Escript-RT: Reactive transport simulation in Python using escript

Abstract

We present escript-RT, a new reactive transport simulation code for fully saturated porous media which is based on a finite element method (FEM) combined with three other components: (i) a Gibbs minimisation solver for equilibrium modelling of fluid–rock interactions, (ii) an equation of state for pure water to calculate fluid properties and (iii) a thermodynamically consistent material database to determine rocks' material properties. Using decoupling of most of the standard governing equations, this code solves sequentially for temperature, pressure, mass transport and chemical equilibrium. In contrast, pressure and Darcy flow velocities are solved as a coupled system. The reactive transport itself is performed using the masses of chemical elements instead of chemical species. In such way it requires less computing memory and time than the majority of other packages. The code is based on escript, a parallelised platform which supports efficient stepwise simulation of realistic geodynamic scenarios at multiple scales. It is particularly suitable to analyse hydrothermal systems involving geometrically complex geological structures with strong permeability contrasts and subject to complex fluid–rock chemical interactions. The modular architecture of the code and its high level Python interface also provide flexibility for modellers who can easily modify or add new feedbacks between the different physical processes. In addition, the implemented abstract user interface allows geologists to run the code without knowledge of the underlying numerical implementation. As an example we show the simulation of hydrothermal gold precipitation in a granite–greenstone geological sequence, which illustrates the important coupling between thermal response and mass transfer to the localisation of gold.