Environ. Sci. Technol. 1986, 20, 939-942
Radon in Unconventional Natural Gas from Gulf Coast Geopressured-Geothermal Reservoirs Thomas F. Kraemer Gulf Coast Hydroscience Center, US. Geological Survey, NSTL, Mississippi 39529
Radon-222 has been measured in natural gas produced from experimental geopressured-geothermal test wells. Comparison with published data suggests that while radon activity of this unconventional natural gas resource is higher than conventional gas produced in the gulf coast, it is within the range found for conventional gas produced throughout the U.S. A method of predicting the likely radon activity of this unconventional gas is described on the basis of the data presented, methane solubility, and known or assumed reservoir conditions of temperature, fluid pressure, and formation water salinity.
Introduction Formation water in deep, hot, high-pressure aquifers of the US. gulf coast is currently being evaluated as an unconventional source of natural gas by the U.S. Department of Energy (DOE). The most commonly proposed production scheme is to bring the water containing dissolved gases (mostly methane) from the aquifer to the surface and pass it through a gas-water separating manifold. The resulting drop in temperature and pressure would liberate the dissolved gases, which would be put into gas distribution systems. The spent water would be disposed of by injection into shallow (- 1000 M) saline aquifers. Several tests have been carried out (Figure 1)to examine the quantity and quality of gas that can be produced from so-called ”geopressured-geothermal” aquifers, and a great deal of study has gone into determining the characteristics of this gas. Because radon gas poses a public health concern, its activity in gas produced from these tests was determined, and a method to predict radon activity of “unconventional” gas was sought. By use of these results, future tasks of assessing the public health impact caused by use of this gas may be made easier, although such efforts will probably await demonstration of commercial feasibility of production. Behavior of Radon Radon is a radioactive noble gas produced by the decay of radium in the naturally occurring uranium and thorium decay series. The only isotope of radon with a significantly long half-life is zzzRn(t,/z= 3.82 days). Because of its noble gas character radon can migrate with relative ease from within matrix grains where it is formed, along microcracks, fractures, and other grain imperfections to the pore space of an aquifer. Considerable excess activities (unsupported by radium) of radon can accumulate in the pore space of water-saturated granular aquifers by this process (1,2).In some cases, especially in the gulf coast, aquifer formation waters are also high in radium (3). This dissolved radium will decay and contribute radon to the liquid phase as well. Ultimately, in a closed system, the total radon dissolved in pore water will be an equilibrium activity determined by the activity of the radium in the water plus the radon diffusing from the solid phase minus the decay of radon in the pore space. Although radon never accumulates in large enough quantities to form a separate gas phase, it will migrate from solid and liquid phases to a gaseous phase if one is present
in an aquifer. It is therefore found in conventional hydrocarbon gas deposits, where an intimate association of solid, liquid, and gaseous phases coexist.
Previous Studies The subject of radon in natural gas and natural gas liquids has received considerable attention over the past 20 years because of the potential for adverse health effects as a result of domestic and industrial use of natural gas contaminated with radon. Johnson et al. ( 4 ) cite radon levels of natural gas at the wellhead for various regions of the U.S. based on an extensive literature search (Table I). They used these data, and related data, to estimate population exposure due to radon from natural gas and natural gas products and to estimate associated health risks to the general population. Their conclusion was that radon in natural gas and natural gas products could lead to 15 deaths a year from lung cancer throughout the entire U.S., which at the time was 0.03-0.08% of normal lung cancer mortality. Gogolak (5)estimated the radon activity of various potential unconventional sources of natural gas, including geopressured solution gas. He concluded that none of the unconventional sources of natural gas reviewed by him (coal-bed methane, geopressured solution gas, eastern shale gas, western tight sands gas) would present “a significantly higher radiological impact on the U.S. population than exists from conventionalrecovery technologies.” However, his data were severely limited due to the lack of sufficient samples from the as-of-then largely untested potential sources. His data included only a few gas samples from geopressured-geothermal wells, since the DOE evaluation program had only just begun as of that date. Measurement of Radon in Gulf Coast Natural Gas During the DOE-sponsored geopressured-geothermal energy evaluation program, high-volume flow tests were conducted on wells throughout the coastal region of Louisiana and Texas. Several tens-of-thousands of liters of formation water per day were passed through gas-water separators to extract the dissolved methane, and then reinjected through a second well for disposal. The produced gas was flared in all cases except for the Pleasant Bayou 2, where the gas was put into a commercial gas distribution system. This represented the only commercial production of unconventionally produced geopressured gas in the U.S. until 1984, when another test well, Gladys McCall 1, also started producing unconventional gas for a commercial pipe line. Samples of gas were taken from each well from a port on the gas line just after the gas-liquid separator. Sampling was done only during periods of high-volume flow through the separator and after thorough flushing of the lines had taken place. Pressure in the gas lines ranged between 0.50 and 4.0 mPa, depending on the well and the flow characteristics of the gas-liquid separator. Samples were taken from conventional high-pressure gas fields in south-central Louisiana that closely resembled the unconventional solution-gas test well environments in temperature, pressure, and lithology. A number of such
Not subject to U.S.Copyright. Published 1986 by the American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 9, 1986 939
+IKOELEMAY.
