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), wastewater injections, and oil and gas operations such as hydraulic fracturing.
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 wastewater injection projects. According to the United States Geological Survey (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.
How is hydraulic fracturing related to earthquakes and tremors?
Reports of hydraulic fracturing causing felt earthquakes are extremely rare. However, wastewater produced by wells that were hydraulic fractured can cause “induced” earthquakes when it is injected into deep wastewater wells.
Wastewater disposal wells operate for longer durations and inject much more fluid than the hydraulic fracturing operations. Wastewater injection can raise pressure levels in the rock formation over much longer periods of time and over larger areas than hydraulic fracturing does. Hence, wastewater injection is much more likely to induce earthquakes than hydraulic fracturing.
Most wastewater injection wells are not associated with felt earthquakes. A combination of many factors is necessary for injection to induce felt earthquakes.