Sunday, April 22, 2012

Triggered earthquakes

The latest issue of EARTH magazine from AGI has an article on  "Managing the seismic risk posed by wastewater disposal," by Stanford geophysicist Mark Zoback, that debunks some of the confusion over hydraulic fracturing ("fracking") and earthquakes.

Mark notes that "No earthquake triggered by fluid injection has ever caused serious injury or significant damage. Moreover, approximately 140,000 wastewater disposal wells have been operating safely and without incident in the U.S. for many decades. That said, we have known for more than 40 years that earthquakes can be triggered by fluid injection. The first well-studied cases were earthquakes triggered by waste disposal at the Rocky Mountain arsenal near Denver, Colo., in the early 1960s, and by water injection at the Rangely oilfield in western Colorado in the late ‘60s and early ‘70s."  [Right, relation of aquifer, producing zone that is frac'ed, and deeper wastewater injection zone.  Credit, NPR]

He concludes that "water injection associated with hydraulic fracturing is not responsible for the triggered seismicity in question. The reason for this is that pressurization during hydraulic fracturing affects only limited volumes of rock (typically several hundred meters in extent) and pressurization typically lasts only a few hours."

Instead, the earthquakes that are generating so much attention result when the "wastewater that flows back out of the wells is disposed of at dedicated injection wells."

Mark lays out a 5-step program for reducing earthquakes induced from injection of wastewater:
Step 1: Avoid Injection into Active Faults
Step 2: Minimize Pore Pressure Changes at Depth
Step 3: Install Local Seismic Monitoring Arrays
Step 4: Establish Modification Protocols in Advance
Step 5: Be Prepared to Alter Plans or Abandon Wells

Mark cautions that "we have to be prepared to reduce injection rates, or even abandon wells if triggered seismicity cannot be stopped by limiting injection rates."

In summary,  he says "Overall, it is important for the public to recognize that the risks posed by injection of wastewater are extremely low. In addition, the risks can be minimized further through proper study and planning prior to injection, careful monitoring in areas where there is a possibility that seismicity might be triggered, and operators and regulators taking a proactive response if triggered seismicity were to occur."

For background, there is no hydraulic fracturing going on in Arizona, and no reports of induced seismicity from wastewater disposal.

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