Since 2000, CO2 has been successfully injected for the purposes of both enhanced oil recovery (EOR) and carbon capture and storage (CCS) at the Weyburn oilfield. A component of the geophysical monitoring program at Weyburn has included the use of downhole geophones to monitor microseismic activity. Microseismic events have already been used to assess the likelihood of CO2 leakage through the caprock at Weyburn. However, in recent years, the focus with respect to CCS and geomechanics has shifted to the concern that fluid injection will trigger induced seismicity. Therefore, in this paper we reanalyse the microseismic observations at Weyburn with respect to concerns regarding induced seismicity. We assess the population statistics of the Weyburn microseismic events, both in terms of the Gutenberg–Richter b-values and the correlation between moment release and injection/production volumes. Our observa- tions serve to corroborate previous studies that considered the geomechanical cause of the microseismic events: namely that the events are not directly triggered by fluid injection, but in response to stress transfer through the rock frame in response to both production and injection. We observe that the b- value at Weyburn is close to 1, the value expected for stress-driven, tectonic seismicity, and significantly lower than values observed in cases where the microseismicity is directly driven by fluid injection (during hydraulic fracturing, for example). We also note little to no correlation between fluid volume changes and induced seismicity – either injection volume alone or net volume change (produced–injected). Finally, we use the observed event statistics to forecast the likelihood that current operations at Weyburn will lead to larger events of sufficient magnitude to be of concern to local populations. We find that the probability of inducing such events is very unlikely.
- Induced seismicity
- Passive seismic monitoring