Microseismics This is an image showing hundreds of earthquakes directly caused by hydraulic fracturi

MicroseismicsThis is an image showing hundreds of earthquakes directly caused by hydraulic fracturing. Scary right? Hundreds of earthquakes set off by a single fracking job?To understand the relationship between earthquakes and fracking, we first need to understand something about this plot. Do you have a pen or a pencil nearby? Or a small marble? Something about that weight? Hold that object out in front of you and drop it to the floor. Would you call that an earthquake?That comparison is important because the energy released by whatever you just dropped hitting the ground is comparable to the energy released in the majority of these earthquakes.Hydraulic fracturing must cause earthquakes – rocks would not crack anywhere if there wasn’t a release of stress and that stress release must cause something that we would call an earthquake. The dictionary definition of an earthquake is “a series of vibrations induced in the earth’s crust by the abrupt rupture and rebound of rocks in which elastic strain has been slowly accumulating.” – any time a rock suddenly breaks and releases enough energy to vibrate the surrounding rocks it fits the definition of an earthquake.However, in a hydraulic fracturing job, the majority of the earthquakes produced are at magnitude -2. Read that again. You’ve heard of a magnitude 7.8 earthquake recently striking Nepal, the majority of these earthquakes are magnitude “Minus 2”. The study that produced this image sampled about a dozen fracking jobs across multiple different gas reservoirs; the most abundant were had magnitudes close to -2 and the largest quake had a magnitude less than 1.0 – about 1000x too small to be even felt at the surface.The amount of energy in these earthquakes is so tiny that it’s almost unfair to call them earthquakes. In fact, because the phrase “Fracking is causing thousands of earthquakes” conjures up images of the devastation large quakes bring, some scientists and engineers prefer the term “microseisms”, a term for ground tremors far too small to feel.Detecting the tiny vibrations of these microseismic earthquakes is difficult even for seismometers at the surface, but if they can be measured, they can give a huge amount of data. Each of these earthquakes, no matter how small, is the formation of a fracture in the rock that gas can flow through. If the quakes are detected, an engineer can tell how far away from the well the fractures are forming, whether the rock is being broken up in the way that it was planned, or even if additional pressure might be required to produce the required fractures.The monitoring wells listed in the image are wells drilled specifically for seismic instruments. To detect events this small, the seismometer must be extremely sensitive and very close to the quake site. These wells aren’t drilled on every site, but for the ones they are, the data literally lets you see 2 kilometers underground.Do you see how there are clusters of quakes that move away perpendicular to the well direction? A single horizontal well is broken into segments that are pressurized in sequence – that way pumps don’t have to pressurize the entire well at once. The earthquake hypocenters record the fractures formed from each segment of the well as it is pressurized – showing that the rock is in fact breaking in the way its supposed to.Perhaps more importantly for the safety of the technique, the lower image shows the depth to the events. The Marcellus shale, the target unit, is just under 50 meters thick, but its buried under 2 kilometers of rock. Most of the earthquakes happen in the Marcellus, a smaller number occur in units within 50 meters above or below the Marcellus, but beyond that there are absolutely zero events.In other words, by detecting these earthquakes, engineers can actually state with 100% certainty that the fractures are not propagating upward anywhere close to aquifers that hold drinking water (typically in the uppermost 500 meters). There is a full kilometer+ of intact rock in-between every fracking job and the water table, leaving it literally impossible for gas or fluids to migrate upwards to contaminate drinking water from these jobs.The plot you’re seeing just shows the type of data publicly available. If this kind of data is made public, then it’s likely drilling operators have much better data – letting them actually monitor the injection process at very fine scale, making sure it is being done both safely and effectively. When this kind of data is made available to scientists, they’ll also be able to use it to characterize the rocks, stress patterns, and fracture patterns throughout the country – giving scientists data that would otherwise not be available.So that’s my first earthquake post – microseismics. This is a really useful technique and it is hugely important from an environmental perspective to verify that fracking does not and cannot directly contaminate drinking water resources above. However, if you Google “Fracking and earthquakes” you will find a very different picture. You won’t hear about the thousands of microseismic events that characterize every fracking job, you’ll instead hear about the growing disaster in Oklahoma and maybe about a small cluster of larger earthquakes in Ohio directly associated with fracking.The fracking process cannot and should not ever cause larger events like that, and in fact as we’ll see the Oklahoma earthquake swarm actually does not have very much to do with fracking. However, there’s clearly more of a story to tell there as well – why are we getting swarms of larger earthquakes, why did fracking cause larger quakes at one site in Ohio, and what is different about each of those cases?If you’ve tuned in this far…that’s the story for my next post.-JBBImage credit and more information (Creative commons licensed paper):http://bit.ly/1P1lPv4References:http://dictionary.reference.com/browse/earthquakehttp://bit.ly/1EafL8Yhttp://bit.ly/1IhzN87http://bit.ly/1FQcArOhttp://bit.ly/1c7dBQdhttp://onforb.es/SLech9http://bit.ly/1GPT4b7Previous articles in series:http://on.fb.me/1yhKcYuhttp://on.fb.me/1IFOJd9http://on.fb.me/1CDW5Nyhttp://on.fb.me/1Gaibdghttp://on.fb.me/1CPPFd3http://fb.me/7huKhsqWrhttp://on.fb.me/1NC8Z5Khttp://on.fb.me/1GD2pVVhttp://on.fb.me/1Iey3fWhttp://on.fb.me/1Qo21iM -- source link
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