| Abstract |
Roosevelt Hot Springs (RHS), located near Milford, Utah, is a magmatic, structurally controlled hydrothermal system in the Basin and Range geological province. The Blundell geothermal power plant has been operating at this site for over 40 years, since 1984. Blundell produces more fluid from the reservoir than it reinjects, leading to a net decline in reservoir fluid and pressure. We hypothesize that this pressure drop is driving the growth of a subsurface steam cap. The purpose of this study is to quantitatively test that hypothesis via subsurface modeling, and to demonstrate how reservoir development has influenced formation and expansion of a subsurface steam cap. The model developed in this study is a regional, three-dimensional, two-phase representation of the hydrothermal reservoir and its surrounding area. The modeling process consists of two main components: native state modeling and production modeling. The purpose of the native state modeling is to establish pre-production conditions in the simulated reservoir. To achieve this, it was necessary to further divide the native state modeling into two stages of development: an initialization stage and an activation stage. These two stages represent a step change in reservoir permeability, indicative of the transient nature of the system permeability over geological timescales. The native state model then serves as initial condition for the production model, which simulates 40 years of reservoir development and the effects of production and injection wells. The primary conclusions of this modeling effort are 2-fold. The first key finding is that native state modeling requires a two-stage approach to replicate observed reservoir conditions. The intermediate permeability of the initialization stage enables high temperatures to be widely distributed at shallow depths. Meanwhile, the high reservoir permeability of the activation stage is essential to support geothermal production, and represents chemo-mechanical processes such as fault activation. Both stages are essential for developing the high-temperature, productive reservoir observed at RHS today, demonstrating the system's dynamic nature over geologic time. Thus, permeability changes must be considered when modeling hydrothermal systems over long timescales. Secondly, production modeling results reveal that the formation and expansion of a subsurface steam cap above the reservoir is in direct response to geothermal production activities, supporting our hypothesis. |