Radium content of oil- and gas-field produced waters in the northern Appalachian Basin (USA): Summary and discussion of data

Radium activity data for waters co-produced with oil and gas in New York and Pennsylvania have been compiled from publicly available sources and are presented together with new data for six wells, including one time series. When available, total dissolved solids (TDS), and gross alpha and gross beta particle activities also were compiled. Data from the 1990s and earlier are from sandstone and limestone oil/gas reservoirs of Cambrian-Mississippian age; however, the recent data are almost exclusively from the Middle Devonian Marcellus Shale. The Marcellus Shale represents a vast resource of natural gas the size and significance of which have only recently been recognized. Exploitation of the Marcellus involves hydraulic fracturing of the shale to release tightly held gas. Analyses of the water produced with the gas commonly show elevated levels of salinity and radium. Similarities and differences in radium data from reservoirs of different ages and lithologies are discussed. The range of radium activities for samples from the Marcellus Shale (less than detection to 18,000 picocuries per liter (pCi/L)) overlaps the range for non-Marcellus reservoirs (less than detection to 6,700 pCi/L), and the median values are 2,460 pCi/L and 734 pCi/L, respectively. A positive correlation between the logs of TDS and radium activity can be demonstrated for the entire dataset, and controlling for this TDS dependence, Marcellus shale produced water samples contain statistically more radium than non-Marcellus samples. The radium isotopic ratio, Ra-228/Ra-226, in samples from the Marcellus Shale is generally less than 0.3, distinctly lower than the median values from other reservoirs. This ratio may serve as an indicator of the provenance or reservoir source of radium in samples of uncertain origin. Introduction Radium forms naturally from the decay of uranium and thorium, elements that commonly occur in sandstones and shales in sedimentary environments. Radium has been documented in the formation waters in many sedimentary basins (for example, Fisher, 1998). In the northern Appalachian Basin, radium has been measured in the water co-produced with gas and oil (that is, produced water3) from reservoirs of Cambrian-Mississippian age. Radioactive isotopes are commonly quantified in terms of “activity concentration” or simply “activity,” which in this context refers to a number of disintegrations per unit time. For consistency with the studies cited, activity units of picocuries per liter (pCi/L) are used here to define the activity of radium in produced water samples. In surface and shallow subsurface environments, radium can be relatively soluble and, therefore, mobile in groundwater over a range of pH and Eh (redox) conditions (Langmuir and Riese, 1985; Sturchio and others, 2001). Radium also may be adsorbed onto clay particles or onto oxide grain coatings (Krishnaswami and others, 1982; Ames and others, 1983; Sturchio and others, 2001). As a radioactive element, radium may represent a potential health hazard if released into the environment. The half-lives of the two principal isotopes of radium, Ra-226 and Ra-228, are 1,600 and 5.75 years, respectively (Akovali, 1996; Artna-Cohen, 1997), and approximately 10 half-lives are required for a radioactive element to decay to negligible quantities. Chemically, radium behaves in a manner similar to calcium and is capable of bioaccumulation in plants and animals. There is a significant body of research aimed at quantification of radium uptake in crops and livestock that make up the human food chain (for example, Tracy and others; 1983; Bettencourt and others, 1988; Linsalata and others, Radium Content of Oiland Gas-Field Produced Waters in the Northern Appalachian Basin (USA): Summary and Discussion of Data By E.L. Rowan,1 M.A. Engle,1 C.S. Kirby,2 and T.F. Kraemer1 1U.S. Geological Survey, Reston, Virginia. 2Bucknell University, Lewisburg, Pennsylvania. 3The term “produced water” in this report represents water produced from an oil or gas well at any point during its life cycle. The term, therefore, includes waters produced immediately after hydraulic fracturing, with compositions close to those of the injected fluid, as well as waters produced after months or years of production, whose compositions resemble formation water. 