Large volumes of highly saline water injected in a deep disposal zone (the Arbuckle formation) lead to rising pore pressure which can penetrate already stressed basement faults and trigger earthquakes (Photo : Stanford University)
A new study suggests that earthquakes are now more common in the state of Oklahoma in the US since it is apparently linked to the increased pumping of waste water into the Arbuckle formation, located some 7,000 feet below the sedimentary soil formation in the state.
Researchers reveal that the main source of the wastewater that supposedly triggers earthquakes in the region is apparently not caused by flowback water that is produced after hydraulic fracturing operations underground.
Earthquakes are made more frequent due to "produced water" which is brine water that is naturally found within oil and gas inside the layers of the Earth. Many oil and gas companies in the US separate this produced water from the oil and gas they extract from the Earth where they practice re-injecting this excess water deeper into the disposal wells.
According to lead author of the study Mark Zoback of Stanford University, this fluid injection into the Earth's crust leads to the recent seismic activity including earthquakes in Oklahoma that is caused by massive amounts of this waste water which is not related to hydraulic fracturing activity but rather as an after effect.
Last year Oklahoma experienced more than 24 seismic events but prior to 2008, the state only experienced four earthquakes of one to two magnitudes every ten years.
Scientists are aware that some of these produced water resulted from wells that were made for hydraulic fracturing however within the regions that experience the most earthquakes, more than 95 percent of the waste water disposal consists of produced water and not including hydraulic fracturing flowback water.
The earthquakes also occurred after months and even years after the pumping of waste water peaked in the areas where the quakes are frequent. The disposal wells were even distant from the seismic activity which is still a puzzle to scientists.
Zoback explains this discrepancy by saying that the pore pressure or the pressure from fluids found in fractures and rock spaces underground in the Arbuckle formation increases due to waste water disposal that is originating from a disposal zone that is located directly above a crystalline basement where the earthquake fault line is found.
The crust already has naturally pre-existing fault lines that are geologically active today however when stress builds slowly on these faults over a long period of time, it experiences friction between the two sides of the fault that are now pressing onto each other. This is when earthquakes occur, when the fault slips against each other.
Zoback adds that oil and gas companies have already injected an immense amount of wastewater during fracking that it created an after effect where pressure is now spreading among the Arbuckle formation. The earthquakes are bound to stop but it will not cease overnight, he adds.
This study is published in the journal Science Advances.
