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Compositional Changes to Gas Migrating through Water-Saturated Rock

Abstract

Due to the low solubility of hydrocarbon gases and inert gases in water, natural gas composition is commonly assumed to be unaltered by migration. This may not be the case. Although gases are sparingly soluble in water, the water–gas ratio is high during migration. Therefore, mass of dissolved gases along a migration pathway may approach those of the migrating gas, especially where the migration pathway is long or where diffusion of dissolved gases increases the effective water-gas ratio. Concentrations in gas-phase and dissolved gas were simulated for gas migrating through a stationary water-saturated medium. Where no gas was dissolved in water, gas concentrations changed as predicted by Rayleigh fractionation. Gas ratios at the leading edge of the gas can show moderate compositional fractionation, but the main result is loss of gas-phase volume as the gases dissolve in water. Where gases are dissolved in water, gas ratios in the leading edge of the gas quickly approach equilibrium with the dissolved gas ratios. Consequently, a compositional front develops that separates gas compositions altered by water interaction near its leading edge from unaltered gas compositions near the location where gas enters the system. This front moves farther behind the leading edge with time and farther away from the gas entry point as the total volume of migrating gas in the system increases. The compositional front velocity is controlled by the concentrations of dissolved gas in the water and the intrinsic water-gas ratio. If migration distances are long, a significant volume of gas with composition similar to that dissolved in water may develop behind the leading edge. The altered gas enters traps first, but it is mixed with later, unaltered gas such that its original altered composition may not be evident from the bulk gas in the trap. These models demonstrate that migrating gas or small gas accumulations may develop different compositions due to water interaction during migration, that the volume of altered gas may be significant, and that compositional differences developed during migration are diluted as gas charges large accumulations. Water interaction may explain a number of compositional phenomena: (1) minor gas compositional variations in gas fields, (2) utility of mud gas isotopes and compositional changes for identifying gas accumulations, and (3) deviations of seep gas compositions from gas compositions in leaking gas pipelines.