In a series of studies, the trace metal geochemistry of the Marcellus Shale and the extractability of select metals through fluid-rock interactions have been determined. The studies were completed on 16 shale samples including outcrop, well core, and well cuttings from western New York and Pennsylvania. Instrumental neutron activation analysis (INAA) was used to determine the concentrations of metals in the shale samples prior to any chemical treatment. The analysis indicates that the Marcellus Shale is enriched in barium, uranium, chromium, and zinc, among other metals (Table 1). While these metal concentrations in the shales are not high enough to be considered dangerous, their solubilities may be high under the conditions introduced during drilling and fracking. The maximum contaminant level (MCL) in the United States for barium, uranium, chromium, and zinc are 2 ppm, 30 ppb, 50 ppb, and 5 ppm, respectively.
" The concentrations of the metals that will occur in the wastewater are a function of the exact chemistry of the fracking fluid, which changes from one supplier to another... "
To determine the extent that these metals could be mobilized during reactions that occur between drilling and fracking fluids and the shale, several chemical extractions have been performed. A dilute solution containing sodium bicarbonate was used to identify the easily exchangeable ions that could be removed from the rock by natural groundwater. This study was completed by reacting a known mass of finely ground shale with the sodium bicarbonate solution for 24 hours. The amount of metal solubilized from the shale to the solution was negligible in repeated studies indicating that little to no metal of interest is weakly sorbed onto the shale minerals, good news to those living near Marcellus outcrops.
A more invasive extraction test was completed to determine how much of the metals could be mobilized by hydrochloric acid (HCl). HCl is a component of fracking fluids used to dissolve carbonate cement and increase flow pathways for the natural gas. In this study, finely ground shale was reacted with 3 M and then 1.5 M HCl for a total of two hours. The amount of metal solubilized by the extraction was measured using inductively coupled plasma mass spectrometry (ICPMS). Interestingly, the results of this study varied depending on whether the sample was outcrop or core; outcrop samples contain 60 percent HCl extractable uranium while core samples contain only about 20 percent HCl extractable uranium. Zinc and chromium were both moderately extractable in both sample types, up to 60 percent and 20 percent respectively. Up to 40 percent of the barium was removed during the extraction but results vary from sample to sample and are probably affected by contamination from drilling muds.
Table 1. Selected Metal Concentrations in Marcellus Shale
The concentration of HCl used in this study is greater than the concentration of HCl typically used on site. However, the duration of time that the shale was exposed to the HCl was likely shorter in these experiments compared to the fracking stages. The results here may represent an upper level to the range of metal extractability to be expected.
A final extraction test was completed to test the association of metals with the organic matter (including the natural gas). A 30 percent hydrogen peroxide treatment was used to oxidize and remove the organic matter from finely ground shale. A nitric acid treatment followed to acidify the samples and solubilize any metal released by oxidation of the organic matter. Under neutral pH conditions, oxidized uranium released by the organic carbon removal would sorb to the shale mineral phases; therefore acidification was used to keep the uranium and other metals of interest to this study soluble. An average of 20 percent of the zinc in the samples was solubilized with an extractable range from 3-80 percent. Additionally, up to 35 percent of the uranium was solubilized, but a greater amount of uranium could be removed from the outcrop samples compared to the core. Up to 30 percent of the chromium was removed, but an average of only 8.5 percent was removed from the shale.
Figure 1. Concentration of Uranium (ppm) versus Total
In addition to the heavy metals that were the foci of this study, several nuisance ions were mobilized during these extraction studies. In particular, high concentrations of chloride make analyses of the aqueous samples difficult. Also, sodium, potassium, and calcium were also recorded in fluids that had reacted with the Marcellus Shale.
This study shows that metals in wastewater produced during natural gas development of the Marcellus Shale may be a concern. The concentrations of the metals that will occur in the wastewater are a function of the exact chemistry of the fracking fluid, which changes from one supplier to another, and the volume and duration of reaction occurring at depth. While theoretical studies like this cannot be used to predict the concentrations of metals in wastewaters produced during drilling, they can be used to understand reactions that are occurring at depth. Also, these fundamental baseline studies can identify the metals that are likely to be the largest concern. A detailed study of the chemistry of wastewater produced at these wells, and a dynamic study that investigates how the chemistry of these fluids changes over time, would be beneficial to those designing treatment plans for the enormous volumes of wastewater that will be produced by this industry.