A Fully Coupled Model of Nonisothermal Multiphase Flow, Solute Transport and Reactive Chemistry in Porous Media

Over the past decades, geochemical reaction has been identified through experiments in different processes, e.g. the CO2 EOR process, the CO2 sequestration, the enhanced geothermal system. Research has gradually led to the recognition that chemical reactions between injected fluid and mineral rock have significant impacts on fluid dynamics and rock properties in these processes. However, for the majority of the reactive transport simulators, the sequential calculation processes of fluid flow, solute transport, and reactive geochemistry result in numerical instability and computation efficiency problems. In this paper, we present a fully coupled computational framework to simulate reactive solute transport in porous media for mixtures having an arbitrary number of phases. The framework is designed to keep a unified computational structure for different physical processes. This fully coupled simulator focuses on: (1) the fluid flow, solute transport, and chemical reactions within a threephase mixture, (2) physically and chemically heterogeneous porous and fractured rocks, (3) the non-isothermal effect on fluid properties and reaction processes, and (4) the kinetics of fluid-rock and gas-rock interactions. In addition, a system of partial differential equations is formed to represent the physical and chemical processes of reactive solute transport. A flexible approach of integral finite difference is employed to to obtain the residuals of the equation system. Jacobin matrix for NewtonRaphson iteration is generated by numerical calculation, which helps the future parallelization of the fully coupled simulator. Finally, the fully coupled model is validated using the TOUGHREACT simulator. Examples with practical interests will be discussed, including CO2 flooding in a reservoir, supercritical CO2 injection into a saline aquifer, and cold water injection into a natural geothermal reservoir. This type of simulation is very important for modeling of physical processes, especially for CO2 EOR and storage, and geothermal resources development. Inroduction Reactive fluid flow and geochemical species transport that occur in subsurface reservoirs have been of increasing interest to researchers in the subjects of CO2 geological sequestration, CO2 EOR process, enhanced geothermal system, or even waterflooding and other EOR processes. The chemical reaction path has been observed in these processes when subjected to fluid injection in the subsurface reservoir. The nonisothermal reactive solute transport phenomena involed in these processes are thermal-hydrological-chemical (THC) processes. However, the reaction paths may be slightly different due to the different fluid flow mechanisms related to these processes. CO2 geological sequestration and CO2 EOR are two effective solutions to store CO2 from burning fossil fuels in geological formations and petroleum reservoirs. Saline aquifers and petroleum reservoirs have the largest capacity among the many options for long-term geological sequestration. They are large underground formations saturated with brine water or hydrcarbons, and are often rich in dissolved minerals. CO2 is injected into these formations as a supercritical fluid with a liquid-like density and a gas-like viscosity. It is believed that geochemical reaction between CO2 and rock minerals in the aqueous–based system dominates the long-term fate of CO2 sequestrated in geological formations. Two types of geochemical reactions between CO2 and rock minerals have been identified by experiments, i.e. reactions between dissolved CO2 and rock minerals, and reactions between supercritical CO2 and rock minerals. The chemical mechanism between dissolved CO2 and rock mineral has been well understood. The acid H2CO3 is formed by the dissolution of CO2 in an aqueous solution, and it dissociates in the brine to release H + . The carbonate minerals are dissolved into the aqueous phase under this weak acid