Mining And Refining: Fracking

Normally on “Mining and Refining,” we concentrate on the actual material that’s mined and refined. We’ve covered everything from copper to tungsten, with side trips to more unusual materials like sulfur and helium. The idea is to shine a spotlight on the geology and chemistry of the material while concentrating on the different technologies needed to exploit often very rare or low-concentration deposits and bring them to market.

This time, though, we’re going to take a look at not a specific resource, but a technique: fracking. Hydraulic fracturing is very much in the news lately for its potential environmental impact, both in terms of its immediate effects on groundwater quality and for its perpetuation of our dependence on fossil fuels. Understanding what fracking is and how it works is key to being able to assess the risks and benefits of its use. There’s also the fact that like many engineering processes carried out on a massive scale, there are a lot of interesting things going on with fracking that are worth exploring in their own right.

Fossil Mud

Although hydraulic fracturing has been used since at least the 1940s to stimulate production in oil and gas wells and is used in all kinds of well drilled into multiple rock types, fracking is most strongly associated these days with the development of oil and natural gas deposits in shale. Shale is a sedimentary rock formed from ancient muds made from fine grains of clay and silt. These are some of the finest-grained materials possible, with grains ranging from 62 microns in diameter down to less than a micron. Grains that fine only settle out of suspension very slowly, and tend to do so only where there are no currents.

Shale outcropping in a road cut in Kentucky. The well-defined layers were formed in still waters, where clay and silt particles slowly accumulated. The dark color means a lot of organic material from algae and plankton mixed in. Source: James St. John, CC BY 2.0, via Wikimedia Commons

The breakup of Pangea during the Cretaceous period provided much of the economically important shale formations in today’s eastern United States, like the Marcellus formation that stretches from New York state into Ohio and down almost to Tennesee. The warm, calm waters of the newly forming Atlantic Ocean formed the perfect place for clay- and silt-laden runoff to accumulate and settle, eventually forming the shale formation.

Shale is often associated with oil and natural gas because the conditions that favor its formation also favor hydrocarbon creation. The warm, still Cretaceous waters were perfect for phytoplankton and algal growth, and when those organisms died they rained down along with the silt and clay grains to the low-oxygen environment at the bottom. Layer upon layer built up slowly over the millennia, but instead of decomposing as they would have in an oxygen-rich environment, the reducing conditions slowly transformed the biomass into kerogen, or solid deposits of hydrocarbons. With the addition of heat and pressure, the hydrocarbons in kerogen were cooked into oil and natural gas.

In some cases, the tight grain structure of shale acts as an impermeable barrier to keep oil and gas generated in lower layers from floating up, forming underground deposits of liquid and gas. In other cases, kerogens are transformed into oil or natural gas right within the shale, trapped within its pores. Under enough pressure, gas can even dissolve right into the shale matrix itself, to be released only when the pressure in the rock is relieved.

Horizontal Boring

While getting at these sequestered oil and gas deposits requires more than just drilling a hole in the ground, fracking starts with exactly that. Traditional well-drilling techniques, where a rotary table rig using lengths of drill pipe spins a drill bit into rock layers underground while pumping a slurry called drilling mud down the bore to cool and lubricate the bit, are used to start the well. The initial bore proceeds straight down until it passes through the lowest aquifer in the region, at which point the entire bore is lined with a steel pipe casing. The casing is filled with cementitious grout that’s forced out of the bottom of the casing by a plug inserted at the surface and pressed down by the drilling rig. This squeezes the grout between the outside of the casing and the borehole and back up to the surface, sealing it off from the water-bearing layers it passes through and serving as a foundation for equipment that will eventually be added to the wellhead, such as blow-out preventers.

Once the well is sealed off, vertical boring continues until the kickoff point, where the bore transitions from vertical to horizontal. Because the target shale seam is relatively thin — often only 50 to 300 feet (15 to 100 meters) thick — drilling a vertical bore through it would only expose a small amount of surface area. Fracking is all about increasing surface area and connecting as many pores in the shale to the bore; drilling horizontally within the shale seam makes that possible. Geologists and mining engineers determine the kickoff point based on seismic surveys and drilling logs from other wells in the area and calculate the radius needed to put the bore in the middle of the seam. Given that the drill string can only turn by a few degrees at most, the radius tends to be huge — often hundreds of meters.

