Numerical modeling of Basin and Range geothermal systems

Abstract

Basic qualitative relationships for extensional geothermal systems that include structure, heat input, and permeability distribution have been established using numerical models. Extensional geothermal systems, as described in this paper, rely on deep circulation of groundwater rather than on cooling igneous bodies for heat, and rely on extensional fracture systems to provide permeable upflow paths. A series of steady-state, two-dimensional simulation models is used to evaluate the effect of permeability and structural variations on an idealized, generic Basin and Range geothermal system of the western U.S. Extensional geothermal systems can only exist in a relatively narrow range of basement (bulk) permeability (10 −15 m 2 to 10 −16 m 2). Outside of this window, shallow subsurface fault zone temperatures decrease rapidly. Mineral self-sealing does not significantly affect the flow system until the flow path is almost completely sealed off. While topography gives an extra “kick” to convective circulation, it is not a requirement for geothermal system development. Flow from the ranges to the fault dominates the circulation, while secondary flow systems exist on the range front slopes. A permeable fault in one valley can also induce cross-range flow if there are no equally good upflow paths in the adjacent valleys. When bulk permeability is high enough, additional deep circulation cells develop in adjacent valleys, diverting heat and fluid from the fault and consequently reducing temperatures in the fault itself. Qualitative comparison between temperature–depth logs from actual geothermal systems and from the generic models is a significant aid to understanding real-world geothermal fluid flow, and suggests new or better interpretations of existing systems.