Background gas has been observed in oil based drilling fluids in contact with gas bearing formations. It has been postulated that such background gas is caused by diffusion. The objective of this study was to quantify the transport of gas by diffusion. The effect of filtrate flow into the formation (against the gas transport) has been evaluated.

The diffusion coefficients of the filter cakes were determined in the laboratory at various temperatures and pressures. The significance of diffusion was mostly decided by the rate of filtration. The flux of gas by diffusion drops sharply at some threshold value of pressure differential in the wellbore, irrespective of the temperature.

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