The effects of adsorption phenomena in the evaluation of vapour-dominated geothermal reservoirs

Previous reservoir engineering studies of vapour-dominated geothermal reservoirs have generally been analogous to conventional-model studies for natural gas reservoirs. In this work, the concept that considerable adsorbed water may exist in a vapour-dominated zone is examined in detail. Experimental and theoretical evidence of adsorption phenomena is described. The implications of adsorption on material balance calculations and on well test analysis are then determined by incorporating adsorption effects into existing models. The resulting new methods of analysis provide a more realistic estimate of the nature and extent of the vapour-dominated zone. In particular, the new methods result in a considerable reduction in the estimated formation thickness and suggest that fracture porosity has been underestimated using conventional models for a naturally fractured reservoir. The importance of the adsorption phenomenon in reserve estimation is of considerable magnitude. This work shows that a reserve estimate based on geologic evidence and the thermodynamic properties of steam could be as much as one order of magnitude lower than the actual mass of water present.