FISHES: A New Numerical Code for Simulating Phase Separation and Multiphase Flow in Seafloor Hydrothermal Systems

K.C. Lewis

Corresponding author: geofleur@sbcglobal.net
University of Chicago, Department of Geophysical Sciences, IL, 60637

Abstract:
FISHES [Lewis and Lowell, 2009a] uses a finite control volume approach to solve equations governing conservation of mass, energy, momentum and salt in order to perform numerical simulations of multiphase flow in a NaClH2O fluid. The goal of this research is to model temperature and salinity variations in seafloor hydrothermal systems in a realistic manner. FISHES is currently implemented in 2D and operates over a PTX range of 8.5100 MPa, 0°800°C, and 0 – 100% salinity. The thermodynamic relations are expressed in terms of equations of state compiled from lookup tables. These tables are based on previously published formulations for the density and enthalpy of NaClH2O fluid in regions of phase space that are relevant to the study of seafloor hydrothermal systems. FISHES has been benchmarked satisfactorily against previously published results for the Elder problem, and for the problem of fluid extraction from a onedimensional, twophase horizontal pipe. FISHES results from a onedimensional vertical salt pipe simulation are found to agree with an analytic solution. At present, several examples of FISHES simulations have been performed to explore different boundary conditions and both homogeneous cellular and singlepass geometries. I am currently collaborating with Bob Lowell and others at Virginia Tech to develop models to explain the vent salinity patterns at the Main Endeavour Field on the Juan de Fuca Ridge. This task will call for upgrading FISHES with the capability to model 3D flows, creating tools to ease the process of generating and manipulating input files, supplying software to assist in visualization of threedimensional code output, and parallelizing the code to speed up computations. The current version of FISHES together with a User’s Manual is available at http://geosci.uchicago.edu/~kaylal/ . Both the code and User’s Manual will be upgraded periodically.

Contributions to Integration and Synthesis:
Modeling of seafloor hydrothermal systems is a key component of RIDGE 2000. FISHES is among the first numerical codes to adequately simulate multiphase flow in seafloor hydrothermal systems and implementations of this code are just beginning. In addition to the simulations for the Main Endeavour Field, I plan to use FISHES to simulate the EPR 9°50′ system. In addition to upgrading to 3D, FISHES is also being incorporated with an algorithm that determines the solubility of quartz as a function of PTX in order to understand the evolution of quartz in phase separating, saline fluids that are often encountered in ore formation processes. Work is also being undertaken to link FISHES with codes describing magma convection and replenishment. At present FISHES has not been linked with a full reactive transport code, but such a connection would represent a significant advance in modeling capabilities.

References:
Lewis, K. and R. Lowell (2009a), Numerical modeling of twophase flow in the NaClH2O system: Introduction of a numerical method and benchmarking, J. Geophys. Res., 114, B05202, doi: 10.1029/2008JB006029.
Lewis, K. and R. Lowell (2009b), Numerical modeling of twophase flow in the NaClH2O system: Examples, J. Geophys. Res. (in press).

Figures:
Figure 1. Simulations of a single-pass model showing the isotherms, fluid velocities (top row), and bulk salinities (bottom row) at 2, 5, and 10 years in the bottom corner of the cell, where phase separation occurs. The two-phase region at the bottom of the cell is outlined in white. The total cell is 1 km high by 2 km wide. The permeability is the 600 m wide recharge zone is 10-¹³ m² and that in the 100 m wide cross flow channel and 50 m wide discharge pipe is 10-¹² m². The central region of the cell has a permeability of 10-¹⁵ m². The pressure at the seafloor is assumed to be 25 MPa. [Lewis and Lowell, 2009b] Lewis_fig1.jpg