Geochemical Evaluation of Produced Water from
Marcellus Shale Hydraulic Fracturing Wells in Greene
County, PA
Héctor Alejandro Ruiz Moreno
Departamento de Ingeniería Ambiental, Universidad de los Andes, Bogotá, Colombia.
Abstract
Shale-gas obtained by hydraulic fracturing is a growing energy resource that has a significant impact on global economy. Its great economic benefits are contrasted with concerns on its environmental impacts including the management of produced water which flows back up of the wells, its holding, potential spills, and suspected migration to groundwater. Elemental analysis of produced water is important to assess environmental exposure risks and evaluate possible management alternatives such as its reuse as base fluid during the fracturing process. In this study elemental analysis is performed by ICP-MS on 71 samples of produced water from Greene County, PA natural gas wells. Results are related to information such as well age and freshwater use during fracturing, compiled from both public and private sources. All evaluated samples came from wells which had been in production for a long time (>150 days). The produced water samples presented a chemical composition similar to typical produced water. Main elements found were Na, Ca, Ba and Sr. Concentration of these metals decreases with the combined effect of increasing well age and increasing percentage of freshwater used during the fracturing process. Each well presents a unique geochemical signature which could be used to track contamination sources, evaluate treatment and management options, and simulate spills.
Keywords: Hydraulic fracturing, fracking, produced water geochemistry, ICP-MS.
Resumen
El gas de esquisto obtenido por fracturación hidráulica es un recurso energético que tiene un impacto significativo sobre la economía global. Su gran beneficio económico se contrasta con crecientes preocupaciones sobre sus impactos ambientales, incluyendo el manejo del agua de producción, su almacenamiento, derrames y sospechas de migración hacia fuentes de agua subterránea. El análisis elemental del agua de producción es importante para evaluar estos riesgos y las posibles alternativas de manejo tal como su reutilización como fluido de fracturación. En este estudio el análisis elemental se realiza por medio de ICP-MS en 71 muestras de agua de producción proveniente de pozos ubicados en Greene County, PA, EE.UU. Los resultados fueron relacionados con información obtenida de fuentes públicas y privadas sobre la edad de los pozos y el porcentaje de agua fresca empleada durante la fracturación. Todas las muestras provenían de pozos que se encontraban en producción por un largo tiempo (>150 días). Las muestras de agua de producción presentaron una composición química similar a aguas de producción típicas. Los principales
2 elementos presentes fueron Na, Ca, Ba y Sr. La concentración de estos metales disminuye por el efecto combinado al aumentar la edad del pozo y al aumentar el porcentaje de agua fresca que se emplea. Cada pozo presenta una marca geoquímica que puede ser usada para determinar fuentes de contaminación, simular derrames, y evaluar opciones de tratamiento y manejo.
Palabras clave: Fracturación hidráulica, fracking, agua de producción, geoquímica, ICP-MS.
Introduction
Productivity is tied to energy use. Rising global energy requirements call for broad and diverse energy resources. Combined with economical, technological and environmental drivers, its cleaner combustion has caused the focus of electrical power generation to shift from coal to natural gas [1, 2]. While renewable and nuclear energy technologies continue to develop, techniques such as hydraulic fracturing (‘hydrofracturing’, ‘fracking’ or ‘hydrofracking’) help maintain a global economic stability based on fossil fuels [3, 4]. Horizontal drilling of wells for fracking in the Marcellus Shale in Pennsylvania began in 2004 and has expanded to 7234 wells as of November 2013 [5]. This expansion is accompanied by growing concerns associated to potential environmental impacts such as groundwater well contamination, drinking water contamination, stray gas, habitat destruction, induced seismicity, overuse of freshwater resources, the fate of injected fracturing fluids, and management of flowback or produced waters [5, 6, 7, 8, 9, 10, 11].
Table 1. Typical range of concentrations for some common constituents of flowback water from natural gas development in the Marcellus Shale [1]
Constituent High2 (mg/L)
Total dissolved solids 261,000
Total suspended solids 3,200
Hardness (as CaCO3) 55,000
Alkalinity 1,100
Chloride 148,000
Sulfate 500
Sodium 44,000
Calcium, total3 31,000
Strontium, total 6,800
Barium, total 4,700
Iron, total 1,600
Manganese, total 7
Oil and grease 260
1. Data obtained from flowback water from several production sites in western Pennsylvania. 2. "High concentrations are highest concentrations observed in late flowback from several wells with similar reported TDS.
