2:04 PM - 2:23 PM
[SVC30-02] A tool for constructing three-dimensional permeability structures for numerical modeling of volcanic hydrothermal systems
Keywords:hydrothermal system, numerical simulation, resistivity structure
Numerical simulation is a useful method for studying the complex behavior of volcanic hydrothermal systems; however, most of the previous modeling studies have assumed a relatively simple permeability structure and have rarely considered the heterogeneity of actual volcanic bodies except for a few pioneering studies. This is because it is often difficult in practice to estimate the subsurface distribution of permeability.
For many volcanoes where borehole data are scarce, except for some geothermal areas, one practical solution to this problem is to estimate the permeability structure on the basis of geophysical exploration results. In particular, electrical resistivity, like permeability, is a geophysical quantity that strongly depends on the presence of fluids and their connectivity in rocks, and therefore, the resistivity structure estimated by electromagnetic soundings could be directly linked to the permeability structure for numerical modeling. It is expected that a comprehensive scheme for conversion will be established in the future. For this purpose, it is necessary to convert the resistivity structure to permeability structure as systematically as possible, and then perform numerical modeling using actual observation data obtained in many volcanoes as a constraint to clarify the general correspondence between the two physical quantities.
In order to provide a computing environment that can perform a large number of simulations under various conditions, we have developed a new GUI/command line tool (WEAK3) that streamlines the multiple tasks required for simulation. WEAK3 includes the generation of permeability structure from resistivity structure, execution of numerical simulations, compilation and visualization of the results, and so on. The tool is intended for use with TOUGH2 and TOUGH3, the widely used multi-component, multi-phase simulators in the field of geothermal studies, and can convert not only the constructed permeability structure data but also various settings about simulation parameters into an input file format for the TOUGH simulators. One of the major features of WEAK3 is that it creates a permeability structure by assigning several predefined rock types to a computational mesh based on conditional expressions that are described by the position and resistivity of each element. The advantage of this approach is that it is easy to create a permeability structure that incorporates information on the subsurface structure, because only a few parameters are required to define the mesh, including the resistivity values defining the boundaries of the rock types.
As a case study, we attempted numerical simulations to obtain a steady-state model of the hydrothermal system of Kusatsu-Shirane Volcano. The permeability structure was constructed based on the resistivity structure estimated in previous studies and their interpretations. The low permeability regions such as basement rocks, clay layers, and sealing layers associated with silica precipitation were assigned to the mesh based on the resistivity values. Volcanic fluids with temperatures and compositions determined based on various observation data were injected from the bottom of a highly permeable conduit which was designed to connect the deep inflation source and the summit area. The simulations of the hydrothermal fluid flow were performed for about 20,000 years, corresponding to the latest magmatically active period. The results reproduced the original resistivity structure and the actual distribution of hot springs around Kusatsu-Shirane Volcano comparatively well. Although the constructed permeability structure model was relatively simple, it reproduced some of the observed data, indicating that the use of the resistivity structure can significantly reduce the uncertainty in the construction of the permeability structure. This suggests the feasibility of integrated analysis combining electromagnetic explorations and hydrothermal simulations.
For many volcanoes where borehole data are scarce, except for some geothermal areas, one practical solution to this problem is to estimate the permeability structure on the basis of geophysical exploration results. In particular, electrical resistivity, like permeability, is a geophysical quantity that strongly depends on the presence of fluids and their connectivity in rocks, and therefore, the resistivity structure estimated by electromagnetic soundings could be directly linked to the permeability structure for numerical modeling. It is expected that a comprehensive scheme for conversion will be established in the future. For this purpose, it is necessary to convert the resistivity structure to permeability structure as systematically as possible, and then perform numerical modeling using actual observation data obtained in many volcanoes as a constraint to clarify the general correspondence between the two physical quantities.
In order to provide a computing environment that can perform a large number of simulations under various conditions, we have developed a new GUI/command line tool (WEAK3) that streamlines the multiple tasks required for simulation. WEAK3 includes the generation of permeability structure from resistivity structure, execution of numerical simulations, compilation and visualization of the results, and so on. The tool is intended for use with TOUGH2 and TOUGH3, the widely used multi-component, multi-phase simulators in the field of geothermal studies, and can convert not only the constructed permeability structure data but also various settings about simulation parameters into an input file format for the TOUGH simulators. One of the major features of WEAK3 is that it creates a permeability structure by assigning several predefined rock types to a computational mesh based on conditional expressions that are described by the position and resistivity of each element. The advantage of this approach is that it is easy to create a permeability structure that incorporates information on the subsurface structure, because only a few parameters are required to define the mesh, including the resistivity values defining the boundaries of the rock types.
As a case study, we attempted numerical simulations to obtain a steady-state model of the hydrothermal system of Kusatsu-Shirane Volcano. The permeability structure was constructed based on the resistivity structure estimated in previous studies and their interpretations. The low permeability regions such as basement rocks, clay layers, and sealing layers associated with silica precipitation were assigned to the mesh based on the resistivity values. Volcanic fluids with temperatures and compositions determined based on various observation data were injected from the bottom of a highly permeable conduit which was designed to connect the deep inflation source and the summit area. The simulations of the hydrothermal fluid flow were performed for about 20,000 years, corresponding to the latest magmatically active period. The results reproduced the original resistivity structure and the actual distribution of hot springs around Kusatsu-Shirane Volcano comparatively well. Although the constructed permeability structure model was relatively simple, it reproduced some of the observed data, indicating that the use of the resistivity structure can significantly reduce the uncertainty in the construction of the permeability structure. This suggests the feasibility of integrated analysis combining electromagnetic explorations and hydrothermal simulations.