Directional drilling has been used since the 1920s, often to steal oil from other claims, and so many techniques have been developed for changing the direction of a drill string deep underground. One of the most common methods used in fracking wells is the mud motor. Powered by drilling mud pumped down the drill pipe and forced between a helical stator and rotor, the mud motor can spin the drill bit at 60 to 100 RPM. When boring a traditional vertical well, the mud motor can be used in addition to spinning the entire drill string, to achieve a higher rate of penetration. The mud motor can also power the bit with the drill string locked in place, and by adding angled spacers between the mud motor and the drill string, the bit can begin drilling at a shallow angle, generally just a few degrees off vertical. The drill string is flexible enough to bend and follow the mud motor on its path to intersect the shale seam. The azimuth of the bore can be changed, too, by rotating the drill string so the bit heads off in a slightly different direction. Some tools allow the bend in the motor to be changed without pulling the entire drill string up, which represents significant savings.

Determining where the drill bit is under miles of rock is the job of downhole tools like the measurement while drilling (MWD) tool. These battery-powered tools vary in what they can measure, but typically include temperature and pressure sensors and inertial measuring units (IMU) to determine the angle of the bit. Some MWD tools also include magnetometers for orientation to Earth’s magnetic field. Transmitting data back to the surface from the MWD can be a problem, and while more use is being made of electrical and fiber optic connections these days, many MWDs use the drilling mud itself as a physical transport medium. Mud telemetry uses pressure waves set up in the column of drilling mud to send data back up to pressure transducers on the surface. Data rates are low; 40 bps at best, dropping off sharply with increasing distance. Mud telemetry is also hampered by any gas dissolved in the drilling mud, which strongly attenuates the signal.

Let The Fracking Begin

Once the horizontal borehole is placed in the shale seam, a steel casing is placed in the bore and grouted with cement. At this point, the bore is completely isolated from the surrounding rock and needs to be perforated. This is accomplished with a perforating gun, a length of pipe studded with small shaped charges. The perforating gun is prepared on the surface by pyrotechnicians who place the charges into the gun and connect them together with detonating cord. The gun is lowered into the bore and placed at the very end of the horizontal section, called the toe. When the charges are detonated, they form highly energetic jets of fluidized metal that lance through the casing and grout and into the surrounding shale. Penetration depth and width depend on the specific shaped charge used but can extend up to half a meter into the surrounding rock.

Perforation can also be accomplished non-explosively, using a tool that directs jets of high-pressure abrasive-charged fluid through ports in its sides. It’s not too far removed from water jet cutting, and can cut right through the steel and cement casing and penetrate well into the surrounding shale. The advantage to this type of perforation is that it can be built into a single multipurpose tool.

Once the bore has been perforated, fracturing can occur. The principle is simple: an incompressible fluid is pumped into the borehole under great pressure. The fluid leaves the borehole and enters the perforations, cracking the rock and enlarging the original perforations. The cracks can extend many meters from the original borehole into the rock, exposing vastly more surface area of the rock to the borehole.

Fracking is more than making cracks. The network of cracks produced by fracking physically connects kerogen deposits within the shale to the borehole. But getting the methane (black in inset) free from the kerogen (yellow) is a complicated balance of hydrophobic and hydrophilic interactions between the shale, the kerogen, and the fracturing fluid. Source: Thomas Lee, Lydéric Bocquet, Benoit Coasne, CC BY 4.0, via Wikimedia Commons

The pressure needed to hydraulically fracture solid rock perhaps a mile or more below the surface can be tremendous — up to 15,000 pounds per square inch (100 MPa). In addition to the high pressure, the fracking fluid must be pumped at extremely high volumes, up to 10 cu ft/s (265 lps). The overall volume of material needed is impressive, too — a 6″ borehole that’s 10,000 feet long would take almost 15,000 gallons of fluid to fill alone. Add in the volume of fluid needed to fill the fractures and that could easily exceed 5 million gallons.

Fracking fluid is a slurry made mostly from water and sand. The sand serves as a proppant, which keeps the tiny microfractures from collapsing after fracking pressure is released. Fracking fluid also contains a fraction of a percent of various chemical additives, mostly to form a gel that effectively transfers the hydraulic force while keeping the proppant suspended. Guar gum, a water-soluble polysaccharide extracted from guar beans, is often used to create the gel. Fracking gels are sometimes broken down after a while to clear the fractures and allow freer flow; a combination of acids and enzymes is usually used for this job.

Once fracturing is complete, the fracking fluid is removed from the borehole. It’s impossible to recover all the fluid; sometimes as much as 50% is recovered, but often as little as 5% can be pumped back to the surface. Once a section of the borehole has been fractured, it’s sealed off from the rest of the well by an isolating plug placed upstream of the freshly fracked section. The entire process — perforating, fracking, recovery, isolation — is repeated up the borehole until the entire horizontal bore is fracked. The isolating plugs are then bored out, and the well can begin production.

