Beiträge zur Isotopengeochemie von Erdgasen

Mühle, K.; Harting, P.; Maass, I.; Missbach, D.; Nitzsche, Horst-Michael; Weise, G.

doi:10.1080/00211919408046745

Cited by 2 publications

(1 citation statement)

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“…Positive IE L values – corresponding to a higher aqueous solubility of the 13 C hydrocarbon gas – are in accordance with literature data for lower pressures and temperatures (Galimov 1975; Harting 1977; Fuex 1980; Mühle et al . 1994).…”

Section: Resultssupporting

confidence: 91%

Molecular transport of methane, ethane and nitrogen and the influence of diffusion on the chemical and isotopic composition of natural gas accumulations

Schloemer¹,

Krooss²

2004

Geofluids

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Laboratory experiments have been performed to determine diffusion coef®cients of natural gas components (methane, ethane and nitrogen) and isotope fractionation effects under simulated in situ pressure (up to 45 MPa effective stress) and temperature conditions (50±2008C) in water-saturated pelitic and coarse-grained rocks. Effective diffusion coef®-cients of molecular nitrogen (0.39 Â 10 À11 to 21.6 Â 10 À11 m 2 sec À1 at 908C) are higher than those for methane (0.18 Â 10 À11 to 18.2 Â 10 À11 m 2 sec À1 at 908C). Diffusive¯ux rates expressed in mass units are generally higher for N 2 than for CH 4 . Both methane and (to a lesser extent) nitrogen diffusion coef®cients decrease with increasing total organic carbon (TOC) content of the rock samples because of sorption processes on the organic matter. This effect decreases with increasing temperature. Effective diffusion coef®cients increase upon a temperature increase from 50 to 2008C by a factor of four. Effective diffusion coef®cients and steady-state diffusive¯ux decrease with effective stress. Stationary diffusive¯uxes drop by 50±70% for methane and 45±62% for nitrogen while effective diffusion coef®cients are reduced by 38% (CH 4 ) and 32±48% (N 2 ), respectively. Isotope fractionation coef®cients of diffusive transport are higher for methane (À1.56 and À2.77%) than for ethane (À0.84 and À1.62%). Application of the experimental results to geological systems show that diffusive transport has only a low transport ef®ciency. Signi®cant depletion of natural gas reservoirs by molecular diffusion is only expected in cases of very poor caprock qualities (in terms of thickness and/ or porosity) and over extended periods of geological time. Under these circumstances, the chemical and isotopic composition of a gas reservoir will change and maturity estimates based on these parameters may be deceptive. To account for these potential effects, nomograms have been developed to estimate diffusive losses and apply maturity corrections.

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Section: Resultssupporting

confidence: 91%