Induced seismicity, once an obscure phenomenon, is currently a red-hot environmental issue in North America, particularly regarding activity in Oklahoma, Ohio, Texas, Colorado, and British Columbia and Alberta, Canada. Many news media reports suggest hydraulic fracturing is the cause, but the occurrence of induced events is complex, and the cause and size of the induced earthquakes varies from area to area.
Background of Induced Seismicity
Induced seismicity refers to earthquakes caused by human activity. A thorough review of the topic is presented by the National Academy of Sciences study titled Induced Seismicity Potential in Energy Technologies published in 2013.
Earthquakes induced by oil and gas operations can occur when changes in the subsurface occur near preexisting faults due to activities such as wastewater injection (Oklahoma), hydraulic fracturing (British Columbia and Alberta, Canada; Ohio), or when large volumes of material are extracted or compacted (Groening field in the Netherlands). The orientation of a fault with respect to the surrounding stresses in the earth and subsurface changes due to human activity, such as pore pressure, may prompt a preexisting fault to slip and cause an earthquake. Wastewater disposal wells and hydraulic fracturing have both been suspected to have induced seismic events as large as magnitude 4.4 (hydraulic fracturing) and 5.6 (wastewater disposal).
Induced earthquakes related to wastewater injection are relatively rare. The United States has approximately 150,000 Environmental Protection Agency Class II injection wells, of which about 30,000 are disposal wells. The disposal wells vary in injection rate and injection target. An injection well is specially drilled to target a rock formation with high permeability. The disposal is typically performed under “vacuum”—i.e., disposing of water without additional pressure, where the weight of the wastewater column in the wellbore is enough to drive the water into the disposal interval. Wastewater injection wells are designed to be used for years and years for safe disposal, and very few of these wells have been associated with any seismic activity.