The July issue of Scientific American carried the best summary of the fracking-earthquake controversy I have seen so far. "Drilling For Earthquakes" by Anna Kuchment reviews the fracking (hydraulic fracturing), the associated water injection, the earthquakes, the science, and government reactions to the problem. In particular, the article shows the very different approaches the states of Texas and Oklahoma have taken to the problem. And I regret to say it doesn't make my native state of Texas look good by comparison. But first, the basics.
As I wrote in this space in 2013, water-injection wells to dispose of the brackish water that comes up sometimes along with oil and gas are nothing new. But the combination of fracking to extract fossil fuels from previously inaccessible formations, horizontal drilling to gain wider access to those formations, and the boom of widespread deployment of these techniques that has gone on in the last six or eight years, have led to a huge increase in the volume of water injected back deep underground. During 2014, in Texas a gallon of water was injected back into the ground for every 100 or so cubic feet of shale gas extracted. That may not sound like much, but Texas produced about 4 trillion(4,000,000,000,000) cubic feet of shale gas that year. Leave off two zeroes and that's how many gallons of water were injected back into the ground. And that ratio probably holds true more or less for the rest of the country as well.
Wastewater injection from fracking doesn't always cause earthquakes. North Dakota has had a lot of fracking and wastewater injection too, but hardly any earthquakes. On the other hand, Oklahoma, a place that was hardly famous for earthquakes before 2005, had 581 temblors of magnitude 3.0 or greater in 2014. Its most severe one recently happened in November 2011, when a 5.6-magnitude quake wrecked more than a dozen houses and injured a couple of people. Less severe but just as widespread quakes have been happening in North Texas, where the Barnett Shale has been exploited for natural gas in a big way, and injection wells are operating there too.
Because of the huge volumes of wastewater to deal with, oil and gas producers don't have too many options that won't make their operations too expensive to carry out. Treating the water to extract the salt and other minerals would mean distilling it, a hugely costly process that would turn them all into water-purification plants with an unprofitable sideline of making oil and gas as a byproduct. So that's not an option. Trucking it to a place where injecting it wouldn't cause earthquakes would be expensive, even if we knew of a nearby place where injecting it wouldn't cause earthquakes. And just throwing it out on the ground, which used to be a common practice in the bad old days before 1950 or so, would cause huge amounts of waterway pollution because of the salts, radioactivity, and other nasty stuff that comes up with the water. So going to the expense of drilling wells typically much deeper than the producing ones and injecting the wastewater downhole at tremendous pressures is the only thing that producers can typically do with it.
The trouble is, rocks are porous—that's the only way you can inject water into them in the first place. So that high-pressure water starts to move, and seeps toward faults, which are just big cracks between intact blocks of rock. Some faults are under shear stress. To envision shear stress, think of holding two old-fashioned chalkboard erasers together face to face and rubbing them back and forth across each other. It's shear stress you put on them that makes them slide. If you mash the erasers together perpendicularly, putting them under compressive stress, it's a lot harder to get them to move with shear stress. So a fault that is under shear stress won't slip and cause an earthquake as long as the compressive stress is great enough.
Then along comes your water injection at high pressure. It seeps through the pores to the cracks and provides an opposing pressure that can counteract the compressive stress that's keeping the fault from slipping. We're not talking lubrication here, but large opposing mechanical forces. I'm sure the technical details involve stress tensors and the whole nine yards of solid mechanics, but the basic picture is simple. When the fluid pressure exceeds a certain threshold, that fault is going to let go, and you've got an earthquake. People have even done experiments in the field to figure out exactly how much stress makes the faults slip, and there is a definite threshold, just as theory predicts.
Both from mechanical analyses and statistical studies, as well as abundant seismological data correlating particular regions of earthquake activity with particular injection-well activity, by now it is clear to all but the most biased observers that, generally speaking, the injection-well activity has caused the increase in earthquakes in both Texas and Oklahoma. The U. S. Geological Survey, which has been issuing long-range earthquake predictions by region for some time now for the convenience of structural engineers, insurance companies, and other interested parties, has had to revise its forecasts for Oklahoma and Texas sharply upward in the last few years. A contour map of earthquake likelihood for Oklahoma now looks like an archery target with Oklahoma City in the bullseye. And the scientific literature abounds with studies showing details of the correlation.
Oklahoma has a long tradition of assertive state government, dating back to the 1930s when it passed laws regulating things like the price of ice. And they have now continued that tradition by shutting down individual wells since 2015 and regulating the volume of wastewater that can be injected. On the other hand, the Texas agency in charge of oil and gas regulation (for historical reasons, it's called the Texas Railroad Commission) still has not been able to bring itself to admit that any earthquakes have been triggered by water injection associated with fossil-fuel production. But recently the Commission asserted its right to shut down wells if it wants to. So far, though, it hasn't wanted to.
To some degree, all this is water under the bridge, or well, as the case may be. Oil and gas markets are glutted right now, and the consensus is that the big fracking boom is over, at least in Texas and Oklahoma. But all that injected water is still down there, slowly diffusing, and some geologists estimate that the effects of water injection on earthquakes can last as long as twenty years. So in that sense, we may be dealing with the aftershocks of the fracking boom for some time.
Sources: Anna Kuchment's article "Drilling for Earthquakes" appeared in the July 2016 print issue of Scientific American, pp. 46-53. I also referred to a U. S. Department of Energy table of shale-gas production available at https://www.eia.gov/dnav/ng/ng_prod_shalegas_s1_a.htm. I blogged on earthquakes and fracking most recently on Dec. 30, 2013.
