Evidences of long-period long-duration (tremor) signals occurring in fluid injection sites are generated due to aseismic fault slip.

Shankho Niyogi, Abhijit Ghosh, Abhash Kumar, & Richard Hammack

Published September 10, 2023, SCEC Contribution #13282, 2023 SCEC Annual Meeting Poster #001

Long-Period Long-Duration (LPLD) signals, observed to be occurring in hydrocarbon fields where fluid injections for tertiary recovery of oil are taking place, have sparked debates regarding their origins and underlying physical mechanisms. Interestingly, these signals often share characteristics with tremors typically seen in natural tectonic environments. While some of these signals stem from non-tectonic, our research dives into LPLD signals detected at the Wellington Field in Kansas during an enhanced oil recovery project involving high-pressure CO2 injection in 2016. The injection was monitored by a dense seismic array of 14 stations distributed around the injection site with a tight (~3 kms) aperture. We observe clear LPLD signals, henceforth called tremor, emerging clearly as the injection starts, continue during the injection, and cease after the injection. The clearest tremor signals are characterized by multiple distinctive gliding spectral lines at 2-6 Hz in spectrogram. They resemble tremor signals reported in volcanic and glacial settings produced by frictional slip. Our investigations using array beamforming, spectral analysis eliminated the possibility of origins from non-tectonic or distant sources. Using a beam back-projection method, we traced the sources of the tremors, revealing an alignment with the tectonic features of the region, namely the Humboldt Fault Zone and Central Kansas Uplift. Interestingly, we do not observe change in microseismic activity during the injection. We also perform modeling to show that series of small earthquakes with varying recurrence intervals can produce tremor signal with spectral lines observed here. We inferred that the tremor is produced by frictional slip on fault planes driven by local changes in stress accumulation rate due to fluid injection (Niyogi et al., Science, 2023). Previous studies showed that high pore pressure due to fluid injection may alter fault frictional property from rate weakening to rate strengthening leading to mostly aseismic deformation and slip. We propose that these tremors are seismic manifestation of aseismic slip induced by fluid injection. It implies that such tremor can act as a benign way of tracking and monitoring deformation and slip in the formation due to fluid injection. Our future direction encompasses additional research at other high-pressure fluid injection sites, aiming to gauge the prevalence of such tremors and understand their characteristics and origins.

Citation
Niyogi, S., Ghosh, A., Kumar, A., & Hammack, R. (2023, 09). Evidences of long-period long-duration (tremor) signals occurring in fluid injection sites are generated due to aseismic fault slip.. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology