| Abstract |
There is evidence suggesting that fluids in the root zone of many hydrothermal systems (at the brittle-ductile transition zone, about 3-4 km) can be highly saline. Therefore, fluid (state) can be expected to be in subcritical conditions and intensive multi-phase convection may occur at those depths. The particular type of fluid-solid interaction and as well as the fluid state depends on the fluid and rock compositions. This can be rather complicated involving several liquid and gaseous phases. As the simplest illustrative example, we consider the NaCl-H2O system assuming the rock matrix to be chemically inert. When limited to three phases (NaCl-L-V), the critical curves and the pressures of the liquid-gas equilibrium in the system seem to be too low when compared with expected parameters of the root zone of the geothermal systems (T=380-450oC). The fluid pressure is not well constrained and can be lower than hydrostatic due to active deformations that can increase rock porosity (fracture). In this work we examine the model for a one-dimensional steady-state, two-phase gravity flow. The work involves derivation of the steady-state solution for mass balance equations for all components and fluid flow equations for the gas and liquid phases. Results show that in a three phase set (salt-water-gas), the plot of the steady-state liquid volume fraction, e, vs. liquid temperature in the convecting column bifurcates into two sets of closed curves. Separation occurs because the maximum of the P3b(T) curve divides the entire P-e plane into domains of solution-like and hydrous NaCl melt liquid phases. For the salt-unsaturated two-phase equilibrium, dependence of the steady-state liquid volume fraction describes a folded surface in the P-T-e space. An estimate of the upper level of heat flux that can be transported through the salt loaded heat pipe yields a maximum of 10-15 W/m2 at a permeability coefficient value, k, of about 10-15 m2. Due to difference in the solubility of salt (or silicate materials) in the liquid and gas phases, heat transfer in salt loaded heat pipes is intrinsically connected to mass transfer. We suggest that the multi-phase convection of aggressive brines at high P-T can cause deep karst in the zone of dissolution. This process can be facilitated by hydrolysis with the acid that separates as a gaseous phase upon boiling. Assuming a solubility level of about 10 wt % and a permeability of 10-15 m2 yields a net dissolution rate of 2-3 cm/yr. This is close to the subsidence rates measured in some geothermal fields. Mechanical deformation in the system may also play an important role in developing fluid pathways through continuously sealed areas separating geothermal fluid reservoirs in the brittle and ductile zones. . |