Fluid Induced Earthquakes: Insights from Hydrogeology and Poro-mechanics
By Dr. Shemin Ge, Professor of hydrogeology at the Dept. of Geological Sciences, University of Colorado-Boulder, gave a guest lecture today, March 24, 2016, at the University of Florida, Williamson Hall.
Dr. Ge was introduced by Dr. Tom Frazer, Acting Director of the University of Florida Water Institute.
Dr. Ge spoke mainly on the correlation of earthquakes and the various types of wastewater injection deep into the earth. The fracking technique employs what is called the “dewatering method,” where extracting water from deep underground lessens pressure, which causes the mixture of oil and gas to flow out of the matrix, which in turn leaves voids underground. Roughly 40 per cent of the water injected is dispersed underground.
There are about 188,070 injection wells in 34 states in the U.S., of these, about 106, 070 are active.
Devising correlations between seismic activity and injection wells is approached by using a document called the Advanced National Seismic System Earthquake Catalog (ANSS). According to the ANSS, in the 1970s, the region had only a very few earthquakes, but by 2014, there were 650. A spatial correlation is recognized if the seismic activity occurs within 15 km of the well, and a temporal correlation is recognized if the quake occurs during the time of injection. Thus we arrive at what is called a spatiotemporal association.
Dr. Ge offered mathematical formulae to prove what is called the poroelasticity governing equations, which are very complicated to non-mathematicians. However, the principal was first described in 1892, which is called the poroelasticity phenomenon, which simply says that water in pores, or in tube well bore, for example, will rise when the earth matrix surrounding it is compressed. This is an example of changes in stress which produces a change in fluid pressure.
Many variables may affect what is called the pore mechanics, which determines whether or not underground strata shifts, resulting in earthquakes. Some of the variables are the rock properties, faults, pore pressure, background stress, and stratigraphy.
Experiments with pore pressure in the earth have resulted in induced quakes and it was found that injection rates may influence the results. Mainly, high rates of injection caused more earthquakes than slower rates. This fact is significant if the goal is to reduce earthquakes.
Dr. Ge’s lecture dealt entirely with the scientific aspect of induced seismic activity, and not the political. During the Q & A period afterward, our writer inquired as to the industry’s reaction to this mounting evidence against fracking. Dr. Ge indicated that the response seemed to be lessened because of the cost involved.
Another query as to “what happens to the injected water that stays underground?” was answered by “We don’t know.”