4:45 PM - 5:00 PM
[SVC42-12] Hydrothermal fluid flow simulation in a seafloor vent area for metal resource estimation
Keywords:seafloor hydrothermal deposit, Iheya North Knoll, TOUGH2, heat flux, temperature
TOUGH2 can analyze gas-liquid two-phase flow and three-dimensional heat flow. The size of study area is 1.2 km from north to south, 4 km from east to west, and 2 km along the vertical direction. The seafloor was set as the top boundary where temperature and pressure were fixed at 4 ℃ and hydrostatic condition, respectively. As the initial condition, we set the hydrostatic pressure and 4 ℃ at the surface with thermal gradient 0.12 ℃/m, which is the average gradient in this area except for the vent sites, and physical rock properties by referring to the field survey data. The values of discharge rate, mass in rate, and permeability (hereinafter called k) were adjusted appropriately through trial and error. To realize the vertical flow of hydrothermal fluid from the deep part, a volcano conduit with k= 10-13 m² was placed along the vertical direction from bottom to surface. Additionally, cap rock with k = 10-16 m² were distributed near the surface to enable lateral hydrothermal fluid flow. Anisotropic k of the volcanic basement was set so that horizontal k was one order larger than vertical k, by considering a fact that thin impermeable layers were observed several times in the volcanic basement in the surveys. Under those conditions, we implemented the natural state simulation.
Through the simulation, we obtained a plausible hydrothermal fluid flow pattern and compared the calculated heat flux and temperature with the measured values. As a result, the calculated heat fluxes generally corresponded with the measured values, even around the discharge zone in which the flows tend to be complicated. As for the temperature profile at one drilling site, calculated temperatures were also consistent with the measured values in general except for the high temperature at 50mbsf. However, at a drilling site apart from the vent, calculations were about 30℃ higher than measured values. This overestimation is caused probably by that the recharge from the surface could not be expressed well in the present model.
The effectiveness of the present model was verified by the following two points. By a model without the volcano conduit, the hydrothermal fluid did not ascend to the seafloor along the vertical direction. Accordingly, the tendency of measured temperature and heat flux could not be reproduced. Furthermore, by a model with isotropic k of the volcanic basement with k = 10-15 m², lateral hydrothermal fluid flow could not be expressed and trends of temperature and heat flux were far away from the measured values. Consequently, the location and physical properties of the volcano conduit and the permeability of volcanic basement were clarified as significant factors controlling the accuracy of hydrothermal fluid flow simulation.