The cover story of the latest issue of Physics Today is part explanation, part discussion of the use of fracking techniques in the oil and natural gas industries in America. As this topic gained traction in the news and online, I was always admittedly ignorant when it came to the actual science and details of these methods. I vaguely knew that fracking could be seen as beneficial in that many US power plants now burn cleaner natural gas instead of coal, but it also seemed obvious that pumping high pressure liquid (which isn’t pure water) into the ground was bound to cause other environmental problems. Still, I neither consider myself strongly for nor against these practices, but I did greatly appreciate the explanations and discussions provided in this article. Below I’ll highlight the parts I found interesting, but I do recommend that the interested reader take a look at the article.
Fractures in siltstone and black shale in the Utica shale, near Fort Plain, New York. (Photograph by Michael C. Rygel.)
The article begins with an explanation of black shale itself. At left is a picture of some black shale, part of the Utica shale in upstate New York. So what is black shale? Well, to quote the article: “Just as sandstones are a rock equivalent of sand, shales are a rock equivalent of mud.” Organic material, oil and/or gas, trapped in the shale gives it the darker color and name, black shale. The oil and gas will only remain in the shale under anoxic conditions. No need to open that extra browser tab, I had to look up what anoxic meant too. Anoxic water is water depleted of much of the dissolved oxygen that is typically in water, this usually happens when water is left stagnant. The dissolved oxygen in normal water would tend to oxidize the carbon in the sediment, destroying the organic material. Under the right conditions, roughly 2-4 km beneath the Earth’s surface, the heat and pressure will convert the organic material into oil. Go a bit further down, roughly 3-6 km, and the temperature and pressure rises, breaking the oil down into gas.
As most people are now aware, the general idea of fracking is to pump liquid into the black shale, causing fractures in the rock which allows the oil and gas to escape its confines and be collected. Three categories of fracking can be distinguished. The first is natural fracking, which is to say, the normal fracturing of shale due to the internal pressure of oil and gas, the fractures in the picture above are due to natural fracturing. Sometimes natural fractures allow oil and gas to escape the shale, the largest such natural seepage area can be found off the coast of Santa Barbara, California. The other methods of fracking are described in the figure below. The main differences pointed out in the article were the volume and viscosity of the water used to carry out the hydraulic fracking. In traditional fracking, water is made viscous by adding guar gum or hydroxyethyl cellulose. Typically about 75-1000 cubic meters of water are used to create a single fracture though which the oil and/or gas may be extracted. High-voulme (or super) fracking, on the other hand, uses a low viscosity water based liquid pumped at a high rate to create many smaller fracture networks along a horizontal well that is periodically plugged to create a number of fracking sites. The water usage is typically 100 times greater in high-volume fracking as compared to traditional fracking. The benefit, of course, is that high-volume fracking is capable of extracting oil and gas from tight shale formations where either few natural fractures exist for the oil and gas to migrate though, or the natural fractures have been sealed over time by the deposition of silica and/or carbonates. For a detailed layout of the environmental concerns surrounding high-volume fracking, see the insert within the main article.
Traditional and high-volume fracking. (a) In traditional fracking treatments, a high-viscosity fluid creates a single hydraulic fracture through which oil or gas (or both) migrates to the production well. (b) In high-volume fracking, or super fracking, large volumes of a low-viscosity liquid create a wide distribution of hydraulic fractures. Fossil fuels can then migrate through the fracture network to the production well. The sketch here shows the result of a sequence of four high-volume fracking injections. Such sequential injections would not be possible without directional drilling, which creates a horizontal production well in the target stratum.
The authors of this article found their way to studying fracking because of the occurrence of small earthquakes associated with high-volume fracking. Some production wells now monitor the seismic activity of the fracking with a series of seismometers distributed along the length of a monitoring well. Better earthquake prediction models would allow for better emergency preparedness by governments, more robust risk analysis by insurers, and possibly even save lives of those living in earthquake prone areas. So, from a research perspective, the earthquakes induced by fracking can provided a useful testbed for earthquake modeling. Below is a map of microseismicity associated with the Barnett shale in Texas. The monitoring well is situated at the origin, and each dot (or I guess + mark) represents a unique seismic event.
Small earthquakes associated with four high-volume frackings of the Barnett shale in Texas. Each tiny “+” symbol on this microseismicity map shows the epicenter of a microearthquake. Collectively, the symbols reveal the distribution of fractures induced by the injected water. The monitoring well is at the origin of the coordinate system shown. The injection well is off to the right; the thin line shows its horizontal extent. (Adapted from: S.Maxwell, Leading Edge 30, 340 (2011))
These small earthquakes are typically very weak and can not be felt on the surface. The frequency of natural earthquakes of a certain magnitude or greater follows a well defined function where the logarithm of the number of earthquakes with magnitude m or greater decreases linearly with m. This is just to say that small earthquakes are common and big earthquakes are rare. Studying both natural and fracking induced earthquakes, the distribution of earthquake magnitudes from high-volume fracking have a steeper fall off than natural earthquakes, meaning that a large earthquake would be extremely rare, but not ruled out. The authors quote that the probability of seeing a magnitude 4 earthquake (minimally damaging) from high-volume fracking is less than on in a billion. An effort has been made by an old acquaintance of mine, J. Quinn Norris at UC Davis to model the fracking earthquakes using “a type of graph-theory analysis called invasion percolation from a point source.” See his paper here.
The last part of the article that I found particularly interesting was the estimates from the Department of Energy in 2011 on the availability of recoverable oil in the 48 contiguous states. The total estimated volume of recoverable oil was 24 billion barrels. Of this, 3.6 billion barrels are attributed to the Bakken shale, mostly in North Dakota, and 15.4 billion barrels are expected from the Monterey shale along the coast of California. As a California native this was surprising to me, and is probably so because efforts to use high volume fracking on this shale have so far proved unfruitful because of the natural fractures which already exist. Maybe think of it like trying to fracture a sponge by pushing water through it, where the water will happily fill every nook and cranny instead of build up any pressure. Still, this source is likely to play some part in future energy discussions as other sources are depleted. Of course, just because this material exists does not mean we must burn it to satisfy our energy needs. Most of this oil and gas has been locked away for hundreds of millions of years and it would gladly remain so if we allowed it to. I for one am optimistic that fossil fuel consumption will significantly decrease within my lifetime and we can get on with solar powered hovercrafts and the like.