3 Flowback water is the fluid that returns to the surface after the fracturing process is completed and pressure is released, but before the well is placed in production. Produced water is water that returns to the surface throughout the life of the well once it has been put in production [9, 12]. Flowback and produced waters are composed of a mixture of high salinity in situ brines and injection fluids [8]. Given their high salinity, high radium and barium content, and other chemical constituents, potential spills and leaks of these waste stream fluids as well as their proper handling and management are a major concern [6, 13, 14].
Constituents of produced water include salts, metals, oils, greases, as well as soluble organic compounds (Table 1). The levels of total dissolved solids (TDS) found in produced water can be 5-10 times the concentration found in seawater [8]. Concentrations of several inorganic compounds exceed EPA maximum contamination levels. Barium concentration in Marcellus Shale flowback water has been found to be particularly high with a mean of 1,200 𝑚𝑔/𝐿 [1]. Some studies have detected alpha particles and other radionuclides [15, 16]. Gross alpha levels, radium-226 and radium-228 have been found to exceed EPA maximum contamination levels. Radium content in some cases exceeds 10,000 pCi/L [6, 15, 17]. Other contaminants including chlorinated solvents, disinfectants, dissolved metals and volatile organic compounds have also been found at exceedingly high levels [16]. High concentrations of radionuclides and TDS make treating flowback and produced waters a challenge.
Hydraulic fracturing processes are not likely to cause rapid flow of contaminants to shallow groundwater. There is no statistical difference in contaminant concentrations between areas with active drilling and hydraulic fracturing and areas where there is no drilling [10]. However, there is geochemical evidence of hydraulic connectivity between deep shale formations and shallow aquifers, observed as areas of high salinity prior to the development of shale gas in the region. This creates a need for studies to confirm the source of brines and the timescale of possible brine migrations [10]. Geochemical analysis of groundwater and produced water can be used to address this problem.
The goal of the current study was to characterize the geochemistry of produced water by elemental analysis. Samples from 71 wells in Green County, Pennsylvania were supplied by a fracking company. These wells are located within a 22 km radius. Information on the wells is publicly available; this includes fracture date, location, and the composition of the fracturing fluid used [18]. Additional information provided by the company which supplied the samples includes data on freshwater used and recycled produced water. This information was studied for the collective group of samples to evaluate the effect of well age and use of freshwater on the geochemistry of the produced water. Characterization of flowback and produced water composition provides information useful to assess the risks of potential release to the environment, evaluate treatment approaches and study the importance of flowback composition on the reuse in fracking processes.
4
Methods
Samples
Samples of hydraulic fracturing produced water were obtained from 71 Marcellus Shale natural gas wells located in Greene County, PA. Samples analyzed in this project were retrieved from separators by on site operators. Samples were shipped to lab facilities and stored within 36 hours at -80°C. Samples were left to thaw in the fridge overnight prior to chemical analysis.
Elemental analysis
Elemental analysis was performed on a Perkin-Elmer Nexion 300x ICP-MS. This analysis provides qualitative information on metals present and quantitative concentration values for metals within detection limits. Thawed samples were collected. Suspended solids in samples were separated using 45 μm filters. Samples were diluted with 2% nitric acid to a dilution factor of 10000 after filtration. Dilution was performed as the TDS concentration of typical produced water is too high for ICP-MS analysis. An internal standard composed of germanium, beryllium and thallium was added to each sample (0.008 mL). Each dilution was performed measuring the mass of each solution to calculate individual dilution factors and reduce error due to pipetting. Duplicate samples were taken each seven samples. The ICP-MS performs blanks and drift checks during the process. Data reduction was carried out for samples below detection limits (BDL). Data with sodium (Na) concentrations far below the typical concentration range found in produced water were also eliminated from the analysis.
Results
Elemental analysis
Table 2. Elemental Analysis of Produced Waters from Hydraulic Fracturing in the Marcellus Shale, Greene County Region.
