Mechanism of fault slippage in the underground gas storage and preliminary application in the Shuang 6 UGS in China

Abstract

The mechanical stability of faults is crucial for the safe operation of underground gas storage (UGS). The complex fault systems, strong heterogeneity and anisotropy of geological formations, associating with uncertainty of in-situ stress state after long-term exploitation of oil or gas reservoirs and multiple cycles of injection and production, making it particularly challenging to ensure the safe and efficient operation of the UGS in depleted gas reservoirs. This study investigates the Shuang 6 UGS in the Liaohe basin, NE China, employing a hydro-mechanical coupling approach through both simplified simulation modeling and field case analysis. Systematic parametric studies are carried out to illustrate the effect of key factors on fault slippage. Based on the results of fluid migration and geomechanical responses, an advanced critical pressure perturbation method is proposed to evaluate the risk of fault failure. The findings indicate the following: (1) The fault stress profile exhibites greater complexity than pore pressure due to geomechanical interactions between formations with contrasting properties on hanging wall and footwall; (2) The ΔP in the advanced critical pressure perturbation method provides quantifiable criteria for slip risk evaluation; (3) Low injection rates, small difference in Young's modulus between laminated formations, a high permeable fault zone, and damage zone, as well as simultaneous injection or withdrawal on both sides of the fault, may reduce the risk of fault failure; (4) Specific risk-prone areas are identified in the Xing II/III formation and the right side of Faults 2/3 in the SX block of the Shuang 6 UGS. These insights offer practical guidance for UGS design and operational safety management.