Origin Of Steam In Geothermal Reservoirs
Authors: McNitt J.R. 
Published: December 2007

Abstract
In geothermal systems, due to the mechanism of mass and heat transport on the boiling point for depth curve, stable steam-water mixtures can occur only at reservoir pressures less than about 30.6 bars and at temperatures less than about 235 deg. C. The presence of a stable steam phase dispersed in a confined water-dominated reservoir tends to stabilize pressure and, therefore, temperature throughout the reservoir. This mechanism is responsible for the common occurrence of water-dominated reservoirs having temperatures in the range of 220 deg. C to 235 deg. C. Structural closure over such two-phase systems is believed to result in gravity separation of the phases and the formation of steam caps. Field data indicate that at least five, and possibly eight, water-dominated fields discovered to date contain steam caps.
Although difficult to detect, steam zones can be identified by a careful testing program that includes flow testing during drilling. Steam zones can be easily damaged with mud, and underbalanced drilling with aerated fluids can aid both in their location and preservation. The additional care needed to complete preservation. The additional care needed to complete wells in steam zones could be compensated by the following advantages.Well productivity will be enhanced due to the absence of two-phase flow in the reservoir.Production will be in the relatively shallow depth range of about 400 to 900 m.Effluent disposal problems will be minimized.The production life of the field will be extended.
The occurrence of vapor-dominated geothermal fields is well documented, and indeed, the primary classification of geothermal fields into water-vapor-dominated appears to be an accepted concept. Also, it is known, from the production history of the Wairakei field, that a vapor-dominated zone can be created artificially by the long-term withdrawal of fluids from a water-dominated field. The effect of this process on reservoir production characteristics have been discussed by Martin. The purpose of this present paper, however, is to describe a present paper, however, is to describe a water-dominated field in which a steam zone occurs under natural undisturbed conditions, to describe the conditions under which such steam zones occur, and to emphasize their roles as contributors to well productivity. productivity.

RECOGNITION OF STEAM ZONES
The presence of a steam zone occurring in a water-dominated field is difficult to detect with temperature and pressure logs alone, which are often the only logs obtained from geothermal wells drilled at remote exploration sites. Unless the hole is open only to the steam zone, which would be fortuitous without prior knowledge of its existence, water will enter the hole from the water-saturated part of the reservoir and effectively mask the presence of a steam zone. Pressure gradients in the well will be those of water, and well temperatures will be controlled in part by the hydrostatic gradient in the well. Even if part by the hydrostatic gradient in the well. Even if steam is found in the hole, it normally would be assumed to have originated in the well by flashing from the water phase.
It remains to be demonstrated whether or not more sophisticated logging techniques, similar to those designed to detect gas zones in petroleum reservoirs, also could detect steam zones. The effectiveness of such techniques is doubtful, however, because cooling the well in order to protect the electronic instrumentation in the logging tool may condense steam in the formation beyond the radius of detection.
In spite of the present inadequacies of geothermal well-logging techniques, a steam zone has been recognized during the exploratory drilling phase of the Olkaria field, located in the Rift Valley of Kenya. The presence of steam was first indicated by the results of flow tests conducted at several depths during the course of drilling, and the position of the zone was indicated by the behavior of the temperature gradient during heating.