Element N Mean SD Min Q1 Median Q3 Max
Na (mg/L) 58 37465.93 20646.44 1242.62 23857.20 33291.16 46993.54 102715.58 Mg (mg/L) 62 1235.62 585.74 48.91 825.21 1176.75 1492.61 2713.84 K (mg/L) 55 390.92 771.10 6.75 72.55 147.40 244.59 3620.13 Ca (mg/L) 62 14120.96 6796.24 763.69 9852.19 13452.55 16955.70 31668.32
Mn (mg/L) 62 5.62 2.54 0.80 4.36 5.18 6.25 13.48
Fe (mg/L) 61 236.58 97.69 11.04 174.54 220.48 287.19 595.13
Co (mg/L) 57 0.06 0.04 0.00 0.03 0.04 0.10 0.20
Ni (mg/L) 62 1.16 1.62 0.18 0.35 0.64 1.47 10.34
Cu (mg/L) 19 0.71 0.36 0.15 0.48 0.65 0.89 1.76
Zn (mg/L) 19 2.57 6.98 0.36 0.52 0.78 1.20 31.20
As (mg/L) 47 0.50 0.34 0.01 0.20 0.46 0.76 1.47
Sr (mg/L) 62 2776.50 1523.04 183.60 1848.17 2485.15 3260.05 8228.76 Ba (mg/L) 62 3053.24 1529.43 65.06 1952.49 2922.77 3770.55 7094.51
5 ICP-MS analysis performed on samples from 70 different wells from Greene County, PA detected the presence of 34 elements (Li, B, Na, Mg, Al, Si, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, W, Hg, Pb, Bi, U). Quantitative analysis showed many of these elements presented concentrations below detection limits. Basic statistics were used for elements within detection limits (Table 2).
The concentration value of elements in each well is different. While elements have different concentrations, the same elements are present in all produced water samples in similar proportions. The metals of interest presenting high concentrations were Na, Ca, Ba and Sr. High concentrations of these elements are characteristic of produced waters. There is a great variability associated to the data. This variability is observed with a high standard deviation between samples compared to their mean values. Lowest variability was observed for by iron concentration in different wells.
Effect of well age on geochemistry
REPORTED DAYS
N 68
Mean 1000.3
SD 444.9
Min 150
Q1 694
Median 1048.5
Q3 1358.5
Max 1847
Figure 1. Basic statistics for well age.
The studied wells were already in production phase at the moment of sampling. The age range of the studied wells ranges between 150 and 1847 days; these wells have been in production for a long period. Chemical composition was evaluated for different well age groups. To study the effect of well age on concentration of different elements group age ranges were selected according to the well age statistics (Figure 1).
The main elements analyzed (Na, Ca, Sr, Ba, Mg, Fe, Mn, As) present high relative variability in the data. A pattern of decreasing barium and magnesium concentration with increasing well age seems apparent at first glance (Figure 2). The concentration data from oldest wells has the highest variance. Observing the high variability in the concentration results it could be seen that the potential concentration values do not vary and are similar and stable at the different well ages studied.
6 Well age (days):
Figure 2. Mean concentration of elements of interest in the wells within different age groups (mg/L).
Fresh water use and chemical composition
FRESHWATER USE %
N 54
Mean 66.55
SD 22.53
Min 28.58
Q1 50.75
Median 64.26
Q3 100
Max 100
Figure 3. Freshwater percentage use basic statistics
The amount of freshwater used in the fracturing process is of great concern. To reduce the volume of freshwater used some wells reuse produced water from other locations. Introduction of reused water could affect the concentration of metals in the produced water of the new well. Information was available on 54 of the wells studied. Out of these 54 wells, 12 wells used 100% fresh water as base fluid for fracturing.
All wells use some fraction of freshwater as base fluid for the fracking process. The distribution of freshwater use shows that wells will either use 100% freshwater or use at least 28.6% fresh water (Figure 3). This means that most of the studied wells reuse produced water in their process; up to 71.42% recycled water is used in certain wells.
