Pore Size and Shape & the Release of Radon Gas in Fractured Rocks in the Marcellus Shale Gas Fields.
Last edited Tue Mar 26, 2019, 12:30 PM - Edit history (3)
The paper I'll discuss in this post is this one: Investigating Effects of Pore Size Distribution and Pore Shape on Radon Production in Marcellus Shale Gas Formation (Sondergeld et al, Energy Fuels, 2019, 33 (2), pp 700–707).
Although it garnered very little attention until the late 1930's, other than as a colorant for stained glass and to make orange glazes for ceramic cookware and serving dishes, uranium was of considerable scientific interest, and some commercial interest. Industrially the ore was mined not for the metal itself, but rather for its decay product, radium, which was widely used in luminescent watch and clock dials. (I had one of these when I was a small kid. I thought it was great.) The discovery of radioactivity was also associated with uranium, and it gathered much interest in what was, again up until the late 1930's, when Lise Meitner discovered nuclear fission while interpreting the experimental data from an experiment conducted in the laboratory of Otto Hahn.
It was not recognized until well after the discovery of nuclear fission that uranium is a very common element, about as common as tin. Because uranium has been present on the planet since its formation and is often fixed in ores, it has had time to come into "secular equilibrium" with all of its decay products except for the final product, non-radioactive lead. Except in the ocean, which contains a little under 5 billion tons of uranium, where the chemical distribution of decay products is driven by solubility and is thus subject to fractionation, the products of uranium decay generally remain in the ores, unless the ores are disturbed.
The Marcellus shale, which is a large producer of dangerous natural gas is, in fact, a low grade uranium ore, and throughout its geological history it has contained all of the decay products of uranium.
Here is the decay chart for U-238, which should be fairly familiar to people in high school science classes:
Radon-222 (Rn-222) is a noble gas. Where uranium is found in surface soils, it can accumulate in people's basements, and can represent a significant health hazard, in paticular because it's decay product, highly radioactive polonium-218, can lodge in people's lungs, go through several fast radioactive decays and remain, ultimately, lodged as lead-210, with a half life of 22 years. (I have measurable radon in my basement, and probably have a few radioactive atoms in my lungs.)
The half-life of uranium-238 is approximately equal to the age of the earth, about 4.5 billion years. There is so much uranium on the surface and subsurface of the Earth that no technology can ever eliminate it.
Here, for completeness, is the decay chart for U-235, which is also found in natural uranium, although its shorter half-life, 703.8 million years, means that it is relatively depleted in this isotope. (About 1.8 billion years ago, the fraction of U-235 found in uranium ores was high enough that natural nuclear reactors operated, most famously at Oklo, in Gabon.)
There is also a related decay series for thorium-232, itself a decay product from historic Pu-244 which has more or less gone extinct on earth.
A fourth decay series, the Cm-249/Np-237 series went extinct early in Earth's history.
Fracking has allowed for the release of radon gas from the Marcellus shale uranium ores which are not being mined for uranium, but for the dangerous natural gas that is mined in ever increasing amounts while we all wait for the grand so called "renewable energy" nirvana that never comes, as I often say, like Godot.
The paper cited here at the opening is about the mechanism of the release of radon from natural gas, and the fate of that radon as it's shipped to end users.
From the introduction:
Radon gas associated with shale gas production has come under the scrutiny of medical and environmental societies because of its potential negative impacts on the public health and environment.4−6 Radon is the daughter product of radium. Its most stable isotope is 222Rn with a half-life of 3.8 days. Radon is commonly found in the gaseous phase, but it can also partition into the aqueous phase such as contaminated brine and flowback fluids from hydraulic fracturings.7−12 Epidemiological and toxicological surveys show that exposure of radioactive radon causes lung cancer.13,14 Considering radon’s hazard to the public, the EPA set the safe level of radon concentration at 4 pCi/L. Picocuries per liter is a unit of radioactivity. Radon production from the Marcellus Shale is particularly more severe than other shale gas reservoirs and it is worth more attention. First, Marcellus Shale contains highly concentrated uranium and radium, inferring possibly high concentration of radon. Uranium concentration in rock can reach about 8.9−83.7 ppm, which is much higher than other US shale formations.15 Laboratory test measured radium concentration in hydraulic fracturing flowback water to be 1.7 × 10^4 pCi/L.16 Kondash et al.17 also pointed out that flowback water from Marcellus Shale contained unusually high levels of radium. Secondly, field measurements confirmed the existence of radon at a wellsite4 and inside a natural gas pipeline.6 Both observations indicated the radon level was higher than the safe standard. Thirdly, Marcellus Shale is close to a highly populated residential area, which implies a short transportation time for radon to decay from wellsite to residential buildings. Consequently, residents would be at risk of being exposed to hazardous radon. Therefore, it is imperative to critically evaluate the potential danger of the produced radon from Marcellus Shale.
