GSA Critical Issue: Hydraulic Fracturing


Seismic Expression from Hydraulic Fracturing Figure 15:
Seismic Expression from Hydraulic Fracturing; Warpinski et al, 2005. Reproduced with permission of SPE; further reproduction prohibited without permission.

Induced seismicity is an earthquake caused by human activities. One way this can occur is from iInjection of fluids deep into the earth. The increase in underground disposal of produced and flowback water from oil and gas wells are associated with a large increase in triggered small and moderate earthquakes in some regions, such as central and northern Oklahoma [69, 70]. Oil and gas operations are responsible for two types of fluid injection: 1) injection of hydraulic fracturing fluids into the reservoir rock; and 2) disposal of waste fluids through deep well injection.

Hydraulic fracturing imparts pressures of several thousand pounds per square inch on reservoir rocks. The resulting fractures may extend several hundred feet away from the borehole (Fig. 15), but generally no more than that due to physical and technological limitations on the hydraulic fracturing process[38, 49]. The hydraulic fracturing process creates very small seismic events or earthquakes. Such microseismicity is generally too small for humans to feel or to cause surface damage [26], although it can be detected by monitoring instruments that are designed to precisely determine where the fractures have propagated. A number of studies, including one by the National Academy of Sciences, have determined that hydraulic fracturing does not create a significant earthquake risk [48]. Alberta and British Columbia, Canada, have had moderate earthquakes that appear related to the hydraulic fracturing process itself [71, 72].

Disposal of large volumes of waste fluids produced from hydraulically fractured rocks through deep-well injection has been documented to produce small earthquakes, generally less than magnitude 2.0 [48,]. However, in areas with high volumes and rates of injection into disposal wells, there have been dramatic increases in earthquakes magnitude 3.0 and greater [50, 70]. Horizontal wells that have been hydraulically fractured typically produce large volumes of waste fluids (produced and flowback water). Deep disposal of any fluids can trigger earthquakes [48, 51]. Most, although not all, of such earthquakes have occurred in areas of long-term or continuous injection of wastewater. Fluids injected near a subsurface fault may reduce the frictional resistance that keeps faults from slipping. These small movements allow energy already stored in brittle rock to be released in earthquakes [52]. In some locations, sites of slowly accumulating forces in the earth resulting from natural geologic processes are already susceptible to seismic events (which is why it is referred to as “triggered seismicity”). The increase in pore pressure on stressed fault surfaces appears to be the main physical reason for injection-induced earthquakes in the central and eastern United States [69, 72]. Deep well injection of fluids has likely caused earthquakes in excess of magnitude 2.0 over the past several decades, including a magnitude 5.7 earthquake in 2011 in Oklahoma[54] and a sharp increase of earthquake frequency from 2012 to 2015 in Oklahoma [65, 72]. Kansas has also experienced a marked increase in seismic activity in the last two years, including the state’s largest earthquake recorded at magnitude 4.9 in November 2014[64]. The potential for triggered seismicity with the increasing volume of wastewater disposal is unknown [48, 40, 55]. States are implementing strategies to mitigate risks of induced seismicity associated with disposal injection wells. This includes a screening protocol to determine what response strategies may be appropriate [69]. Mitigation actions can include changing the allowable rates and pressures of injection, partial plugback of the injection well, and stopping all injections and shutting the well [69].

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