ABSTRACT
Partial differential equations describingmultiscale, multicomponent, multiphase reactive flowand transport equations are developed and applied to CO2 sequestration and geothermal energy recovery from an EGS (Enhanced Geothermal System) facility using the massively parallel computer code PFLOTRAN. In this formulation mass and energy conservation equations which determine the stable phase assemblage and state variables pressure, temperature, and phase saturation, and velocity are sequentially coupled to multicomponent reactive transport equations that account for aqueous complexing, precipitation and dissolution of minerals, and sorption. Multiscale processes, ubiquitous in porous media and essential to account for the bimodal distribution in material properties characteristic of fractured rock, are included through a multiple continuum approach. However, for this approach to be of practical use to describe such processes numerically, a computationally efficient numerical algorithm is needed. Such an approach is implemented in PFLOTRAN following previous work which allows rigorous decoupling of the finite volume discretized equations for primary (fracture) and secondary (matrix) continua. This approach results in a so-called embarrassingly parallel implementation for the secondary continuum calculations in which there is no communication between processors. Two applications are considered involving CO2 sequestration and modeling an EGS facility. The effects of convective instability on CO2 sequestration are investigated for a highly permeable, deep sandstone aquifer with calcite cement. The simulations take into account density variations caused by CO2 dissolving into the formation fluid and chemical interaction of the acidified fluid with calcite and silicate minerals. Single and multiple interacting continuum models are applied to an EGS facility to model heat extraction from fractured hot rock using both H2O and CO2 as working fluids. Parallel scalability is investigated for 3DEGS simulations with different grid resolutions. Results are compared using the single and dual continuum formulations for temperature and net power production, and the sustainability of an EGS facility is explored.