0 10000 20000 30000 40000 50000 60000 70000 80000 Na Ca C o n ce n tr at io n ( mg /L ) 0 1000 2000 3000 4000 5000 6000 Sr Ba C o n ce n tr at io n ( mg /L ) 0 500 1000 1500 2000 2500 Mg Fe C o n ce n tr at io n ( mg /L )
7
Figure 4. Concentration of elements of interest in wells with different freshwater use percentage (mg/L).
The use of recycled water from other wells in the fracking process could affect the composition of produced water. Results of concentration values for the four main metal constituents of produced water show some ambiguous results. Observing the whole data, concentration values appear to be constant and independent from the fraction of water used that comes from other wells or from freshwater sources (Figure 4).
Figure 5. Calcium concentration (mg/L) in wells with different well age and fresh water use percentage during fracturing.
The concentration of metals in produced water is dependent on many factors. In this case there are only two factors to explore: freshwater use percentage and well age. The combined effect of these two factors can cause produced water to present different concentrations of calcium (Figure 5) or other constituents. All elements present the same general behavior.
0 20000 40000 60000 80000 100000
0% 25% 50% 75% 100%
C o n ce n tr at io n ( mg /L )
Freshwater use percentage
Na 0 10000 20000 30000 40000
0% 25% 50% 75% 100%
C o n ce n tr at io n ( mg /L )
Freshwater use percentage
Ca 0 2000 4000 6000 8000
0% 25% 50% 75% 100%
C o n ce n tr at io n ( mg /L )
Freshwater use percentage
Ba
0 2000 4000 6000
0% 25% 50% 75% 100%
C o n ce n tr at io n ( mg /L )
Freshwater use percentage
8
Discussion
Characterizing produced water is important to assess the impact of potential spills of produced water and to evaluate possible treatment alternatives. The main elements found in the samples analyzed from 70 different wells were Na, Ca, Ba and Sr. Sodium ion is the largest fraction of metal ions present and accounts for most of the TDS. The concentration of these metals changes for every well as they directly depend on the contact with in situ brines.
Barium and magnesium concentration appears to decrease with increasing well age, while sodium, calcium, and strontium concentrations do not change considerably with well age (Figure 3). This could happen if the in situ brines from these metals originate had decreased in volume over time as they emerge to surface with produced water. However, there is a large variability in the concentration of metals between samples from different wells. Taking this variability into account, the concentration of metals found is stable or stationary and could have reached a peak. Concentration values are found to increase over time during the first few weeks, first during flowback period, then in the early days of production. The age of the wells studied affects the concentration of metals found. However the youngest wells studied are 150 days into production. The concentration of metals in this wells are therefore found to have stabilized.
The concentrations at which metals were found in the samples analyzed is close to the ranges of concentrations found in typical produced water when compared to samples from older wells. The concentrations of the different metal ions have the same behavior regarding well age and freshwater percentage use. The chemical profile of each well could be suitable as a “chemical fingerprint” [8]. This could help analyze the origin of contaminants in near freshwater bodies as well as in potential spills as long as a base line is available [10].
Figure 6. Surface plot of calcium concentration (mg/L) in wells with different well age and freshwater use percentage. Wells which use 100% freshwater as base fluid are included.
The different wells also used different proportions of freshwater and water recycled from other wells. The first impression is a lack of relationship between the percentage of freshwater that is
9 used and the concentration of the produced water from the well. In the cases where 100% of the base fluid employed is fresh water, the concentration of metals is dependent only on the contact it has with in situ brines and therefore could many values. After removing these data points, an actual decrease in concentration as the percentage of freshwater used increases is observable. Seeing it from another other point, using more recycled water produces flows with higher metal concentration. There is a possibility that metal ions from the new well accumulate to those present in the recycled produced water.
If the effect of the two factors studied is observed at the same time, previously observed trends appear to be more evident. However, wells in which there is no recycled water used as base fluid during fracturing (100% freshwater) fall out of the trend (Figure 6). The cause of this could be that this is the first contact the water has with the well and brine formations and therefore could be strongly dependent on the composition of shale brines.
