A published method for calculating excess steam is modified in order to make it applicable to the case of a well producing fluid with a high non-condensable gas content. The method is applied to data from a number of wells from Los Azufres. The results show the expected linear relation between excess steam and the molar fraction of C02 in the total discharge. The modified method also provides an estimate for the molar fraction of C02 in the liquid phase of the reservoir. It is found that this concentration is relatively homogeneous in the southern section of the field, and seems to be slightly smaller in the deeper zones of the northern section. The calculated values of excess steam agree qualitatively, but not quantitatively, with values of steam quality in the undisturbed reservoir calculated by other authors. The calculation of excess steam tends to overestimate steam quality in cases where steam saturation in the reservoir is low, and has the opposite tendency when the steam quality in the reservoir is high. The first type of deviation is explained in terms of the effects of higher mobility of steam in the two-phase zone induced by pressure drawdown. The second type of deviation is explained as the result of boiling of liquid phase, and the consequent addition of (non-condensable) gas-poor steam to 4. the flowing mass, also caused by the pressure drawdown. A necessary extension of the reasoning leading to these explanations, is the prediction that the non-condensable gas content in fluids produced from a reservoir with a high steam saturation, will either remain stable or decrease with time during exploitation.

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