Application of 4D Geomechanical Modelling for Fault Critical Re-Active Stress Evaluation in Underground Gas Storage

Abstract Fault stability is the risk of reactivation under dynamic stress conditions. The reactivation of faults in the oilfield is mainly caused by the increase of fluid pressure in the reservoir zone, with the quantitative evaluation index of the critical reservoir pressure required for fault reactivation under the current pore pressure condition. When the formation pore pressure reaches the critical stress, the corresponding fault part will be in a critical stress state. The slippage of the critical stress fault tends to cause fluids leakage. Therefore, the study of fault stability is of great significance to oilfield production; In order to guarantee national natural gas peak regulation and supply, the YH underground gas storage (UGS) has been proposed and is carried out with the project of expanding storage capacity and increasing production. The operator hopes to effectively guide the optimization of the limit operation pressure and ensure its long-term safe operation. It is urgently required to carry out fault stability evaluation for the YH underground gas storage. The operator plans to find out the conditions for the activation of the faults, with studies about the stability of the fault under the impact of mining and the impact of the system parameters on the stability of the fault. The results suggest that: whether the fault is in a stable or active state depends on the magnitude relationship between the apparent friction factor (k1) and the fault friction factor (k). When k1 < k, the fault will be in a self-locking state. However, when k1 ≥ k, the fault is in a reactive state. The apparent friction factor reflects the stress risk level of the fault under the collective impact of the in situ stress (including σ1 and σ3), the cohesion of the fault plane (c) and fluid pressure of fault (pi). Higher k1 indicates higher tendency of fault re-activation. k is a quantity factor determined by the friction angle (φ) within the fault. The larger friction angle of fault indicates higher friction factor and the more stable state. The system parameters (includingφ, c, pi, σ1 and σ3) will affect the stability of the fault after the change of initial stress conditions: the smaller cohesion of the fault plane and greater fracture fluid pressure indicate the fault will be easier to reactivate. This paper established the 4D dynamic geomechanical model of the YH underground gas storage and took the fault stability as the judgement basis to analyze the in-situ stress characteristics of different faults. The research results could be used to evaluate the UGS operation safety quantitatively under the impact of the dynamic stress conditions, which will provide technical guidance for optimizing the operation plan of the UGS.