Estudo numérico de ensaios triaxiais aplicado à perfuração de poços em rochas salinas

Abstract

This work presents a numerical study about triaxial creep tests applied to wellbore drilling in salt rocks using the Finite Element Method. The discovery of the pre-salt brought numerous challenges to the oil industry in Brazil, including the need to drill wellbores in thick layers of salt rocks. These rocks have almost zero permeability and porosity, facilitating the containment of hydrocarbons. However, they also present the creep phenomenon that can cause the wellbore closure during drilling. Thus, the development of constitutive creep models for salt rocks and their calibration from the results of triaxial creep tests is important, as these models are used in computer simulations of drilling wellbores through these rocks. One of the main constitutive models applied to Brazilian salts is the double mechanism of deformation that considers only steady-state creep. Models are found in the literature that enrich the double mechanism, proposing alternatives to describe transient creep. In this context, it is observed that the constitutive models applied to Brazilian salts do not consider accelerative creep, leading to underestimated strains, mainly in higher strains scenarios. In the study of accelerative creep, it is common to introduce a damage variable. In addition, triaxial creep tests under certain levels of confining and deviatoric stresses, these models do not reproduce the experimental behavior well, even for transient and steady-state creep. To achieve the proposed objective, the methodology adopted is based on four macro steps: i) studies of constitutive models for salt rocks calibrated to Brazilian salts and with coupled damage; ii) numerical modeling of triaxial creep tests; iii) implementation and verification of constitutive models for salt rocks; iv) application of the implemented constitutive models. Based on the studies carried out, a constitutive model with potential to address the deficiencies of the models used in Brazilian salt rocks is identified. The main contribution of this work is the comparison of different constitutive models for salt rocks in the computational modeling of different scenarios of triaxial creep tests.