Three-dimensional geomechanical model of creep behavior on wellbore casing in the composite salt–gypsum formation

According to statistics, a certain proportion of the world’s oil and gas resources are stored under the salt rock in the sedimentary basin. These reservoirs are located in formations called “pre-salts,” which are below the salt formations. They are widely found in the Sichuan, Tarim, and Changqing oilfields and in West Africa, offshore Brazil, the Gulf of Mexico, and the North Sea. The salt–gypsum layer exhibits a rheological property, which has a significant influence on casing damage. Creep deformation of the composite salt–gypsum formation occurs over time. Moreover, it applies excess stress on the wellbore casing, which can lead to casing damage. In this study, triaxial experiments are conducted on samples of salt–gypsum composite rock, and a power-law creep constitutive model is created. In the experiment, room temperature and humidity conditions are applied. Three types of loading, including axial and confining pressures, are separately applied on the samples, including the confining pressure (5 MPa) and the axial pressures (20, 25, and 30 MPa). From the experimental results, the relation between creep strain and time can be obtained, and a power-law creep model can be matched as the constitutive equation, which indicates that the strain rate is dependent on differential stress and temperature. The Abaqus finite element method software is used for establishing a 3D geomechanical model with formation stress and wellbore pressure to evaluate the life expectancy of wellbore casing. According to the triaxial rock experiment in the laboratory and according to field data, the mechanical parameters of the salt–gypsum formation and the parameters of the pre-salt reservoir are obtained, respectively. The results demonstrate that the von-Mises stress on the wellbore casing in the salt–gypsum layer increases during the production period. Meanwhile, the creep deformation of the formation also increases and applies extra loading on the casing, which is prone to exceed the strength and damage. Moreover, in this research, the discontinuous stress and deformation at the interface of the composite salt–gypsum formation, on which few researchers have studied until now, can be quantitatively described. The results indicate that the stress on the wellbore casing in the pre-salt layer is different, that is, the stress abruptly changes at the interface between pre-salt and salt–gypsum formation. The results also show the discontinuous creep deformation at the interface and that the casing is prone to shear collapse. Considering the sudden change in the stress at the interface of the formation is important. These results can be utilized to establish a casing design and for the analysis of wellbore integrity. For instance, the composite salt–gypsum layer should pass through the design of the double-layer casing and enable the thick-walled casing to increase its strength.