The Quantification of Diffusive Hydrocarbon Losses Through Cap Rocks of Natural Gas Reservoirs—A Reevaluation

A calculation scheme is presented to estimate the quantities of diffusive gas loss through the water-saturated cap rock of natural gas reservoirs over geologic time. Reexamination of earlier calculations in a 1982 paper by Leythaeuser et al. revealed that the estimate of the rate of diffusive hydrocarbon losses through the cap rock of the Harlingen, Netherlands, natural gas reservoir was based on oversimplified assumptions concerning the boundary conditions at the reservoir–cap rock interface. According to this recalculation, the time required to reduce the present-day methane content of the Harlingen gas field by one-half is approximately 70 m.y., not the previously published value of 4.5 m.y. Consequently, diffusion losses through the cap rock appear to affect the long-term stability of natural gas reservoirs to a lesser extent than suggested by the earlier results.