Assessing the effect of intermediate principal geostress on the caprock integrity for underground gas storage

Abstract

It is critical to evaluate the integrity of caprock for underground gas storage under 3D anisotropic geostress conditions to improve the operation performance. The constitutive model, named as Triple-Mean-Normal-Stress model (TMNS), is embedded into a FEM code to investigate the integrity of caprock for the storage. The results obtained using Mohr-Coulomb, Mogi-Coulomb and TMNS models were compared in analyzing the pore pressure, stress and displacement in the gas storage throughout the injection stage. The results show that the rock at the interface of reservoir-caprock is more prone to damage during gas injection. The vertical displacement and pore pressure at the reservoir-caprock interface adopting Mohr-Coulomb model are 0.021 m and 3.01 MPa higher than those adopting TMNS model, respectively. The integrity of the caprock was evaluated, and the ultimate upper limit pressure for the storage at the critical failure state was formulated. When the caprock adopts TMNS model and uses TMNS damage criterion to determine if shear damage occurs in the rock, the reservoir will receive a greater upper limit pressure. Comparing the upper limit pressure calculated by the TMNS model and the Mohr-Coulomb, it can be found that the TMNS model can effectively improve the upper limit pressure by 13.7% compared with the upper limit pressure calculated by the Mohr-Coulomb model. The research will provide a theoretical foundation for future research on the integrity of caprock for underground gas storage and the geological storage of CO2.