Creep constitutive modeling applied to the stability of pre-salt wellbores through salt layers

摘要

Salt rock mechanics has long been recognized as important in potash mining. Currently the petroleum engineering industry in Brazil is facing challenges related to this rock. Discoveries of carbonate reservoirs sealed by thick salt rock layers motivated researchers to consolidate background and workflows. Accessing pre-salt reservoirs means drilling through salt rock layers, hence special methodologies are required to evaluate the stability and integrity of wellbores. Salt rock has a different behavior than common sedimentary rocks, requiring special attention to deformation and dissolution. From a mechanical point of view, creep is one of the most important deformation mechanisms of salt rock in mining and petroleum engineering. State variables such as stress state and temperature are determinant for creep response. A representative constitutive model for creep in salt rock is particularly important in order to predict and prevent operational problems due to excessive displacements, such as the necessity of new drillings, stuck pipe or even wellbore loss. Three power-law-based creep constitutive models are investigated based on finite element simulations:Norton's Power Law, Double Mechanism Creep Law and Multi Mechanism Deformation Creep Law. The commercial finite element code ABAQUS? is adopted as the simulation tool, in which both Double Mechanism and Multi Mechanism creep models are coded as user subroutines. A creep analysis along an interval of pre-salt wellbore located at Campos Basin is carried out to compare the creep laws. Numerical results are compared to caliper data. The performance of the Multi Mechanism Deformation Creep model is highlighted.