Induced Earthquakes Before and After Cessation of Long‐Term Injections in Rongchang Gas Field

• Published in: Geophysical research letters Vol. 47; no. 22
• Main Authors: Wang, Zhiwei, Lei, Xinglin, Ma, Shengli, Wang, Xiaolong, Wan, Yongge
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.11.2020
• Subjects: Depletion; Earthquakes; Epidemics; Fault lines; induced earthquakes; Injection; Injection wells; long‐term injection; Oil and gas fields; Overpressure; postinjection seismicity; Probability theory; Reservoirs; Seismic activity; Seismicity; Statistical analysis
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL089569

For more than three decades, the depleted Rongchang gas reservoir in China's Sichuan Basin was used for the disposal of unwanted water, which resulted in induced earthquakes, with magnitudes as high as 5.2. After all wells were closed, the frequency of seismic activity was observed to decay following a modified Omori law, and since April 2015, seismic activity again began to increase, and a M4.9 earthquake occurred on 27 December. The results of an epidemic‐type aftershock sequence (ETAS) model analysis show that forced seismicity accounted for more than 70% of the total events. For most M ≥ 3.5 earthquakes, including two M ≥ 4 events, the estimated overpressure was lower than the maximum injection pressure. These results, coupled with the fact that postinjection seismic activity has similar characteristics to seismicity during injection, indicate that the injected overpressure fluid was still the driving factor for postinjection seismic activity. Plain Language Summary Prior to 2013, unwanted water was intermittently pumped into the depleted Rongchang gas reservoir at a depth of 3 km for approximately three decades, inducing many earthquakes with a magnitude up to 5.2. Although all nearby injection wells were closed in 2013, the seismicity in the area has remained relatively high. At this location, M4.9 and M4.0 earthquakes occurred in December 2016. In order to assess the situation, we carried out an integrated study focused on the area's seismicity before and after the injections ceased. The results, including the overpressure required for major earthquakes and various statistical features, suggest that the postinjection seismicity, which was the same as that during injection, was caused by the delayed rupture of preexisting faults within the reservoir and its surrounding faults. Therefore, at an injection site, at which significant injection‐induced earthquakes occurred during injections, there is still a high probability of continuous occurrence of relatively large earthquakes after injection ends. Key Points Long‐term injection for wastewater disposal into the depleted Rongchang gas reservoir induced earthquakes with magnitudes as large as 5.2 After shutting down of injection, seismicity decayed following a modified Omori law with some major events Postinjection seismicity demonstrates features similar to that during injection except for a change in event rate