Pore Scale Processes Associated with Subsurface CO2 Injection and Sequestration

Abstract

The injection of CO2 into the Earth’s subsurface drives the fluid-rock system into “far-from- equilibrium” conditions, which means that the fluxes that return the system to equilibrium are nonlinearly related to the generalized driving forces (e.g., chemical affinities and gradients in the fluid pressures and chemical potentials). The nonlinear response results in emergent structures and self-organization (Prigogine 1968, 1980; Ortoleva 1994; Lasaga 1998; Jamtveit and Meakin 1999), of which the reactive infiltration instability is a well-known example in the Earth Sciences that is directly applicable to CO2 injection and caprock integrity (Ortoleva et al. 1987; Hoefner and Fogler 1988; Steefel and Lasaga 1994; Steefel and Maher 2009). Flow, solute transport, colloid transport, mineral dissolution and mineral precipitation combine within the mechanical framework of the porous medium to generate precipitate structures in individual pores correlated over many thousands to millions of pores, and immiscible fluid structures with fractal geometry over scales from millimeters to kilometers (Feder 1988). The focus in this chapter is on processes, emergent and otherwise, taking place at the pore scale, defined here as the scale where individual grains and fluid interfaces can be resolved. The chief focus of research in recent years has been to improve the understanding of the nonlinear dynamics specifically associated with biogeochemical and microbially induced processes coupled to physical processes such as flow and diffusion at the pore scale. When coupled, whether in field-scale applications, bench-scale experiments, or in models, emergent behavior may result, including changes in permeability, diffusivity, and reactivity. The pore scale is an important scale to analyze these emergent behaviors given that it provides the architecture within which the smaller scale (nano and molecular scale) processes occur. For the individual processes to interact, transport is typically necessary and the resulting behavior …