MCCALL
\
1 2R , SALDANA
Y
Flgure 1. Location of unconventional(geopressured-geothermal) gas test wells sampled for this report.
Table I. Radon-222 Activity i n Natural Gas at Production Wells, a s Reported by Johnson et al. (4) radon-222 level, pCi/L average range
area Colorado, New Mexico Texas, Kansas, Oklahoma Texas Panhandle Colorado Project Gasbuggy area Project Gasbuggy area California gulf coast (Louisiana, Texas) Kansas Wyoming
5 100 10
overall average
37
25
lo0 g/L) salinity. Thus, a relation between water salinity and radon activity of produced geopressured solution gas can be established for high-salinity brines by use of a relatively simple technique, To illustrate the use of the technique, Table IV has been constructed. In this illustration temperature and formation fluid pressure are held constant at 120 “C and 103 mPa, respectively, and salinity varied from 25 to 250 g/L. The methane content of formation water under the various salinities at the constant temperature and pressure were calculated by using the algorithm of Haas (12).As salinity of the formation water increases, methane solubility decreases. The radon activity in this volume of gas is then calculated by using the radium-salinity relation found by Kraemer and Reid (3),assuming that all methane and radon is removed from the water a t the surface (actual recoveries are greater than 90% for both). Thus, formation water of 100 g/L TDS salinity brought to the surface will exsolve 4.93 L of methane/L of water, assuming methane saturation of the water. The radium activity of this water can be estimated from Kraemer and Reid (3) to be 315 dpm/L of water, resulting in dissolved radon of the same activity. When the dissolved methane is extracted, it will scavenge the radon, resulting in a radon activity in the solution gas of 64 pCi/L at STP. Similar calculations could be made for each potential geopressured-geothermal aquifer, given its known or assumed physical conditions of temperature and fluid pressure. The dashed line in Figure 2 shows the data
Table IV. Calculated Radon Activity of U.S.Gulf Coast Solution Gas from Formation Water at 120 “C and 103 mPa” salinity, g/L 25 50 75 100
125 150 175 200 225 250
methane solubility at 120 OC and 130 mPa, ppmb
CH,, mol of gas/ L of water
CHI, L of gas/ L of water
zzzRngenerated from 226Ra: pCi/L of water
222Rn in gas, pCi/L
5127 4561 4032 3540 3085 2666 2285 1939 1628 1351
0.32 0.285 0.25 0.22 0.193 0.167 0.143 0.121 0.102 0.084
7.17 6.38 5.60 4.93 4.32 3.74 3.20 2.71 2.29 1.88
32 108 225 315 518 721 1036 1216 1487 1802
4 17 40 64 120 193 324 449 649 958
OAll values at STP. *From ref 12. cFrom ref 3. Environ. Sci. Technol., Vol. 20, No. 9 , 1986
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presented in Table IV and is probably suitable for estimating the radon content of solution gas from most geopressured-geothermal aquifers since most aquifers likely to be utilized for dissolved-gas production would be close to these approximate temperature and pressure values. Actual data points for solution gas from Table I1 are also plotted in Figure 2. At high salinities there is reasonable agreement with calculated values, but below 100 g/L divergence can occur. This is possibly because a greater percentage of the total radon in low-salinity water comes from mineral grain exhalation as opposed to decay of radium in the water itself. Because of this exhalation, it appears that some activity of radon will be present in solution gas regardless of how low the radium content of the water becomes. The actual magnitude of this “exhaled” radon in the formation water depends on many factors such as reservoir