Improving Models of Vapor-Dominated Geothermal Fields: the Effects of Adsorption

Abstract

Adsorbed water on the rock surfaces in vapor-dominated geothermal fields has long been thought to provide a major source of fluid within the reservoir. Over the past several years, the Stanford Geothermal Program has conducted a series of investigations into the phenomena of water adsorption in geothermal systems,and their effects on reservoir performance. The results and conclusions of the most recent studieswill be summarized in this paper.

The studies include: (1) an experimental investigation in which measurements of the adsorption and desorption isotherms on actual geothermal cores were made, including a study of adsorption characteristics in The Geysers field in California; (2) a theoretical investigation into how adsorption relates to capillary condensation, and how the combined phenomenon can be expected to act; (3) a theoretical, numerical and data investigation into how adsorption and desorption will affect reinjection at The Geysers field, including a study of adsorption effects evident in tritium tracer observations.

It has been found through these studies that the amount of (liquid) water adsorbed in geothermal fields is very considerable, even at pressures well below the boiling point pressure. Adsorbed water represents the major fraction of fluid stored in the reservoir and can be the most important source. Reservoirperformance forecasts are governed by the amount, and the rate of release of adsorbed water. Adsorption seems only modestly affected by the presence of noncondensiblegases. During reinjection, water adsorbs more easily than it subsequently is able to desorb, which results in a reduction in the efficacy of reinjection.