The 5.6 magnitude earthquake that rocked the state of Oklahoma on 5th November, 2011, has once again raised some serious questions about the increasing use of hydraulic fracturing to tap natural resources, i.e. oil and gas, from beneath the Earth's crust. Even though it has not yet been ascertained whether this quake was caused as a result of weakening of the Earth's crust as a result of fracking, most of the people believe so. The experts at the United States Geological Survey (USGS) though, are of the opinion that fracking cannot trigger an earthquake of this magnitude. According to them, it was the fault line existing in this region that triggered the Oklahoma earthquake - and not fracking.
Link Between Hydraulic Fracturing and Earthquakes
While the presence of a fault line in this region of the United States can be an apt explanation for the 5.6 magnitude Oklahoma earthquake, what about the sudden rise in seismic activity here? Between 1972 to 2008, an average of 2-6 earthquakes were recorded in the state of Oklahoma every year. In 2009, the number of earthquakes recorded reached 50, and further increased to a whopping 1047 in 2010. One cannot ignore the fact that more than a thousand drilling wells and more than a hundred injection wells have cropped up in this region over the course of time. Back in August itself, the region experienced a series of tremors, all ranging between the magnitude of 1 and 2.5, and now the 5.6 magnitude quake. While environmentalists cite a link between fracking and earthquakes to oppose such projects, those in this business refute these allegations as baseless.
Man-made Hydraulic Fracturing
Hydraulic fracturing is basically the process wherein a fluid - which is basically a mixture of water and some chemicals, is pumped underground at high pressure to develop cracks in the sediment layers which have the natural gas and oil trapped within. While breakdown of sediment layers due to the development of veins and dikes does occur, it is not as rampant as man-made hydraulic fracturing - a technology used to harness natural oil and gas trapped within the Earth's crust. Man-made hydraulic fracturing is more often referred to as 'fracking' or 'hydrofracking'. While many people tend to associate fracking with drilling, the fact is that the two are totally different concepts, and fracking is used only after the process of drilling is complete.
Even though there is abundance of natural gas and oil in shale, its characteristic low permeability can hamper the flow of these natural resources. In such circumstances, one convenient method of releasing these trapped resources is to make cracks in the rock bed - and this is exactly what is done in the process of hydraulic fracturing. After vertical drilling is complete, horizontal drilling is done in the targeted sediment layer, i.e. the hydrofrac zone or hydraulic fracturing zone, to make passage for the 'fracfluid'. When the fracfluid is released at high pressure, it causes the targeted layer to crack, and the gas which is released as a result of this makes its way to the surface through the bore-well. At times, hydraulic fracturing is also used to restore the cracks within the formation to ensure unabated flow of natural oil and gas in the existing wells.
Does Hydraulic Fracturing Trigger Earthquakes?
Until recently, hydraulic fracturing was opposed for its tendency to pollute ground water, but the series of earthquakes in the United States and the United Kingdom has once again brought it to the headlines. This time it is the sudden rise in seismic activity in Oklahoma and surrounding regions, followed by the 5.6 magnitude earthquake, that has brought the alleged link between fracking and earthquakes to the forefront. Environmentalists argue that the sudden rise in seismic activity and the rise in the number of injection wells in this region cannot be a coincidence, and there has to be some link between the two.
If the experts at the USGS are to be believed, it is not possible for such intense earthquakes to be caused as a result of human activities such as fracking. These experts further add that such intense earthquakes are usually attributed to plate tectonics. However, they don't deny the chances of human activities causing minor earthquakes. In fact, deep injection of fluid and nuclear detonations are the two prominent human activities with the ability to trigger tremors by the USGS. That being said, they do make it a point to add that the earthquakes caused as a result of these activities are not of such high magnitude.
Previously there have been cases of earthquakes being triggered by deep injection of wastewater and deep injection of water for the production of geothermal power. One prominent example is that of the Rocky Mountain Arsenal (RMA) deep injection well which was built by the RMA in 1961 for disposing liquid waste, but only used till 1966 as it was eventually revealed that the fluid injection was triggering earthquakes in this area. As the regions with abundance of natural oil and gas are usually located along the fault lines, it gives rise to the misconception that energy exploration and production makes us vulnerable to hazards such as earthquakes.
While that does rule out the chances of major earthquakes, like the one in Oklahoma, being triggered by hydraulic fracturing, it also suggests that less-intense seismic activity can be linked to the practice of harnessing natural gas and oil. However, one also has to note that major earthquakes can be triggered as a result of fluid injection when it is injected at the wrong place - an unknown fault for instance. In such circumstances, earthquakes with a magnitude of 5 or more cannot be ruled out.