Geological controls on supercritical fluid resources in volcanic geothermal systems

Abstract:

Large-scale fluid convection in conventional volcanic geothermal systems is driven by the hydrothermal cooling of shallow intrusions. Recently, there has been increased interest in tapping supercritical fluid resources in volcanic geothermal systems, since such fluid reservoirs could provide a roughly order-of-magnitude greater potential for electricity production than conventional geothermal wells drilled to temperatures of 250-300 °C. The potential of supercritical geothermal reservoirs was demonstrated in 2010, when the Iceland Deep Drilling Project (IDDP) drilled into liquid magma at 2 km depth and encountered an overlying permeable, high-temperature (~450 °C) fluid reservoir capable of more than ~30 MWe of electricity production. However, a conceptual model describing the main factors governing the extent and structure of target reservoirs has remained elusive. Here, we present the first systematic investigation of the role of rock permeability, the brittle-ductile transition temperature, and the depth of magma chamber emplacement on the development of supercritical fluid reservoirs. We use the numerical modeling code CSMP++ to model two-phase flow of compressible water around an initially elliptical, 900 °C intrusion. Our models indicate that potentially exploitable supercritical fluid resources are an integral part of many magma-driven geothermal systems. Hotter and more extensive reservoirs are promoted by a brittle-ductile transition temperature higher than ~400 °C, an intrusion depth less than 3 km, and a host rock permeability of 10^-14 to 10^-15 m². The systematic dependence of the size, location and hydrologic behavior of supercritical reservoirs on these factors aids the development of exploration models for different volcanic settings. In addition, by serving as the main agents of heat transfer at the interface of an intrusion and the overlying hydrothermal system, supercritical fluid reservoirs play a decisive role in determining the overall thermal histories of shallow intrusions and in shaping the overall character of hydrothermal systems.