Modeling multiphase flow of H2O-NaCl fluids in volcano-hydrothermal systems by combining CSP5.0 with SoWat2.0

Following up on our earlier developments, we have recently improved (a) our model of the system H2O- NaCl to describe accurately the phase relations [1], volumetric properties, enthalpies and heat capacities [2] now to 1000°C, 500 MPa, and 0 to 100% NaCl, and (b) algorithms for multiphase fluid flow in this system. The fluid properties are formulated entirely in terms of temperature, pressure, and composition, and have been implemented as the C++ library SoWat2.0 (for Sodium chloride Water). The new flow algorithms modify our earlier decoupled temperature-pressure scheme [3,4,5] towards an enthalpy-pressure-salinity based approach. Using a generalized finite element finite volume scheme for hybrid finite element meshes, transport can now be simulated in 3D. It is also possible to include capillary effects. Parallelization using a domain-partitioning approach can alleviate the large CPU requirements for high-resolution 2D and 3D simulations. Enthalpy conservation and correct distribution of enthalpy between multiple fluid phases and rock is obtained in the isobaric step by a simple Newton iteration. The iteration is carried out by using novel expressions of the apparent heat capacities of all possible fluid mixtures in the systems. The algorithms have been incorporated as C++ modules into our simulation platform CSP5.0. During the iteration, calls to the SoWat module ensure that the phase state is detected correctly and the respective apparent heat capacity is chosen automatically in each iteration step. In this way, the iteration can be carried out correctly even across phase boundaries at moderate computational costs. The combination of geometric flexibility, accurate numerical solution, precise calculation of thermodynamic properties, and parallelization provides an efficient tool to simulate complex thermohaline multiphase flow occurring in oceanic and continental hydrothermal systems. [1] Driesner T and Heinrich CA, Geochimica et Cosmochimica Acta, in press. [2] Driesner T, Geochimica et Cosmochimica Acta, in revision. [3] Geiger et al., Journal of Geophysical Research, 110, B07101, 2005. [4] Geiger et al., Transport in Porous Media, 63, 399-434, 2006. [5] Geiger et al., Transport in Porous Media, 63, 435-461, 2006.