Wednesday, May 7, 2014

Earthquakes Triggered By Fluid Injection Along Faults

A friend sent me this abstract published in the Seismological Society of America 2014 annual meeting-

Triggered Earthquakes Far From the Wellbore: Fluid Pressure Migration and the 2008-2014 Jones Swarm, Central Oklahoma

KERANEN, K. M., Cornell University, Ithaca, NY, USA,; WEINGARTEN, M., University of Colorado, Boulder, CO, USA,; BEKINS, B., USGS, Menlo Park, CA, USA,; GE, S., University of Colorado, Boulder, CO, USA,; ABERS, G. A., Lamont-Doherty Earth Observatory, Palisades, NY, USA,

Earthquake relocations and hydrogeologic modeling show that the Jones earthquake swarm, occurring near Oklahoma City since 2008, is linked to disposal wells injecting high volumes of water along the Nemaha Fault. Felt and recorded earthquakes in the Jones swarm began in 2008, approximately 15 km from four high-volume wastewater disposal wells. These wells dispose of ~2-3 million barrels per month (4-5 million barrels per month cumulatively) in two adjacent locations on the downthrown side of the Nemaha fault. Earthquakes are observed to migrate away from these high-volume disposal wells up the structural dip and down hydraulic gradient. Hydrogeologic modeling shows that the increase in subsurface pore pressure resulting from the fluid injection is of sufficient magnitude to trigger slip on pre-existing faults. The region of increased pore pressure grows outward through time with injection. The larger, mapped faults in the subsurface may act as conduits or guides to fluid flow, and may transmit fluid pressure into basement. Our results demonstrate that wastewater disposal can raise fluid pressure and trigger earthquakes at tens of kilometers from the wellbore on existing faults.

Fracking for shale gas by itself has not been shown to trigger biggish earthquakes but the wastewater disposal that follows fracking has.

How does increasing pore pressures increase the chances of slip along a fault? High pore pressures reduce the effective normal stresses acting perpendicular to faults. These normal stresses resist shear movements i.e. fault blocks from sliding past each other. With increased pore pressure the effective normal stresses decreases, allowing shear movements.  Geophyicist Mark Zoback explains in more detail about the risks and management of seismic risk posed by wastewater disposal.

No comments:

Post a Comment