--> Abstract: Estimated Diffusion Rates of Inorganic Gases from Southeastern Colorado Reservoirs, by J. S. Nelson and E. C. Simmons; #91008 (1991)

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Estimated Diffusion Rates of Inorganic Gases from Southeastern Colorado Reservoirs

NELSON, JON S., and E. C. SIMMONS, Colorado School of Mines, Golden, CO

Helium, argon, and nitrogen are small, chemically unreactive molecules with relatively large effective diffusion coefficients compared to most hydrocarbons. If these gases have existed in gas fields for even the shortest geologic time spans, steady-state diffusion must be at least approximated, and the diffusional flux of these gases through a reservoir cap rock may be estimated using Fick's First Law of Diffusion. Diffusional loss represents a minimum loss rate since mass transfer along fractures and faults would be faster.

Under a steady-state condition, the rate of diffusional loss must be balanced by an equal influx of the gas into the reservoir. Using a natural gas field's estimated reserves, natural gas composition, area, and the estimated flux of a given gas through the field's cap rock, the turnover time of the gas is estimated.

Southeastern Colorado gas fields producing from the Morrow Formation often contain anomalously high concentrations of nitrogen (to 70%), helium (to 5%), and argon (to 0.5%) and yield turnover times for these gases of generally less than 100,000 years. Unless the N2, He, and Ar were emplaced within the last 100,000 years, there must be a continuing large flux of gas into these fields to balance the diffusional loss.

The large fluxes of inorganic gases required to maintain their concentrations in natural gases raises questions about the age and longevity of gas fields. Extension of these calculations to light hydrocarbons implies that catagenesis is a more recent and ongoing process than is often believed.

 

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)