52 thoughts on “Mining And Refining: Fracking

    1. I used ChatGPT to finish the sentence: ‘The advantage to this type of perforation is that it can …… be built into a single multipurpose tool which can access and stimulate multiple underground reserves. This eliminates the need to drill separate wells for each layer, saving time, money, and minimizing the surface footprint of the operation. However, concerns remain about the environmental impact of fracking, including potential water contamination and induced seismic activity.’

      1. Doubt it. You might reuse the perforation tool, but you’d have to pull it out and run it down another bore hole. That crap spewed by the chatbot says you can use on perforation tool so as not to have to drill multiple bore holes – pure BS.

        Thanks, but I’ll wait and see what the author meant to write.

          1. It’s the internet.

            Only bet can be a public forfeit, with vid posted as proof.

            I suggest loser has to wear sign saying ‘I was wrong on the internet’ walk back and forth on city hall steps, lunchtime.
            Wearing a prisoners uniform (typically ‘warning orange’ in USA) w skidmarked underpants on outside.
            Then post vid.
            Honor bound.

            This should be the next TikTok trend.
            No chance of that.

            I think we should do more nuclear fracking.
            Just because the first two experiments failed, doesn’t mean we should stop trying.
            Look it up. We did two test shots. Then we quit. :-(

            Also: All of northern Germany and Netherlands are prime fracking territory. But they haven’t the politics.

      2. Looks like i won the bet. The article now reads: “The advantage to this type of perforation is that it can be built into a single multipurpose tool.”, which is EXACTLY what ChatGPT gave us. I seem to remember HaHa offering to strip naked in front of city hall with a video camera for a selfie?

        1. Nope. You lost.

          The chatbot went on to say a bunch of other things in the same sentence that the author didn’t. It also provided another whole paragraph.

          All of the additional stuff is simply incorrect.

          Thus do fortune tellers and chatbot proponents work:
          Only mention the occasional randomly correct “hit” and ignore all the other times when it was completely wrong.

    2. The advantage of solid fuel plasma perforators and abrasive perforators are. 1 you don’t have to deal with hazmat / explosives, which all require more licenses/ safety/ job cost. and 2 you get better control of the perforations. With a shape charge you load up a perf-gun and it pops several round holes at a set distance apart. With the other methods you can cut slots or spirals, and can vary the distance between perforations without an additional trip to the surface.

  1. Typically a perforating gun has multiple shaped charges such as form the warhead of an RPG7.

    Where the analogy to water jet cutting comes from is rather strange. I seriously doubt that would work as the abrasive would simply plug the pores in the formations. As far as ChatGPT it’s pure BS.

    Perforating guns have been around ever since shaped charges were invented. Prior to that they lowered a container holding 10-20 gallons of nitroglycerin down the hole and then dropped a weight down hole to set it off.

    The front cover of a book on the Ohio oil fields around 1900 shows 3-4 guys standing about 10-15 ft from the “shooter” as he is pouring the contents of a tall narrow nitro can into the tool that he was filling.

    Very different safety protocols.

    1. My grandfather grew up in western Pennsylvania in the Titusville — Oil city area around the turn of the last century. He would tell me stories of going around to the oil wells with some of his friends and collecting the empty nitroglycerin cans and draining the last few drops of nitroglycerin into a bottle until they had accumulated enough nitroglycerin.l to amuse a group of small boys .This they would use to play practical jokes like pouring out a little nitroglycerin on the trolley tracks or applying it to buggy springs where it would explode to general amusement.

      As they say, times change.

        1. My circle of boyhood acquaintances had less than 10 fingers, on average. Paul, Jimbo and somebody else whos name escapes me had ‘misadventures’. At least a couple more. Good times.

          BTW times have changed!
          Nitroglycerine is dangerous and unstable.
          Nitrocellulose is much better and available in the reloading sections of guns stores in free nations. ‘Smokeless powder’. Also FFF black! Also Tannerite!

  2. Based on the discovery of oil wells producing more, after an earthquake. So let’s make shockwaves down there. I worked with a physicist on it. Pressure pulsations are not easy to make with so much hose/tubing/pipe.
    Fracking also uses very strong acids injected to dissolve formations as well.
    Water-injection is also common, to displace the oil and bring it up. Water is cheap, free. It’s almost emptied a few lakes here. The environmental costs are high.

    1. There are a wide range of methods for increasing rate and total recovery from a well. You are conflating them into a meaningless hodgepodge. Choice of method is very varied and entirely governed by the reservoir characteristics.

