Induced Seismicity: Drilling Activities and Earthquakes

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The Number of Earthquakes Has Multiplied in Oklahoma

The Great Plains are better known for their tornadoes than for earthquakes being a region far from the edges of continental plates, and where few fault systems are active. There are centers of sporadic seismic activities in Missouri and Arkansas, but Oklahoma has few geological drivers of earthquakes. When weak tremors started occurring in swarms in 2009 and 2010, they were noted.

 While highly unusual in the past, the average rate of earthquakes in the state quickly ratcheted up to 600 times the historical average. In 2014, 585 earthquakes of magnitude 3.0 or greater were experienced, more than what California experienced that same year. Most quakes were centered around and north of Oklahoma City. One quake of magnitude 5.6 in November 2011 damaged some homes and other infrastructure.

Drilling Activities Suspected

It quickly became apparent that there was a geographical correlation between the location of these earthquakes, and areas of intense oil and gas drilling activity. Drilling wells into bedrock has been done for a long time with no indication that it triggers earthquakes, so something new had to be involved. The beginning of the earthquake swarms coincided with the increased use of a technique called hydraulic fracturing, or hydrofracking, to exploit unconventional sources of oil and gas.

This technique involves the use of explosives deep underground to create openings in a horizontal well casing – but these blasts are much too small to cause earthquakes. Perhaps, then, we could point at the high pressure with which fracking fluids are pushed into the well to open up shale fractures and release oil and gas?

That reasoning, though, would not explain why thousands of those wells drilled with the same hydraulic fracturing techniques in several other states have generally failed to create significant seismic activity.

Scientific Evidence Is Mounting

Several groups started studying the earthquakes seeking possible causes, and converged in a likely explanation. The hydrofracking process produces high volumes of wastewater, after fracking fluids, and brine from the rock formation coming back up to the surface. Few options exist for the disposal of these liquids, so a common method is to inject them, under pressure, in deep disposal wells. In the largest disposal wells, millions of gallons of wastewater are sent over a mile deep each month. Pressure from the liquid interferes with stressed faults, providing enough force to release that stress and rupture the fault. This “slipping” of the fault is felt on the surface as earthquakes in a phenomenon called induced seismicity. The observed swarms of earthquakes have occurred in faults positioned at an angle and orientation favoring interferences from some the largest disposal wells.

Earthquakes elsewhere have been tied to disposal wells, and also to the hydrofracking process itself, but none as large in magnitude as the Oklahoma quakes.

Sources

Keranen et al. 2014. Sharp Increase in Central Oklahoma seismicity since 2008 induced by massive wastewater injection.

Oklahoma Secretary of Energy and Environment. Earthquakes in Oklahoma.

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Beaudry, Frederic. "Induced Seismicity: Drilling Activities and Earthquakes." ThoughtCo, Jan. 10, 2017, thoughtco.com/induced-seismicity-drilling-activities-and-earthquakes-1203815. Beaudry, Frederic. (2017, January 10). Induced Seismicity: Drilling Activities and Earthquakes. Retrieved from https://www.thoughtco.com/induced-seismicity-drilling-activities-and-earthquakes-1203815 Beaudry, Frederic. "Induced Seismicity: Drilling Activities and Earthquakes." ThoughtCo. https://www.thoughtco.com/induced-seismicity-drilling-activities-and-earthquakes-1203815 (accessed October 19, 2017).