Sealing integrity in faulted underground gas storage: A case study of the Nanpu-1 gas storage, Eastern China

Underground gas storage (UGS) is one of efficient way for natural gas peak shaving regulation, with which sealing integrity during the gas injection and withdrawal is the key issue for the UGS operation. In this study, a one-way geomechanical modeling as well as rock mechanical test was performed in the Nanpu-1 depleted oil reservoir which located in the Eastern China Nanpu sag. The modeling is coupled with a dynamic model to investigate the geomechanical behavior of faulted reservoirs during gas injection and production as well as estimate the reservoir and caprock integrity and evaluate the risk of fault reactivation. The results show that horizontal stresses and formation pressure in the sandstone reservoirs are distinctly affected by the gas storage while change slightly in the caprocks. The caprocks have a good mechanical integrity before and during the UGS construction, with the shear safety factor ranging from 0.35 to 0.62. The slip index indicates that faults in the UGS is stable with the FSI value less than 1.0 even at the maximum pressure. Furthermore, the critical formation pressure for the fault activation is estimated as 34.5 MPa. The study implies that the Nanpu-1 UGS can be safely operated at least 20 years without any risk from the caprocks or the cross-cutting faults. The tight caprocks are insensitive to the pressure changes in contrast with the reservoirs due to the weak hydraulic communication. In addition, evaluation of fault reactivation should overall consider stress state along fault planes in the whole UGS.