Casing is designed to provide well bore stability and withstand imposed loads such as external pressure, internal pressure and mechanical load due to compressive forces on casing. To ensure integrity and reliability of casings, a cost effective approach is hereby presented using finite element (FE) methods. At ordinary conditions of temperature and pressure, casing design can be achieved by the use of available grades of casing, but for HTHP wells it becomes critical to investigate the integrity and reliability of casing grades provided. Thus, numerical methods such as FE become invaluable as opposed to standard casing selection procedures. This work models stability of casing in the wellbore using a commercial industry accepted FE package – Abaqus 6.12. The model was developed using simple plane stress and plane strain elements with corresponding boundary conditions. Various mechanical properties of steel casing, concrete, surrounding rock and interfaces were modeled to mimic a full scale industry scenario. The model investigated casings under HTHP, mechanical failure criteria in terms of elasticity, plasticity of material, geometric non-linearity and compared with other existing analytical models in literature in other to validate the model. It was observed that the model becomes highly unstable under HTHP environment, hoop and radial strains in the wellbore casing increased as the temperature and pressure of the transported fluid in the wellbore increased. Areas of stress concentration in the casings can easily be observed in the model and it shows an efficient procedure for investigating the stability of casings in harsh conditions. Casings were subjected to HTHP to predict optimum configurations for different scenarios.
Casing failure has become a common occurrence especially in injection wells, which often affect development objectives of intended reservoirs. Stresses induced by injection of fluids (e.g. water, CO2, steam) in a well may affect the integrity of the casing, cement and surrounding formations. Thus, a simplified and reliable assessment of the thermal stresses induced in casings are necessary in predicting the integrity of well in complex and unconventional production activities. The injected fluid cause fracturing of the formations and this induces local compression and tensile stresses in the various casing strings landed and cemented in the well. This work presents a quick and cost effective procedure for obtaining appropriate grades of casing using finite element (FE) method. The model is developed for quick assessment of well integrity using a well-known commercial FE package- ABAQUS 6.12-1. The ABAQUS’ plane stress and plane strain elements with associated material properties were used to model the thermal stresses in casing, cement and near well bore. The effect of varying the thickness of casing was investigated and an optimum thickness corresponding to various fluid conditions was obtained. The developed model was compared to existing models in other to assess the accuracy of the model. A parametric study was conducted on the effect of casing dimensions on the stresses and strains developed in the steel casing. The results of the study showed that strains and stresses on the casing vary directly with increasing temperature and pressure of transported fluid in the wellbore casing. A strain based failure criteria was developed to aid the well design engineer in selecting appropriate casing thickness and diameter. The model presented shows a quick and reliable procedure for obtaining dimensions of casing subjected to unconventional environments.
High temperature variation in wells used in thermal oil recovery processes, deep gas wells, offshore wells with significant riser lengths and wells completed in abnormally hot reservoirs cause compaction of formations and this induces local compression and tensile stresses in the various casing strings cemented and landed in the well. Cemented wells are under various stresses and strains in various directions which can result in significant deformations and fracture. Therefore, casing failure or damage is inevitable largely due to thermally generated strains and stresses in the casing and the surrounding formation. This research studied the effect of the non-uniform loading (poroelastic effect) on stresses and failure of casing and cement by developing a Finite Element (FE) method for the analysis of temperature-stress interaction in three dimensions (3-D) for casing, cement and formation with all its complexities using ABAQUS simulation tool. The tool was used to develop temperature distribution and thermal stresses in the casing and near wellbore formation. This enabled possible casing failure predictions and provided theoretical basis for casing thermal stress failure by coupled temperature field and thermal stress field of casing.
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