What is Induced Seismicity?

In general, induced seismicity refers to seismic events that are a result of human activity.  There are many different ways in which human activity can cause induced seismicity including geothermal operations, reservoir impoundment (water behind dams), waste water injections, and oil and gas operations such as hydraulic fracturing.    

Microseismic induced event

In the oil and gas industry, induced seismicity has become synonymous with larger magnitude seismicity events which are larger than what was observed historically. Although rare in North America, induced seismicity is most often associated with waste water injection projects.  According to the USGS, the recent rise in seismicity in the central and eastern US may be related to the injection of waste water from oil and gas operations into deep disposal wells in states such as Arkansas Colorado, Ohio, Oklahoma, and Texas.  This has led to legislature being implemented in certain jurisdictions which requires companies to monitor seismic activity during and shortly after the injection period.

When large volumes of fluid are injected into underground formations for storage or disposal, some fluid may travel into faults.  These fluids can affect the pressures that are acting on the faults. Fluid pressure in the fractures and pores of rocks is called “pore pressure.” If pore pressures are low compared to the natural forces holding the rock together, then only natural tectonic forces could cause an earthquake. However, if pore pressures increase, then it would take less of an imbalance of stresses to cause an earthquake.  There is the potential that when fluids are injected into the rock, the fluid may increase the pore pressure on a possibly active fault.  This could cause a sudden slip that releases stored energy, generating seismicity and potentially an earthquake. 

Characterizing Induced Seismicity

It is important that the right tools are used to monitor induced seismicity to accurately observe the number of seismic events associated with injection and most importantly, their location and magnitude.  In particular, it is important to record events of high magnitude that locate near the injection zone or are associated with a known fault in the area of interest.  Many companies adopt a traffic light system where seismic events are classified by a series of magnitude thresholds.  Green light events are smaller events that locate in their expected zone and are not cause for concern; operations can continue as normal.  Yellow light events are larger magnitude events and/or clusters of events that are moving out of the expected zone; operations can continue but conditions are observed carefully and operators must be ready to shut-down if conditions change.  Finally, red light events exceed pre-determined thresholds and require the immediate shut-down of injection operations.

Seismic events of various magnitudes exhibit different characteristics based on their signal frequency.  Smaller microseismic events emit seismic waves at higher frequencies, while large magnitude events exhibit longer wavelength signals at much lower frequencies.  By using various types of sensors that are tuned to detect events with different frequencies, operators can be confident that seismicity is accurately assessed. ESG’s Hybrid™ Downhole-Surface Seismic Acquisition System combines 15 Hz geophones with 4.5 Hz geophones and force-balanced accelerometers, which can detect lower frequency events associated with high magnitude seismicity, into one time-synchronized system, ensuring that events with moment magnitudes between from Mw -3 to Mw +3 will be fully detected and accurately characterized. 

Microseismic hybrid sensors for induced seismicity