Abstract
This paper deals mainly with diffusion of methane from the oil-free dissolved-in-water type gas reservoirs in the Kazusa group which is widely distributed in the Southern Kanto region, Japan. Diffusion of methane in water-saturated sediments is ruled by Fick's laws as expressed by the equations [2.1] and [2.2]. Several solutions of the differential equation [2.2] under some simple boundary conditions, that is, simplified geologic conditions, are shown in the equations [4.1], [4.2], [4.3] and [5.1]. Concerning diffusion coefficient of methane in water-saturated sedimentary rocks, (Nernst-) Einstein's law (expressed by the equation [2.3]) is valid. Moreover, assuming that adsorption effect of rook-forming minerals is negligible, Klinkenberg's formula (equation [2.4]) is applicable. Consequently diffusion coefficient of methane in water-saturated clean loose sand of porosity 30% at 40°C, is estimated at 8.2×10-6cm2/sec at most. Diffusion coefficient of methane in water-saturated muddy sand or sandy mud can be assumed to be 1×10-6cm2/sec; in water-saturated compact mudstone, 1×10-7cm2/sec. Calculations by putting these values of diffusion coefficient of methane and a value of sodium chloride into the equations [4.1], [4.2], [4.3] and [5.1], were carried out, and the results were tabulated. Through these calculations, the writer has reached the following conclusions:(1) Migration of methane by diffusion in water-saturated formations in several million years is slight; therefore, natural gas deposits of some scale and some depth cannot be largely deteriorated by diffusion.(2) Although diffusion of sodium chloride is less than that of methane, the difference between these two is not large. Hence, positive correlation between chlorine and methane in formation water, a remarkable tendency found in the Southern Kanto region, cannot be largely disturbed by diffusion.(3) In general, migration of methane caused by the pressure difference of formation fluids- migration caused by ground-water flow or by buoyancy of gas bubbles-, is larger than that by diffusion.(4) In the marine Kazusa group, areas of formation water of very low salinity and very low gas potentiality are widely developed. This fact is ascribed to the percolation of meteoric water into the formations; moreover, this shows that the velocity of meteoric water percolation has been much larger than the diffusion velocities of methane and salts.(5) In case thick sediments, generating and containing enough methane, are overlying a gas reservoir under transgressive condition without meteoric water percolation, the quantity of methane which diffuses out of the gas reservoir may be negligibly small.
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