Stress State Change and Fault-Slip Tendency Assessment Associated with Gas Injection and Extraction in the Hutubi (China) Underground Gas Storage

Summary The Hutubi (HTB) gas reservoir in northwest China was active in production from 1998 to 2012; beginning in 2013, it was used for gas storage. Several earthquakes occurred during the early operation of the Hutubi underground gas storage (HTB-UGS) project, so it is important to explore their relationship with gas injection and extraction from the viewpoint of stress state change and faulting susceptibility. We study the evolution of the stress state and slip tendency associated with the operation of the HTB-UGS to understand the relationship between pore pressure variation and induced seismicity. We constrain the stress state in the reservoir at the time of cessation of production in 2012 using image-logged wellbore breakouts. We then estimate stress state variation over time as a result of pore pressure changes, based on the assumption of poroelastic coupling between horizontal elastic stresses and pore pressure. The maximum principal stress in the reservoir region before the HTB-UGS operation is oriented N23°±6°E, which is consistent with that of the prevailing tectonic stress derived from earthquake focal mechanisms. Our results show that the vertical stress (Sv) and the minimum horizontal principal stress (Shmin) were initially similar in magnitude before petroleum production; the magnitude of Shmin gradually decreased during petroleum production. Gas injection, initiated in 2013, raised the Shmin magnitude, returning it to approximately that of Sv. We use the estimated variations in the reservoir stress state over time to calculate temporal changes in slip tendency of the main faults in the reservoir. The fault-slip tendency decreased continuously with petroleum production and then increased with gas injection. The first earthquake swarm associated with gas injection occurred approximately 2 months after the start of injection, possibly due to the slow pore pressure diffusion. Thereafter, earthquakes were induced whenever gas was injected. Our assessment of slip tendency suggests that earthquake swarms were induced during increasing pore pressure phases when slip tendency reached a value of 0.43 ± 0.04. The maximum allowable pore pressure for avoiding earthquakes (ML > 1.5) is 29.0 ± 4.5 MPa, which might increase through stress adjustments and a newly attained balance. This study provides a possible method for induced earthquake analysis associated with gas injection and extraction from the perspective of stress state change and faulting susceptibility.