porosity, grain size, and uranium content of matrix grains. However, from data collected to date it appears that, overall, exhaled radon in low-salinity water will produce a minimum value of about 40 pCi/L of solution gas a t the well site. Much higher “exhaled”radon activities in solution would not be expected unless porosities were much lower and/or grain sizes were much smaller. In either case the reservoir would probably not support large volume water withdrawal and could not be utilized for geopressured-geothermal solution-gas production. Higher uranium content of deeper reservoir material cannot be ruled out, but in the large majority of areas where geopressured-geothermal solution-gas production is likely, the reservoirs are expected to be clean, well-sorted sandstones whose only uranium content is that contained within the sand grains or clay particles themselves, and this usually does not greatly exceed 1 or 2 ppm (23). Figure 2 or the calculations used in constructing it can be used to estimate the likely activity of radon in solution gas produced from gulf coast geopressured resevoirs if the formation water salinity is above about 100 g/L TDS. Below this value it would be prudent to estimate the radon activity as a minimum value or “floor” of approximately 40 pCi/L, due to exhalation from matrix grains.
Acknowledgments Grateful appreciation is given to the US.Department of Energy for permission to sample the geopressured-
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Environ. Sci. Technol., Vol. 20, No. 9, 1986
geothermal wells and to personnel from Fenix and Scission/Technadril and Eaton Operating Co. (DOE contractors) for assistance in obtaining samples. Superior Oil Co., Phillips Petroleum, and May Petroleum Co. permitted and gave assistance in sampling the conventional gas wells. Registry No. 222Ra,14859-67-7.
Literature Cited Michel, J.; King, P. B.; Moore, W. S. “228Ra,226Ra,and zzzRn in South Carolina Ground Water: Measurement Techniques and Isotope Relationships”; Report No. 95; Water Resources Research Institute, Clemson University: Clemson, SC, 1982. Krishnaswami, S.; Graustein, W. C.; Turekian, K. K.; Dowd, J. F. Water Resour. Res. 1982, 18(6), 1633-1675. Kraemer, T. F; Reid, D. F. Isot. Geosci. 1984,2(2),153-174. Johnson, R. H., Jr.; Bernhardt, D. E.; Nelson, N. S.; Calley, H. W., Jr. “Assessment of Potential Radiological Health Effects from Radon in Natural Gas”; EPA Report 520-173-004, 1973. Gogolak, C. V. “Review of “‘Rn in Natural Gas Produced from Unconventional Sources”; Report EML-385, 1980; Department of Energy, Environmental Measurements Laboratory. Broecker, W. S. In Symposium on Diffusion in Oceans and Fresh Waters;Ichiye, T., Ed.; Lamont Geological Observatory: Palisades, NY, 1965. Schink, D. R.; Guinasso, R.; Charnell, R.; Sigalove, J. IEEE Trans. Nuc1. Sci. 1970, NS-17, 184-193. Chung, Y. C. Earth Planet. Sci. Lett. 1972, 14, 55-64. Reid, D. F. Ph.D. Dissertation, Texas A&M University, College Station, TX, 1979. Lucas, H. F. Reu. Sei. Instrum. 1957, 28, 680-683. Kraemer, T. F. Proceedings of the Fifth Conference on Geopressured-Geothermal Energy; Bebout, D. G.; Bachman, A. L., Eds.; Louisiana State University: Baton Rouge, LA, 1981; pp 201-204. Haas, J. L. “An Empirical Equation with Tables of Smoothed Solubilities of Methane in Water and Aqueous Sodium Chloride Solutions up to 25 Weight Percent, 360”C, and 138mPa.”; U.S. Geological Survey Open-File Report 78-1004, 1978. Kraemer, T., U.S. Geological Survey, unpublished data, 1986.
Receiued for review October 25, 1985. Accepted April 8, 1986. Use of trade names does not imply endorsement of the product by the US.Geological Survey.