Once the data for the wells with 100% freshwater use is removed from the analysis, the trends are easier to observe. There is a combined effect of well age and freshwater use which causes produced waters to have higher concentrations of metals in the younger wells with higher proportion of recycled water used during fracking. On the contrary, older wells with more freshwater use present lower concentrations (Figure 7). This would confirm that as water is recycled there is an accumulation of metals in the fluid.
Figure 7. Contour plot of freshwater use and well age effect on calcium concentration (mg/L). Wells which use 100% freshwater as base fluid are excluded.
The geochemical signature of produced water varies according to each well. There are however, trends in the composition of produced water. Among many different factors which affect the composition of produced waters, well age (in old wells) and freshwater use have a small but observable effect. In early production, concentration is expected to increase as the well is in
10 production more time. In this case, the wells are older and the concentrations of metals begin to decrease.
Conclusions
The geochemical composition of the wells sampled is similar to typical ‘old’ well values. All wells studied were ‘old’ wells as they had been in production longer than four weeks. The main elements presents in produced water samples were Na, Ca, Br and Sr. Most of the studied Green County wells use some fraction of recycled water as base fluid during the fracturing process.
The composition of produced water depends on multiple factors. For studied Greene County wells, as well age increases the concentration of Ba and Mg decreases slightly, while Na, Ca, and Sr concentration do not change significantly. As the percentage of recycled water increases the concentration of metals in produced water increases as metals accumulate.
The location of the wells and the section of the geological formation involved could be factors which have a greater effect on the geochemistry of produced water than freshwater use or well age.
The composition of produced water has a characteristic composition in each well. This geochemical signature could be used to evaluate contamination of freshwater bodies, simulate contaminant flow in case of spills, or asses the origin of contaminants if a base line concentration is known.
Future work
The results from this project are limited to wells within the Marcellus Shale geological formation and the Greene County region. Further studies are proposed to relate the location of the wells to both the well age and the concentration of elements found in produced water from these wells. Additionally, an analysis of the suspended solids separated from the sample should be performed as the solid phase fraction could contain important information while currently it is discarded and thought of as mostly sand. Suspended solids could be of interest in sedimentation during holding and possible physicochemical treatment alternatives.
Acknowledgements
This project was conducted at the University of Pittsburgh’s Environmental Engineering Department under advice from Kyle Bibby. ICP-MS analysis was performed in the Department of Geology and Planetary Science with the help of Daniel Bain. Thanks to the Bibby Lab Group for their assistance and support.
11
References
[1] K. Gregory, R. Vidic and D. Dzombak, "Water management Challenges Associated with the Production of Shale Gas by Hydraulic Fracturing," Elements, pp. 181-186, 2011.
[2] S. Holditch, K. Perry and J. Lee, "Unconventional Gas Reservoirs - Tight Gas, Coal Seams, and Shales, Working Document of the National Petroleum Council on Global Oil and Gas Study," National Petroleum Council, 2007.
[3] MIT, "The future of natural gas. An Interdisciplinary MIT study.," 2011. [Online]. Available: http://mitei.mit.edu/publications/reports-studies/future-natural-gas.
[4] G. King, "Hydraulic Fracturing 101: What Every Representative, Environmentalis, Regulator, Reportes, Investor, University Researcher, Neighbor and Engineer Should Know About Estimating Frac Risk and Improving Frac Performance in Unconventional Gas and Oil Wells.," Society of Petroleum Engineers, The Woodlands, TX, 2012.
[5] S. Brantley, D. Yoxtheimer, S. Arjmand, P. Grieve, R. Vidic, J. Pollak, G. Llewellyn, J. Abad and C. Simon, "Water resource impacts during unconventional shale gas development: The Pennsylvania experince," International Journal of Coal Geology, no. 126, pp. 140-156, 2014.
[6] M. Engle and E. Rowan, "Geocchemical evolution of produced waters from hydraulic fracturing of the Marcellus Shale, northern Appalachian Basin: A multivariate composition data analysis approach," International Journal of Coal Geology, vol. 126, pp. 45-56, 2014.
[7] A. Farag and D. Harper, "A review of environmental impacts of salts from produced waters on aquatic resources," International Journal of Coal Geology, vol. 126, pp. 157-161, 2014.
