Geothermal Reservoir Evaluation Considering Fluid Adsorption and Composition

Abstract

Summary The concept that adsorbed water is a major source of production fluid in vapor-dominated geothermal reservoirs is presented. The implications of adsorption on material-balance calculations and on well test analysis are determined by incorporation of adsorption effects into existing models. New analysis methods introduced here provide a more realistic estimate of the nature and extent of the vapor-dominated zone. The methods result in a considerable reduction in the estimated formation thickness and suggest that fracture porosity has been underestimated with conventional models for a naturally fractured reservoir. The presence of noncondensable gases in the geothermal fluid has a profound effect on the thermodynamics associated with vapor/liquid equilibrium and adsorption. Noncondensable gases can cause the dewpoint pressure of a noncondensable gas/water mixture to be elevated substantially, above the vapor pressure for pure water at reservoir temperature. Hence, the presence of gas in geothermal steam extends the pressure range where vapor adsorption phenomena are in effect. Monitoring of gas production in the produced geothermal fluids provides additional data useful in evaluating adsorption effects. The adsorption phenomenon in reserve estimation is of considerable importance. This work shows that a reserve estimate based on geologic evidence and the thermodynamic properties of steam could be as much as an order of magnitude lower than the actual mass of water present.