A Geomechanical Approach to Assess the Effects of Stress State on Fault and Caprock Stability in Underground Gas Storage (UGS): A Case Study from Northwest China
Authors: Zhang Ci , Liu Bowen , Wei Leipeng , Kang Liuxu , Xu Shengchi , Wang Shanshan , Ong Hong , Zhou Yongsheng , Yu Huijuan 
Published: February 2024

Abstract
Underground gas storage (UGS) is an important segment in the China gas market. Because of the supply-demand misbalance of natural gas, the gas in underground storage is made available during the peak winter season. The Hutubi (HTB) facility began its UGS operations in a depleted gas field located in Northwest (NW) China in 2013. The reservoir is overlaid with a caprock and bounded by two penetrating faults. The stability of caprock and faults during depletion and subsequent injection must be comprehensively evaluated for formulating storage site characterization and reservoir management strategies.
Many factors can contribute to fault sealing/leakage. Among fault juxtaposition, fault gouge and the stress state acting on fault surfaces, the latter is probably the most important consideration. Geomechanically, fault sealing is commonly related to fault stability/reactivation, which is a function of the fault orientation with respect to the Earth’s principal stress tensor, and the coefficient of sliding friction between the fault surfaces. During depletion and injection, changes in both the normal and shear stresses on the fault plane can affect the stability of the fault. Additionally, the stability of the cap rock can also be breached when the injection pressure exceeds the fracture gradient (Shmin), resulting in the propagation of the fracture. The maximum injection pressure can be calculated based on the stability criteria for faults that are in pressure communication with the reservoir, and the fracturing potential of the 847-m cap rock above the reservoir. To assess the effects of stress state on the stability of fault and caprock for the UGS project, a detailed geomechanical model was constructed to delineate the in-situ stress and rock mechanical properties of the field. The geomechanical parameters provide the required information to be used together with each fault’s properties to analyze the stability of reservoir bounding faults, and the fracturing potential of the caprock. In the process, the maximum injection pressure for the UGS project can be ascertained, providing an invaluable data input for the designs of UGS’ production operations.
A case study is presented in this paper. We studied the evolution of stress state associated with the geometry of faults in order to understand the relationship between pore pressure variations and the stability of fault and caprock. The results showed that there is no risk of fault reactivation for the two reservoir-bounded faults at the early stage of gas storage operations. However, there exists a risk of fault instability when the pore pressure is about 15MPa higher than the original reservoir pressure.
This paper demonstrates an integrated geomechanical simulation approach for assessing caprock integrity and fault instability of an UGS project in NW China. The work lays the foundation and provides the basis for developing and optimizing UGS operational plans during the project life cycles.