Recent temporal changes in the stress state and fault reactivation assessment of HTB underground gas storage in China associated with gas injection and extraction

Abstract

The stress state variations due to seasonal injection and extraction of gas in HTB underground gas storage (HTB-UGS) were estimated, as well as the fault-slip analysis of the main fault, to understand how gas injection and extraction result in earthquakes. The stress state at different depths at the time of a complete depletion in 2012 was first estimated using image-logged borehole breakout data and stress inversions of focal mechanism solutions. Further, the temporal variations of the stress state caused by changes in the pore pressure due to gas injection and extraction were estimated based on a simple pore pressure-stress coupling assumption. The fault reactivation/slip tendency temporal changes and their possible relation with seismicity in this area were analyzed. Results revealed that the stress orientation in the reservoir layer was around N21°E−N23°E, which was consistent with the tectonic stress prevailing direction. The stress regime varied between reverse and strike-slip faulting stress regimes during the gas injection and extraction cycles. The main fault-slip tendency initially decreased continuously with gas production and then increased with gas injection. Earthquake sequences appeared when the slip tendency increased to a critical value of 0.45 ± 0.03.