The Development of A Higher-Order Finite Element Multiphase Reactive Transport Model For Unstructured And Fractured Grids - Modeling of Soluble Gas Components

This work presents a continuing effort to develop a higher-order finite element (FE) reactive transport model for unstructured and fractured grids [1]. The research focuses on the interdependence of soluble gas components (e.g., carbon dioxide, CO2, and light hydrocarbons such as methane) and aqueous-rock geochemistry in multi-phase reactive transport. Osures is an advanced research simulator for flow and transport in subsurface fractured porous media. It models fluid flow with the Mixed Hybrid FE method, which provides smooth velocity fields even in highly heterogenous formations with discrete fractures. Multicomponent species transport is solved by a Discontinuous Galerkin FE method. This higher-order method not only offers strict local mass conservation, but also significantly reduces numerical dispersion. To compute the compositions, compressibility, and other thermodynamic properties of all fluid phases, Osures adopts the Cubic Plus Association (CPA) equation of state, which is an improvement over cubic relations such as Peng-Robinson. As the geochemistry engine, we leverage the well-established PHREEQC package developed by the US Geological Survey, which provides a large library of equilibrium and kinetic aqueous and fluid-rock reactions. We implement both the older iPhreeqc and the more efficient PhreeqcRM interfaces in a sequential iterative operator splitting scheme. Our proposed higher-order FE reactive transport model can simulate geochemical reactions in the aqueous phase including cation exchange, electrochemical migration, etc. from lab to field scales and allowing from highly heterogeneous and fractured formations [2].

A number of numerical experiments are presented to validate the accuracy and robustness of this new model, including comparisons with other reactive transport codes such as TOUGH2 [3]. Advanced capabilities of the higher-order FE approach are demonstrated on more complex geometries with discrete fractures, discretized by unstructured 2D and 3D grids, as well as in relation to the large volume CO2 injection pilot project near Cranfield, Mississippi [4].

References

[1] Moortgat, Joachim, Amin, Amooie, and Di, Zhu. "A Higher-Order Finite Element Reactive Transport Model for Unstructured and Fractured Grids - Benchmark Studies, Electrochemical Migration, and Applications to CO2 Sequestration." AGU Fall Meeting 2019. AGU, 2019.

[2] Moortgat, Joachim, Li, Mengnan, Amin, Amooie, and Di, Zhu. "A Higher-Order Finite Element Reactive Transport Model for Unstructured and Fractured Grids - Benchmark Studies, Electrochemical Migration, and Applications to CO2 Sequestration." 2020 (under review).

[3] Pruess, Karsten, et al. "Code intercomparison builds confidence in numerical simulation models for geologic disposal of CO2." Energy 29.9-10 (2004): 1431-1444.

[4] Soltanian, Mohamad Reza, et al. "Simulating the Cranfield geological carbon sequestration project with high-resolution static models and an accurate equation of state." International Journal of Greenhouse Gas Control 54 (2016): 282-296.