Stability of Two Phase Zones Below Fissured Caprock

Abstract

Many geothermal reservoirs contain two-phase (waterfsteam) flow regions under natural-state conditions. Vapor-dominated resetvoirs are characterized by the predominance of such regions (and occasionally dry steam zones), but even liquid-dominated reservoirs frequently contain two-phase regions which overlie part or all of the liquid filled volume below. Production-induced reservoir pressure decline will cause the two-phage flow region to grow. Owing o compressibility effects, the response characteristics of two-phase regions and of liquid zones are very different. Consequently, it is important to properly understand the development and flow characteristics of reservoirs containing two-phase zones. The purpose of this paper is to study the conditions required for the development of a two- phase zone in a particular Japanese geothermal reservoir.