The widespread application of hydrofracturing to develop petroleum resources has increased the need to dispose of flowback fluids in deep injection wells. In several geographic areas this appears to have triggered earthquakes in locations where none had been reported historically. Texas has had a very active petroleum industry for about a century, and at present there are more than 10,000 active disposal wells in Texas. Thus Texas serves as a huge natural experiment where we can explore how earthquakes are related to both fluid injection and petroleum extraction.
The passage between 2009-2012 in Texas of EarthScope's USArray Transportable Array seismograph stations provided an opportunity to identify and accurately locate earthquakes with magnitudes as small as 1.5-2.5. Information about locations of wells and monthly volumes of water, oil or gas extracted and injected are archived by the Texas Railroad Commission and available to the public. With these data we have been evaluating the relationship between seismic activity and fluid injection and extraction in Texas.
For five different areas in Texas I discuss seismic sequences that appear to have been induced or triggered by injection or extraction. In two areas there is strong evidence that water injection has triggered earthquakes; in two areas there is evidence that the extraction of oil and water has triggered earthquakes; and in one area CO2 gas injection may have triggered earthquakes.
Although these observations provide strong evidence that injection and extraction do induce/trigger earthquakes, there are thousands of active wells in Texas that do not trigger seismic activity. The principal outstanding unsolved question is why earthquakes are induced/triggered near some wells and not at others. Fluid injection for waste disposal seems to cause earthquakes more often than fluid extraction. I am aware of no instances in Texas where hydrofracturing itself has caused earthquakes large enough to be felt at the surface.
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