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Wells that are used to recover hydrocarbons from reservoirs are subjected to large amounts of complex geomechanical loads that may compromise the well's integrity over its operating lifetime. While the reservoir depletes, the pore pressure in the rock reduces and the effective stress in the rock formation increases. As a result, the rock deforms and the reservoir experiences an overall volume reduction (compaction). Under such compaction, any well casing experiences a substantial amount of compressive and shear stress/strain in all directions. In order to ensure safe operation of a well, it is necessary to impose operational limits (drawdown and depletion) so that stresses imparted on the casing do not exceed the casing strength. This problem has been studied in the past [1,2,4,6,8,10] and due to computational complexity and cost, past methodologies have used simplified analytical solutions or decoupled modeling by separating the large scale deformation of the rock (3D model) from the casing behavior (2D model). These simplified methodologies rely on choosing an a priori failure mechanism and also make assumptions on how the load transfer happens between the rock and the well hardware. Based on our experience, this approach is conservative and leads to poor approximations. In this study, a fully detailed two-scale 3D Finite Element (FE) model is used to simulate various drawdown and depletion combinations derived from a reservoir model simulation. The large-scale model captures the far-field effects of drawdown and depletion as well as the effects of neighboring producer wells. The small-scale model includes details such as casing, cement, perforations, and surrounding rock. This approach is not constrained to a specific failure mechanism (axial compaction, pipe collapse, bending or shear), but rather includes all possible deformations based on rock mechanical behavior, stress, pressure change, and geometry. Thus, complex casing deformations with different failure locations can be observed. This technology has been applied to a number of business units within ExxonMobil, allowing us to establish practical drawdown/depletion limits. In this paper, we discuss a field example of this approach. |