2 Radium Content of Oiland Gas-Field Produced Waters in the Northern Appalachian Basin: Summary and Discussion 1989). Most of these studies were conducted in areas where uranium mining had previously taken place; however, it is not known whether similar investigations have been conducted in regions where oiland gas-field produced waters are the source of radium. The purpose of this report is to compile and present data from multiple sources to facilitate ongoing research. Activity data for radium-226 (Ra-226) and radium-228 (Ra-228) in oiland gas-field produced waters from New York and Pennsylvania have been compiled from publicly available sources and combined with new data for six wells (tables 1 and 2, p. 19–31). Measurements of total dissolved solids (TDS) and of gross alpha and beta activities were also tabulated when available. Unstable (radioactive) isotopes decay by emitting alpha and beta particles; therefore, alpha and beta activities can serve as rough indicators of the presence of radioactive elements. The publicly available radium data were obtained from the New York State Department of Environmental Conservation (NYSDEC), the Pennsylvania Department of Environmental Protection (PA DEP), and the Pennsylvania Geological Survey. Most of these data are available online, although the most recent Marcellus Shale produced water data were available only from the regional PA DEP offices. Three of the studies, Gilday and others (1999), Pennsylvania Department of Environmental Protection (1992), and Dresel and Rose (2010), provide data from wells producing from reservoirs of Cambrian-Devonian age. In contrast, the analyses reported by the New York State Department of Environmental Conservation (2009) and by the Pennsylvania Department of Environmental Protection (unpub. data, 2009–2010) are for produced waters predominantly from the Devonian Marcellus Shale. Background The Appalachian Basin comprises a vast accumulation of sedimentary rock west of the Appalachian Mountains, extending from Quebec and Ontario south through New York, Pennsylvania, Ohio, West Virginia, to Alabama. Hydrocarbons are produced throughout the basin from reservoirs of Cambrian-Pennsylvanian age (Legall and others, 1981; Milici and others, 2003). In recent years, however, the Middle Devonian Marcellus Shale has become the focus of gas exploration and production, particularly in Pennsylvania, New York, and West Virginia. A regional comparison of produced water salinities indicates that Appalachian Basin salinities are high relative to other oiland gas-producing basins in the United States (Breit, 2002). The compilation yielded a median TDS of about 250,000 milligrams per liter (mg/L) for the Appalachian Basin (USA), which was exceeded only by the median salinity for the Michigan Basin (about 300,000 mg/L). The data presented here indicate a wide salinity range for water produced from the Marcellus Shale, from less than 1,500 mg/L to greater than 300,000 mg/L. The lower salinities may be attributed in part to dilution with less saline fluid injected during hydraulic fracturing, but the upper end of the salinity range is comparable to the waters produced from the underlying Lower Devonian and older reservoirs as well as some of the overlying Devonian reservoirs (Rowan and others, 2010). The Marcellus Shale is an organic-rich shale that is both the source rock and the reservoir for an extensive natural gas resource (Harper, 2008). Shale-gas accumulations, such as the Marcellus, are termed “unconventional” or “continuous” because the gas is dispersed within a stratigraphic interval rather than confined by a conventional structural or stratigraphic trap. The process of “hydraulic fracturing” commonly is used to access the gas in a continuous reservoir. In this process, water is pumped into a well at pressures high enough to fracture the rock, and the newly created fracture network allows gas that is tightly held in micropores or adsorbed onto clay particles to be released. The injected fluid may be freshwater or relatively dilute, or alternatively, it may have been recycled, that is, produced from one well and then used to hydraulically fracture a new well. The water flowing from hydraulically fractured wells initially reflects the composition of the injected fluid, but with time shifts toward salinities and inorganic chemical compositions similar to the fluids in adjacent formations (for example, Rowan and others, 2010). Hayes (2009), for example, examined the chemistry of produced water samples collected from 12 Marcellus Shale wells at 1-, 5-, 14-, and 90-day intervals following hydraulic fracturing. The water injected into these wells was essentially fresh, with a median TDS of less than 1,000 mg/L, but within 90 days, the salinities had increased to a median value exceeding 200,000 mg/L TDS. Ra-226 and Ra-228 are the decay products of U-238 and Th-232, respectively (fig. 1; Ivanovich, 1992). Once formed, radium may remain within the original host mineral or other solid phase, or may be released into the adjacent pore water. Lithologies that contain substantial amounts of uranium and (or) thorium can, therefore, have measurable amounts of radium dissolved in their pore waters. The data compiled in this report span most of the oiland gas-producing regions of the Appalachian Basin in Pennsylvania and New York (fig. 2), and show significant levels of radium in produced water samples from Cambrian-Mississippian reservoirs. Dissolved radium occurs predominantly as the Ra+2 ion, but also forms complexes with chloride, sulfate, and carbonate ions (Rose and Korner, 1979; Kraemer and Reid, 1984; Langmu