect, and HPHT substantially lessens casing remaining strength; meanwhile, cement defect level should be taken into consideration when designing casing strength in deep salt formation, and apart from the consideration of temperature on casing the effect of temperature on cement properties also cannot be ignored. This study not only provides a theoretical basis for revealing the failure mechanism of well casing in deep complicated salt formation, but also acts as a new perspective of novel engineering applications of the multiphysics coupling FEM.
Drilling and completing wells through complex salt formation is technically challenging and costing. Field data demonstrates that well casings designed by traditional safety coefficient criterion occurring failure in deep salt formation though their safety factors are greater than 1. To reveal the failure mechanism, a probabilistic computational model coupled with salt formation, defective cement and worn casing is established and analyzed using Monte Carlo simulation method. On the basis of reliability theory, the results calculated by 5000 times simulations show that the traditional safety coefficient criterion has been unable to adapt to the safety assessment of well casings under salt creep conditions. To gain a sophisticated evaluation, a new assessment criterion is established and applied to assess the security of well casings under salt creep conditions. This study provides a new perspective for revealing the failure mechanism and solutions of evaluation on well casings under salt creep conditions, which may be an alternative method to study and predict the life of well casings in deep complicated formation.
Drilling through and completing wells through deep and acid environment regions are technically challenging and costly. Based on twin shear unified strength theory (TSUST), a new high collapse (HC) oil country tubular goods (OCTG) collapse strength model, involving the manufacturing imperfections and significant anisotropy of the material, was developed in this paper. Comparisons of numerical calculations with full-scale test collapse data show that the new HC OCTG collapse strength model gets higher calculation accuracy for predicating HC OCTG collapse strength than both American Petroleum Institute (API) Bulletin 5C3 and ISO/TR 10400. Thus, the new HC OCTG collapse strength model will provide a more scientific method and exciting possibility for deep and acid environment wells design and construction.
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