The measurement of the quantity of adsorbed water on geothermal reservoir rocks allows a more realistic estimation of reserves for vapor-dominated geothermal reservoirs. This study measured adsorptioddesorption isotherms of water vapor on rock samples from Calpine Co.'s well MLM-3, both core fragments and well cuttings from Coldwater Creek steamfield and a number of well cuttings from well Prati State 12, Northwest Geysers steam field. Surface areas of these rock samples were measured using nitrogen adsorption at 77K. The results of these measurements suggest that surface area is a crucial factor in determining the amount of water adsorption. Analysis of the water adsorption data indicates that adsorption is the dominant phenomena in the matrix of the reservoir rock at relative pressures below 0.8. Depending on the structure of the rock, capillary condensation contributes considerably to the total water retention at relative pressure between 0.8 and 1.0. However, there is no clear distinction between adsorption and capillary condensation and it is difficult in the experiments to determine when complete saturation occurs. A significant result of these experiments was the demonstration that well cuttings show adsorption characteristics very much like those obtained from core fragments. This should allow further adsorption measurements to be made more extensively and at lower cost.

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