Man-made earthquakes in Oklahoma are strongly linked to fracking methods such as the injection depth of wastewater, according to a major study published in Science.
The study aimed to provide targeted evidence in proving that there has been a substantial decrease in induced seismicity in the Oklahoma region, with possible applications in other parts of the world.
An average of 2.3 billion barrels of fluids per year have been injected into the ground by Oklahoma's well operators since 2011. Disposal of wastewater is typically at depths of one to two kilometers below the ground surface. In addition, saltwater is injected deep underground for recovery of oil and gas.
There has been an approximately 800-fold increase in the annual number of earthquakes in Oklahoma since 2011, which is associated with the connection between the frequency of earthquakes (seismicity) and the depth of fluid injection into underground rock formations.
The researchers examined the connections between injection volume, depth, location, and geological features over a six-year period, using an innovative software that is capable of analyzing large and complex data sets. Called the Uninet, it incorporated injection well records and earthquake data from the U.S. Geological Survey (USGS).
The research team found that seismic activity may be influenced by the depth and volume of injected fluids at depths where layered sedimentary rocks meet crystalline basement rocks. According to analysis, deeper wells allow easier access for fluids to seep into fractured basement rocks, which are prone to earthquakes.
The team concluded that raising the injection depths to above the basement rocks could significantly reduce the energy released by earthquakes, which in turn may reduce the instances of larger, more devastating quakes.
The researchers stated that the findings warrant a broader understanding of how factors such as operational, spatial, and geologic parameters influence seismic activity. The team noted that their analysis may grant opportunities to evaluate regulatory actions on a rational, quantitative basis in terms of seismic effects.