A study from the University of Texas found a correlation between injection wells – wells used to dispose of wastewater from hydraulic fracturing – and clusters of small earthquakes.
The study looked at injection wells located in the Barnett Shale, a region in North Texas with many active natural gas wells. Much of the natural gas is being produced with the use of fracking, or hydraulic fracturing. Fracking involves pumping water and chemicals into a gas well to cause faults in the shale where the natural gas is trapped. The natural gas is then able to flow through the rock and up the well.
Much of the water and chemical mixture also flows back up the well, where it is separated from the natural gas and sent off site for disposal. Different areas use different methods to dispose of the wastewater. In some places, the wastewater had been sent to water treatment plants, until the facilities determined they didn’t have the technology to clean the water. In North Texas, injection wells are the usual method.
Injection wells pump the wastewater deep underground where the water is expected to stay for a very long time. They are designed to be situated away from aquifers. In theory, the wastewater will eventually move up through the rock and be filtered by the rock, and therefore drinkable. (Natural aquifers are also filtered by rocks.)
The study examined seismic data gathered between November 2009 and September 2011. It found more very small earthquakes than other studies had noticed. These small earthquakes are too small for humans to feel; they don’t knock pictures off walls or cause damage to buildings. Nevertheless, they are clustered around injection wells with high injection rates.
It is interesting that not every high volume injection well had an associated earthquake cluster. It is probably some interplay between geology, the pressure and volume of the injections, and possibly the chemicals in the wastewater. More research is needed to determine exactly why the small earthquakes are clustering near certain high volume injection wells.
Fort Worth, Texas photo via Shutterstock