Decompression boiling and natural steam cap formation in high-enthalpy geothermal systems

Abstract

Many high-enthalpy geothermal systems exhibit vapor-rich boiling zones at shallow depths (<1 km), commonly known as steam caps. While the spatial extent and vapor content of steam caps often increases in response to fluid extraction and subsequent pressure decline, the geologic factors controlling steam cap formation and development are poorly understood. Numerical simulations of groundwater convection driven by a transiently cooling intrusion elucidate a natural mechanism by which steam caps can develop from initially liquid-dominated boiling zones. In the simulations, intensive decompression boiling and steam cap development occurs after the heat of a subsurface magmatic intrusion is exhausted by prolonged hydrothermal convection. A reduction in the strength of the upflow leads to a fluid pressure decrease of 2–4 MPa in boiling zones beneath an impermeable cap rock. As the vertical pressure gradient decreases, approaching vapor-static, zones of liquid-vapor counterflow (i.e., heat pipes) develop at the base of steam caps, efficiently isolating overlying vapor-rich zones from underlying liquid. The reservoir rock permeability and the cap rock thickness are main controls on the enthalpy and spatial extent of steam caps, with thicker and higher enthalpy (potentially superheated) steam caps developing in intermediate permeability reservoir rocks (∼ 10−15 m2). Due to the importance of transient changes in geothermal system structure to steam cap development, the dynamics of natural steam cap formation may not be captured in standard approaches to natural state reservoir modeling, which express the role of the heat source in terms of fixed boundary conditions.