I’ve been bothered by the adverse reports on hydraulic fracking to stimulate hydrocarbon production, including pollution of water (surface and subterranean) and air and radon contamination. Most reports cite problems in New York or Pennsylvania, where the Marcellus shale is the source for natural gas (NG). What’s the problem? After all, hydraulic fracturing has been successful in producing hydrocarbons in Texas and North Dakota.
A recent report on the geochemistry of the Marcellus shale seems to explain the geochemical origin of the problems. So far, the problems seem to be unique to the Marcellus formation.
A paper by Haluszczak et al. focuses on the composition of brine co-produced with NG from the Marcellus shale.1 Flowback is defined as liquids produced from the well during the first 90 days after completion of fracking. After 90 days, the liquids are called “production water.”
The study was first commissioned to determine the fate of hydraulic fracking cocktails in the flowback liquid. The fear was that the suspension used in fracking would contaminate subterranean water deposits, including wells producing drinking water.
Data from time course studies show that about a quarter of the fracking fluid flows back up the well immediately after the fracking process is terminated. Part of the flowback is attributed to relaxation of the formation. The remainder of the flowback is liquid native to the formation.
In the Marcellus, brine production generally increases with time after fracking, probably due to dissolution of solids along the flow path to the well bore. The chemistry is complex, but fears that the high salt is due to acids potentially used in the fracking cocktail are inconsistent with the brine content including neutral pH. The fracking fluid is often acidic.
Within the first 10 days post-fracking, the brine from the fracked wells starts to resemble brine from vertical bores that had no history of fracking, i.e., it is increasingly similar to production water. Major features include neutral pH; high chloride; barium and radium; and low sulfate, carbonate, and uranium. This is consistent with geochemistry involving evaporation of a seabed, followed by rehydration with either fresh or seawater.
The major problem with the flowback and probably production water is that the concentration of barium and radium exceeds the U.S. EPA’s Maximum Contaminant Level (MCL) by 10–100 times. Plus the chloride is so high that any leakage to surface water would bring severe contamination including algae blooms or destruction of existing vegetation.
The Marcellus formation contains uranium (Uo), which radioactively decays to radium and on to gaseous radon. The U is not mobile due to the neutral pH. Radium is an alkaline earth. Recall that alkaline earths form insoluble precipitates with sulfate or carbonate anions. Since both [CO3=] and [SO4=] are low, Ra is dissolved and mobile in the brine. However, the high level of 226Ra raises concern about its daughter [222Rn] in the NG. Radon is the second leading cause of lung cancer in the U.S.2
Three issues seem to dominate discussions on producing NG from the Marcellus formation: 1) What can be done with the brine co-produced with the gas? 2) What is the greenhouse gas impact of NG production and consumption, especially the combustion products? 3) How can we mitigate the danger from 226Ra, 222Rn, and daughter decay products?
Water, water everywhere, but not a drop to drink
Marcellus brine is simply too concentrated in chloride, barium, and radium to release into the environment. Reinjection into the source formation is probably the best solution. Injection into other formations runs the risk of contaminating them with Ra and daughter products. Injection wells should be located away from production wells to minimize communication between the bores.
Natural gas is primarily methane. The greenhouse gas effects of methane are about 30 times worse than CO2. Obviously, the risk of leaks to the atmosphere increases with transporting the gas from the wellhead. So the NG should be converted to electricity or other energy-intensive products such as fertilizer, near the wellhead. Gas turbine electrical generators appear to be attractive since they are efficient and easy to run. Wires have a lower environmental footprint than gas lines.
If the combustion exhaust gas is recycled to the producing formation in a closed-loop manner, then one may also recycle the 222Rn back to the producing reservoir. This would be ideal, since radon’s decay daughters (Po, Tl, Pb, Hg) are highly toxic as well as radioactive.
If uncontained, the radionuclides are adsorbed onto particles and transported to lungs, where the daughters produce tumors. To be safe, these should be sequestered from the surface environment. The danger is not trivial. A study by Resnikoff2 estimates that if NG from Marcellus shale gas is placed directly in the gas transmission lines to NY, the small fraction that is burned openly in gas cooking stoves will result in 1100–30,000 cases of lung cancer. Thus, the safest approach is to convert Marcellus NG to electricity and sequester the fumes.
However, it is probably important not to mix the CO2-rich combustion product with the reinjection of brine. After all, Ba(II) and Ra(II) ions probably form insoluble carbonate precipitates at neutral pH that could clog pores in the receiving formation.
New technology may help
Water acquisition and disposal has become a major cost of production, especially in the Marcellus.3 Trucked water for fracking can cost as much as $5/barrel to buy and $2/barrel to dispose of, which corresponds to $500,000/well. One solution is CleanWave® Frac Flowback and Produced Water Treatment, developed by Halliburton (Houston, TX). After clarification, the water is suitable for further fracking or reinjection back into the formation.
Another possibility using switchable water is described in a paper by Mercer and Jessop.4 They found that tertiary amino alcohols form switchable solutions controlled by [CO2]. When CO2 is added to a solution of N,N,N,N’ tetramethyl-1,4,diaminobutane, the amine groups are converted to diammonium carbonates ((–N–R,R’H)+ (HCO3_)). As expected, the conductivity of the solution increases to over 20 mS/cm. The high ionic strength causes the organic molecules to “salt out” and form a two-phase system, which can be separated physically.4 Sparging with air removes the CO2, yielding a homogeneous solution with low ionic strength. The process can be easily cycled. Recovery of the organics is in the range of 61–85%. A two-stage process could probably improve recovery. Recovery of polyamines is generally better than 90%. Possibly, the polyamines could be incorporated into membranes or beads.
How hydrocarbon production is changing the economic landscape
Production of hydrocarbons from tight formations is changing the economic landscape. There are new issues that need to be addressed. Separation science has a large role to play. We need to take the data from studies such as Haluszczak et al. and use them to propose, develop, and implement new processes that can make a difference.
My goal in writing this is to stimulate creative minds to study the problem. I’m sure that human creativity will respond with ideas that will help us improve the lot of our global society and particularly those in the Northeast U.S.
References
- Haluszczak, L.O.; Rose, A.W. et al. Geochemical evaluation of flowback brine from Marcellus gas wells in Pennsylvania. U.S.A.; http://dx.doi.org/10.1016/j.apgeochem.2012.10.002.
- Resnikoff, M. Radon in natural gas from Marcellus shale. Radioactive Waste Management Associates; http://www.nirs.org/radiation/radonmarcellus.pdf.
- Sider, A.; Gold, R. et al. Drillers begin reusing “frack water”‒energy firms explore recycling options for an industry that consumes water on pace with Chicago; http://online.wsj.com/article/SB10001424052970203937004578077183112409260.html.
- Mercer, S.M.; Jessop, P.G. ChemSusChem2010, 3, 467–70.
Robert L. Stevenson, Ph.D., is a Consultant and Editor of Separation Science for American Laboratory/Labcompare; e-mail: [email protected].