Objectives and Scope: Injecting carbon dioxide (CO2) into the formation results in fluid pressure accumulation, leading to changes in the effective stress field. Stress changes can activate preexisting faults, compromise sealing behavior of caprock, and trigger ground surface uplift. Therefore, conducting mechanical analysis and stability assessment is crucial for the successful industrial implementation of CO2 geological storage.
Methods, Procedures, Process: The Gao89-1 block of Shengli Oilfield is the target site in this study. A full 3D field model of Gao89-1 block was constructed using hexahedral finite elements. The changes in fluid pressure and the stress evolution during CO2 injecting were simulated and analyzed. Caprock stability and fault reactivation were assessed based on Mohr-Coulomb criterion. For caprocks, a safety factor was employed to assess the trend of failure. Similarly, the stability of faults was determined using the Coulomb failure function.
Results, Observations, Conclusions: To accurately simulate the stress distribution, it is essential to convert the corner-point grid of the target geological model into a standard finite element mesh. This process involves reading the target reservoir model and faults data, aligning the vertices of grids, handling degenerate grids, assigning valid depth to invalid grids, geometric optimization, domain extension, and fault embedding or equivalence. As CO2 is injected, both fluid pressure and shear stress gradually increase injection time, while the mean effective stress gradually decreases. Consequently, the stress state tends towards the failure envelope, increasing the risk of shear failure. However, once the injection is halted, the stress state moves backward, but stabilizes to the left of the initial stress state. Through calculating the safety factor, it has been determined that the formation remains in a relatively safe and stable state, with an expected absence of shear failure. This favorable condition can be attributed to the implementation of a relatively low injection rate and the careful selection of simulation parameters.
