by Emil Morhardt
We hear a great deal about the economic benefits of hydraulic fracturing, and even more about its potential liabilities, but seldom very much about exactly how fracking works. A fascinating new paper just published by the American Society of Mechanical Engineers (Bazant et al. 2014) combines an extremely clear explanation of the process in non-technical language with a detailed mathematical analysis of the mechanics involved (a combination uncommon in engineering papers). The question at hand is why, with pipes just three-inches in diameter, spaced half a kilometer apart, it is possible to get so much gas out of shale beds. The first thing to know is that even this technology gets only about
5–15% of the gas embedded in the shale, so it’s likely they’ll be going back for more as the technology improves. They know about this percentage because of how much gas they can extract from the rock samples they get out of the well cores.
The reason fracking is useful in the first place is that the gas, in the form of solid kerogen, is trapped in nanovoids, and the natural cracks that would let it migrate to a borehole are either squeezed shut by the weight of the three kilometers of rock above them, or filled up with calcite or other minerals. The trick is to open up the existing cracks, form new ones, then keep them open so gas can flow out. Through a lot of detailed engineering calculations the authors determine that the way to open up the most cracks (and in the process to keep the most fracking fluid stuck in the shale, rather than returning to the surface where it needs to be treated or re-injected) is to gradually increase the hydraulic pressure to a point where it gradually opens up vertical cracks, while the sand they also inject gradually fills an ever-widening array of them. The acid included in the fracking fluid apparently breaks up the rough edges (asperities) of the cracks into pieces small enough to act like the sand, increasing the amount of crack propping. Since the injection pressure of the fracking fluid seldom exceeds the pressure exerted by the overburden, horizontal cracks in the natural bedding plane are equally seldom opened up. The whole procedure at this point in its evolution is dependent on vertical cracks.
Methane, which makes up most of the content of natural gas, does not dissolve in water (nor in fracking fluid, which is 99% water) so it migrates through the cracks and into the well casing in the form of gas bubbles, which, when they reach the vertical borehole are aided in coming to the surface because of their buoyancy.
The graphs included in the paper show that the highest rate of gas flow occurs soon after a well is drilled, with exponentially decreasing flow over the four- or five-year life of the well.
So, in summary, fracking usually occurs in shale layers 20 to 150 meters thick, lying on the order of three kilometers below ground—a long way below any aquifers that might be susceptible to contamination, but of course the well shaft must pass through these aquifers. From each vertical borehole, several horizontal ones several kilometers long are drilled about 500 meters apart. These horizontal holes are lined with three-inch diameter steel pipe that is then perforated explosively at five to eight locations along their lengths. Several million gallons of water with a little sand, acid, and other chemicals is injected under great pressure. (Although this sounds like a lot of water it corresponds to only 1-2 millimeters of rain falling over the well field.) The injection initially opens up natural vertical cracks in the shale, which are typically 15 to 50 centimeters apart. Continued pressure increases the array of vertical cracks, ideally about 10 centimeters apart. The methane forms bubbles and flows along the pressure gradient toward the horizontal pipe, then through the pipe to the vertical borehole, and up to the surface to be captured.
If you’re not familiar with materials science and engineering technical writing, I’d suggest looking at the full paper (link below). It opens a window into the thought processes of engineers that most of us never encounter.
Bazant, Z.P., Salviato, M., Chau, V.T., Viswanathan, H., Zubelewicz, A., 2014. Why Fracking Works. Journal of Applied Mechanics 81. Full paper at: http://imechanica.org/files/Bazant_fracking.pdf.