Modelling the Mechanical Integrity of Caprock and Faults to Optimize the UGS Injection Upper-Pressure Limit in XGS Field

Abstract

A finite element model is prepared for the XGS filed located in Sichuan province, onshore China. The model is simulated under different storage pressures to investigate the geomechanical risks associated with depletion and injection operations during the underground gas storage (UGS) seasonal cycles. For this purpose, a detailed analysis is carried out to study the stability of the faults and integrity of the storage and its caprock. The numerical model is designed to capture the complex geometry of the XGS field and the property variation in the 3D space with a focus on the targeted storage layer for the UGS operation. The result of the reservoir dynamic flow simulation is transferred to the geomechanical model to run the finite element simulation. The changes in the stresses and strains are calculated at the end of each coupled simulation step and used for different geomechanical analyses. These analyses are the reservoir compaction, displacement of ground surface, integrity of the caprock and the stability of the natural fractures and faults during the depletion and injection phases of the UGS operation. The targeted storage rocks are stiff with Young's modulus more than 50GPa. The calculated storage deformations are insignificant during the injection and depletion operations. Consequently, the stress changes in the overburden are relatively low. The analyses show that the caprock fails at injection pressures higher than 6MPa above the initial storage pressure. The results of the modelling were used to optimize the upper-pressure limit of the UGS injection for safe operations.