[8] L. Haluszczak, A. Rose and L. Kump, "Geochemical evaluation of flowback brine from Marcellus gas wells in Pennsylvania, USA," Applied Geochemistry, pp. 55-61, 2013.
[9] R. Vidic, S. Brantley, J. Vandenbossche, D. Yoxtheimer and J. Abad, "Impact of Shale Gas Development on Regional Water Quality," Science, vol. 340, 2013.
[10] N. Warner, T. Jackson, S. Osborn, A. Down, K. Zhao, A. White and A. Vengosh, "Geochemical evidence for possible natural migration of Marcellus Formation brine to shalow aquifers in Pennsylvania," PNAS, 2012.
[11] J. Chermak and M. Schreiber, "Mineralogy and trace element geochemistry of gas shales in the United States: Environmental implications," International Journal of Coal Geology, pp. 32-44, 2014.
12 [13] B. Rahm, J. Bates, L. Bertoia, A. Galford, D. Yoxtheimer and S. Riha, "Wastewater management and Marcellus Shale gas development: Trends, drivers, and planning implications," Journal of Environmental Management, vol. 120, pp. 105-113, 2013.
[14] B. Rahm and S. Riha, "Toward strategic management of shale gas development: Regional, collective impacts on water resources," Environmental Science & Policy, vol. 17, pp. 12-23, 2012.
[15] E. Rowan, M. Engle, C. Kirby and T. Kraemer, "Radium content of oil-and-gas-field produced waters iin the northern Appalachian Basin (USA): summary and discussion of data," U.S. Geological Survey Scientific Investigation Reports, 2011.
[16] N. Abualfaraj, P. Gurian and M. Olson, "Characterization of Marcellus Shale Flowback Water," Environ. Eng. Sci. , 2014.
[17] K. Skalak, M. Engle, E. Rowan, G. Jolly, K. Conko, A. Benthem and T. Kraemer, "Surface disposal of produced waters in western and soutwestern Pennsylvania: Potential for accumulation of alkali-earth elements in sediments," vol. 126, pp. 162-170, 2014.
[18] IOGCC & GWPC, "FracFocus," 2014. [Online]. Available: http://fracfocus.org/.
[19] A. Mohan, A. Hartsock, K. Bibby, R. Hammack, R. Vidic and K. Gregory, "Microbial Community Changes in Hydraulic Fracturing Fluids and Produced Water from Shale Gas Extraction," Environ. Sci. Technol, pp. 13141-13150, 2013.
[20] L. Strong, T. Gould, L. Kasinkas, M. Sadowsky, A. Aksan and L. Wackett, "Biodegradation in Water form Hydraulic Fracturing: Chemistry, Microbiology, and Engineering," J. Environ. Eng., 2013.
[21] USGS, "Water Resources and Shale Gas/Oil Production in the Appalachian Basin - Critical Issues and Evolving Developments," 2013.
[22] P. Ziemkiewicz, Y. He and J. Quaranta, "Characterization of Waste Waters from Hydraulic Fracturing," Shale Energy Engineering , pp. 66-73, 2014.
[23] J. Arthur, B. Bohm and D. Zampogna, "Evolution of Best Management Practices and Water Treatment in High Volume Hydraulic Fracturing Operations," Shale Energy Engineering, pp. 33-44, 2014.
[24] T. Fitzgerald, "Frackonomics: Some Economics of Hydraulic Fracturing," Case Western Reserve Law Review, vol. 63, no. 4, pp. 1337-1362, 2013.
[25] A. Vikram, D. Lipus and K. Bibby, "Produced Water Exposure Alters Bacterial Response to Biocides," Environ. Sci. Technol., 2014.
13 [26] A. Vengosh, N. Warner, R. Jackson and T. Darrah, "The effects of shale gas exploration and hydraulic fracturing on the quality of water resources in the United States," Procedia Earth and Planetary Science, vol. 7, pp. 863-866, 2013.
[27] E. Chapman, R. Capo, B. Stewart, C. Kirby, R. Hammack, K. Schroeder and H. Edenborn, "Geochemical and Strontium Isotope Characterization of Produced Waters from Marcellus Shale Natural Gas Extraction," Environ. Sci. Technol., vol. 46, pp. 3545-3553, 2012.
