Regular variations of the mercury concentration in natural gas

Direct on-line measurements of the mercury (Hg) content in natural gases by a newly developed Zeeman atomic absorption spectrometer RA-915+ have revealed a short-term variability of the Hg concentration, which has not earlier been known. The significant variability of the mercury concentration in a hydrocarbon gas was found in all of the gas wells studied. The variation magnitudes range from 10 to 80% of the average mercury content in the gas. The fluctuations have regular components with periods from few minutes to several hours and can be represented as a set of harmonics with different spectral intensity and stability in time. The most stable harmonics are characterised by intermediate frequencies with periods of 0.3-2.5 h. These 'basic' frequencies are characteristic of individual gas wells even within a single productive horizon. The remarkable features of these harmonics are the multiplicity of their periods and good agreement with phases of the Earth's own oscillations. A relatively low-frequency oscillation with a period of 25 h corresponds to a lunar tidal cycle. An oscillatory mechanism of the adsorption-desorption of mercury in a porous gas-bearing medium is assumed to be responsible for these effects.