In this paper, the author's obtained some fracked rock from a well, and also used certain kinds of computational analysis to consider how the radon escapes into the gas stream and flowback water.
An important thing to understand is that a nuclear decay is a very energetic event. The decay of radium-226 which gives rise to radon-222 occurs roughly at 4.87 million electron volts. Much of this energy is contained in the helium atom (alpha ray) ejected from the nucleus, but the conservation of momentum requires that the recoiling radon atom also has considerable energy, and can in fact travel quite far even in a solid matrix.
From the text:
The authors consider two sources, radium already in the pore or on the surface of the pore, and radon that travels through the rock as part of the alpha decay.
Some remarks on the mathematical modeling of how the radium/radon system works in pores:
(1)
where, ARa is the radioactivity of radium and ARn is the radioactivity of radon, both in unit of pCi/L. Ve is the grain volume in which the radon generated from radium has nonzero possibility entering pore space, in unit of L^3. e is the emanation efficiency of recoil and Vp is pore volume in unit of L^3. Emanation efficiency e consists of two parts (eq 2). First, not all produced radon near the grain-pore surface will be emitted into pore space (fe). Some of the produced radon atoms remain inside the grain due to the inappropriate recoil direction. Second, radon atoms that enter pore space may maintain sufficient kinetic energy so that they could enter neighboring grains eventually (1 − f i). Both of these factors should be included in evaluating efficiency e
(2)
The slit pore shape is one commonly used pore geometry, defined by two parallel planes (grain surface).26 Andrews20 analytically calculated the radon release fraction from grains into pore space (fe) for slit pores. Fleischer21 further studied the fraction of radon atoms ejected from grains that are trapped in pores ( f i). Tian et al.22 investigated how much radon produced from radium in pore space will remain in slit pores after alpha recoil. Besides the slit pore shape, spherical pores also occur in shale, which require different formulas to calculate radon in situ concentration. Emanation efficiency, e, is defined in eq 2. The point O1 is the center of the spherical pore with the radius of R, as shown in Figure 1. The radium atom is initially located at O2. The radon recoil range inside fluid-filled pore space is Rf and the recoil range in solid material is Rs. The solid circle in Figure 1 represents the pore wall and, therefore, the inside of the circle is pore space. If the trajectory of radon after recoil is O2AB, it is helpful to convert the stopping power in fluid to solid.21 In other words, the distance b in the pore filled by fluid is modified to an equivalent distance bRs/Rf if the pore space is assumed to be filled by the solid. Radon particles could possibly be ejected and trapped into the pore if the following criteria are satisfied
(3)
where
The noble gases, including radon, are known to form clathrates with water, and water transport is an important feature. The fracturing of fracking is accomplished with water laced with a number of interesting chemicals, and this water, called "flowback" water is brought to the surface.
Once the shale reservoir development starts, radon escapes to the surface through conductive hydraulic fractures, being entrained in shale gas and formation water. The alpha decay of radium and radon in the reservoir was simulated by first-order chemical reaction because the decay rate was dependent on their concentrations (eq 11). During the simulation, fresh water was injected into the formation for 0.5 day to mimic the hydraulic fracturing process. The injected fracking fluid did not contain any radon or radium. The well was then brought back to production under a constant bottom-hole pressure after 0.5 day shut in. This work adapted model setup from Tian et al.22
where N is the concentration or radioactivity and λ is the exponential decay constant.
Some diagrams and graphics:
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Some other graphics:
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Figure 3. Synthetic model configuration. The horizontal well is located at the top. It is perforated at hydraulic fracture at the left side. The stimulate reservoir is divided into two sections: the near -fracture zone and far-formation zone.
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The concern is that the radon will persist long enough to make it to consumers. I'm sure it does.
Should...should...should...
Could...could...could...
Enjoy what's left of the evening.
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