      Upstream petroleum engineering has historically always been the highest paid field. Mines and/or A&M have two derrick drill ship simulators.

  3. Fracking is finding a new purpose these days…. geothermal energy. Just drilling a hole into hot rocks won’t produce a very efficient return of energy. Fracking increases the surface area for hotter water/steam and allows more send/return wells in a field. The only problem with fracking is the increase in earthquakes and contamination of local wells.

    1. I believe most readers enjoy a bit of variation on these pages – I certainly do. There’s a load of space stuff that is truly fascinating IMO. I’m sure the editors and sponsors have done extensive research to see what brings readers in the most and the winning combination would seem to be a mixture of DIY hacks and more industrial or high tech stuff. Keep up the good work !!

    2. This is an article in a series of articles describing industrial processes for material acquisition. I have enjoyed all of them, as I’m sure many others have.

      Not hacks, but still interesting. I count 12 articles in the series, so maybe it’s a part of the spirit of the website now, if not the name /shrug

  4. Fracking also has ruined many biotopes, potable water sources etc. This article feels a little like propaganda. Mining in general can have serious consequences like artificially induced earthquakes, destroyed houses etc. A local magnesium mine close to where I live purported to have been filled with diesel fuel in an effort to stabilize the mine, don’t ask me why they did that. It’s a bio hazard waiting to happen when that stuff comes to the surface.

    1. Yeah I was less impressed by the talk about lining below the lowest aquifer, as we have all seen US kitchen taps spouting hydrocarbons and setting fire. Maybe that part is optional.

      But hey – free diesel in a few year’s time, what’s not to like?

  5. This is bad. Such overexploitation ruins whole environments.
    I heard stories on TV/radio about villages having cracks in their houses because of fracking attempts and/or companies trying to gain thermal energy.
    Mining had caused similar issues, too. After there was nolonger something to exploit from mother nature, mines often had been abandoned. The electric water pumps had been cut from power, too. The result was that whole landscapes had been imploded, because the mine shafts collapsed. Reading such articles here feels like reading the wonderful optimistic advertisements in old Windows installers, such as that from Windows Me.

      1. The issue with your comment is that saying “there is no third choice” implicitly precludes criticism of extraction. It may be necessary to mine and drill, but that does not mean we should not reduce the scale, find alternatives, and more thoroughly regulate the process.

        1. Or fill up those ol’ mines, eh? I don’t know much about mining but you’d think an old mineshaft would be put to some use… It’s a shame. Up here where I live (Ishpeming, Michigan) we got a lot of old iron mines that, as far as my knowledge goes, were abandoned and left to fill up with water. It’s a shame, but as for the scale of the mines, what else could a man or two do?

  6. It is not the technologies fault
    It is the way in which it is used
    You would think that fracking near housing or aquifers would be a “no-brainer NO”
    If you find a useful hack or work around for corporate greed let me know

    1. So where do you want to do it?
      Nature reserves? Under the sea?

      What I know of fracking, they only take out a small part of the oil. I guess it’s better to let it sit where it is, and maybe extract it all with a more decent method, even when that would not happen until oils becomes 10x as expensive as it is now.

  7. Sometimes using diplomatic language is entirely inappropriate.

    Fracking has “potential environmental impacts” the same way forest fires do.

    It’s always bad.

    The only thing in question is if someone can convince people that it is minimal enough to do it so they can have their sweet ground juice this week.

    1. “Fracking has “potential environmental impacts” the same way forest fires do.”

      Forest fires are an essential element of a healthy forest. The Jack Pine bird only builds its nest in a 17 year old Jack pine tree. Without a fire to clear out the old trees there would be very few 17 year old trees in the forest.

  8. Thank you for an informative article. I’ll make two comments, one geophysical, the other, political.

    Perhaps you could answer a question I’ve posed elsewhere: fracking sometime causes earthquakes. What is never mentioned is that the fracking has released the stress that was already in that region. That release is, in fact, reducing the intensity of some future movement.

    Now, I suggest, that after a major earthquake has occurred, that fracking should be done regularly along the fault line so that there will not be a future build up of stress to a damage causing level. Of course, to do this without tests would be dangerous. But there are areas in the Pacific where there is strong seismic activity but no people for thousands of kilometres. The idea could be tested there.

    The second comment is much briefer: if Europe had fracked like America Crimea would still be part of Ukraine and we would have no war. I’ll leave the reader to work out why.

  9. Interesting, but no mention about the efficiency of the process. Last I heard, for every 3 gallons of shale oil extracted, 1 gallon (from the underground) is burned. I wish this article would explain why.

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