Numerical modeling of the evolution of two-phase zones under a fractured caprock Original Research Article

The conditions necessary for the evolution of two-phase zones in a geothermal reservoir located below a caprock that presents a permeable fracture were investigated. The two-dimensional reservoir model used in this numerical simulation study was based on data from the Ginyu reservoir of the Kirishima geothermal field, Japan. The upper part of the reservoir is characterized by its vapor-static pressure distribution, and by the leakage of fluids to the surface. The reservoir can be represented as a two-phase permeable zone overlain by a fractured caprock. If the fracture in the caprock is highly transmissive, two-phase conditions may be found in the reservoir (a function of the rate of deep fluid recharge) but not necessarily a vapor-static pressure profile. If the fracture transmissivity and the rate of deep recharge are small enough, a vapor-static two-phase zone can develop.