1977 Midwest Gas Storage and Production Indianapolis Indiana - April 13–15, 1977
Identifying gas that has leaked from underground storage reservoirs for natural gas can be a difficult problem in areas where bacterially produced methane is common in the ground water. The absence of ethane or heavier hydrocarbons in methane-rich gas is generally interpreted as indicating that the gas was formed by bacteria because methane is the only hydrocarbon produced by bacteria in significant quantities. The data presented in this produced by bacteria in significant quantities. The data presented in this paper. however, suggest that the presence or absence of hydrocarbons paper. however, suggest that the presence or absence of hydrocarbons heavier than methane may not be a reliable indicator of gas origin.
Radiocarbon dating of methane can be used to identify bacterially produced gas that was formed from organic materials less than 50,000 years produced gas that was formed from organic materials less than 50,000 years old. This technique, however, requires relatively large samples and cannot distinguish between storage gas and bacterially produced gas from older materials. Determination of the C13/C12 ratio of methane is a simpler and more versatile tool for identification of leakage gas. Bacteriogenic methane from Illinois generally has C13 values in the range of -64 to -90 o/oo relative to the Peedee Belemnite (PDB) standard. The 11 samples from pipelines and storage reservoirs that have been analyzed have all had C13 values in the range of -40 to -46 o/oo.
Three storage reservoirs have been studied by analyzing samples from the reservoirs and from shallow water wells overlying the reservoirs. At the first reservoir, the C13 value of methane from the storage gas was -46 o.oo, whereas the C13 value of methane from overlying water wells was about -76 o/oo - Radiocarbon dating confirmed that the gas in the water wells had been formed by bacterial decomposition of materials within the glacial drift. At the second reservoir, the C 13 values of storage-gas samples ranged from -41 to -45 o/oo Samples from waterwells ranged from -78 o/oo off the margin of the reservoir to -41 o/oo directly over the reservoir, suggesting an increasing concentration of leakage gas over the reservoir. Although the gas in the reservoir contained approximately S percent hydrocarbons heavier than methane, no hydrocarbons heavier than methane were detected in the samples from the water wells. At the third reservoir studied, leakage gas was being withdrawn from the units overlying the reservoir and reinjected into the reservoir. Samples from several gas wells at various depths had C13 values ranging from about -41 to -43 o/oo. and contained from 5 to 7 percent hydrocarbons heavier than methane. A sample from a water well percent hydrocarbons heavier than methane. A sample from a water well directly over the reservoir had C13 -60 o/oo and contained 0.8 percent heavy hydrocarbons. It is estimated that this gas contains from 15 to 40 percent leakage gas and that the remainder is bacterially produced gas. Although further testing is necessary, isotopic analysis of methane has proven so far to be a reliable technique for differentiating leakage gas from gas produced by bacterial action.