Background to the Study

Induced seismicity is a well-known phenomenon and has been associated with a number of subsurface activities including, mining, geothermal energy, waste water disposal and, more recently CO₂ storage. 

Key messages

·        Investigation into the link between induced seismicity and very large-scale waste water disposal has advanced significantly since the 2013 IEAGHG review of induced seismicity.  Seismic monitoring of CO₂ storage sites has also led to a better understanding of the phenomenon especially the propagation of microseismic events.

·        The use of sophisticated monitoring techniques has been refined and enabled enhanced event location and improved model calibration.  Evidence from microseismic detection techniques at demonstration CO₂ storage sites has also revealed temporal correlations with periods of high injection rate and bottom-hole pressure.  There is no evidence of felt seismicity at any of the 36 CO₂ storage sites reviewed in this study with the exception of one CO2-EOR (enhanced oil recovery) site.

·        Monitoring microseismicity at CO₂ storage sites has revealed events are more common during perturbations in flow, including shutdowns, than they are during injection.

·        The analysis of geomechanical responses in reservoirs can contribute to the assessment of the risk of felt seismicity caused by pressure perturbations within a project area.  Geomechanical modelling is therefore of critical importance and requires verification with measured parameters. 

·        Large-scale waste water disposal has a clear association with induced seismicity across some regions of the south - central US.  However, it is important to recognize that natural pre-existing tectonic stresses can be triggered by pressure changes induced by fluid injection.  Some regions of the US, such as North Dakota near the Canadian border, and in the northeast US,  exhibit a relative lack of felt seismicity, despite waste water disposal.

·        Historical evidence, that predates large-scale waste water disposal, also shows that natural tectonic activity has been responsible for seismicity across the south – central and other regions of the US.

·        Waste-water disposal intervals at a depth of ~2km, for example in Oklahoma, can have related seismic hypocenters at an estimated depth of ~5km in the crystalline basement. 

·        Other factors can complicate when seismic events might occur.  Stress can be transferred to deeper fault zones that are critically stressed and susceptible to slip.  This mechanism may have been responsible for the Castor event off the north-east coast of Spain.  Earthquake to earthquake interaction is also possible where accumulated stress from previously seismically non-active regions can be affected.

·        In response to concerns associated with induced seismicity in the US a series of precautionary operational measures were introduced.  These have included mandatory injection into higher stratigraphic formations, significant volumetric reductions in waste water injection or, in some cases, complete cessation and a ban on new disposal wells in close proximity to known regional faults.  Seismic monitoring plans also need to be implemented and include tests to detect the presence of faults.  Specific regulations depend on each state.

·        Regulatory authorities have defined rules and expectations for permits, incorporating in most cases, past earthquake data (distance of seismicity from the well and magnitude of events) and characterization of subsurface hazards.  In some cases (Canada, Oklahoma, Ohio, UK), the operator has to follow a traffic light system that “controls” its actions and provides guidance on risk mitigation based on an earthquake magnitude threshold.  This depends on each country or state.

·        Causes of seismicity at specific locations need to be clearly explained especially to local communities that may be affected by CO₂ storage sites.  For example, in Japan natural seismicity is a regular occurrence, and felt events are not uncommon, consequently the origin of such events needs to be conveyed so that they cannot be associated with a CO₂ storage site.  In another example, and in contrast, geothermal energy produced from the Geysers field is actively monitored because of the link between the field’s operations and induced seismicity.  Successful proactive outreach policies at the Tomakomai CO₂ storage demonstration site in Japan, and the Geysers geothermal field in California, have demonstrated how seismic origin, and its potential impacts, can be effectively communicated. 

This report is available to download.