Emergent Processes - Thrust Area 3
The injection of CO2 into the subsurface environment drives the fluid-rock system into “far-from-equilibrium” conditions where new behavior may emerge that is not predictable by considering processes in isolation. Under these conditions, the strong coupling between flow, transport, and reaction may result in emergent structures and pattern formation that develop within the porous structure of the subsurface at scales ranging from the nano-pore and macro-pore all the way up to that of a sedimentary basin. These emergent dynamics must be understood in order to predict how CO2 will behave when injected into the subsurface.
The injection of CO2 into water-saturated sediments drives the system into “far-from-equilibrium” conditions in which the fluxes that return the system to equilibrium are nonlinearly related to the generalized forces driving them (e.g., chemical affinities and gradients in the fluid pressures and chemical potentials). Under these far from equilibrium conditions, a range of new multi-scale behavior may emerge, potentially leading to pattern formation and self-organization. These new and complex, scale-dependent phenomena often involve interactions between transport and reaction processes—the reactive infiltration instability, in which flow-driven solid phase dissolution increases porosity and permeability, thus providing a positive feedback to the flow, is a well-known example in the Earth Sciences. As the complex interdependent processes of flow, solute transport, colloid transport, mineral dissolution and precipitation play out within the mechanical framework of the porous medium, the emergent structures include precipitate structures in individual pores, precipitate structures correlated over many thousands to millions of pores, and immiscible fluid structures that exhibit fractal geometry over many orders of length scale ranging from grain scales to reservoir dimensions.
The nature of the final equilibrium states and how many years it takes the Earth to return to equilibrium after injection is far from known at this point. Key to making such predictions is having proper governing equations for the physics and chemistry of the processes at scales far greater than grain sizes. The porous-continuum laws are themselves emergent from the underlying continuum laws that hold in the fluids, solids and at interfaces, just as these classical continuum laws and boundary conditions are themselves emergent from the underlying molecular dynamics. By carefully understanding processes at both the nano and pore scales, the overall goal of this research is to bring such knowledge to bear on the macroscopic scale of the subsurface in which CO2 is injected.
To unravel the coupled dynamics at the pore and large scale, a new generation of experimental, imaging and modeling tools will be required. This thrust area will make considerable use of the rapidly developing field of 3D imaging techniques (X-ray, neutron, and magnetic resonance) that allow the characterization and observation of dynamic geological and hydrological processes on unprecedented temporal and spatial resolutions. X-ray computed tomography will be carried out at the Lawrence Berkeley National Laboratory Advanced Light Source (ALS), while Neutron Computed Tomography (NCT) and Small Angle Neutron Scattering (SANS) studies will be conducted at Oak Ridge National Laboratory. Complementary 2D imaging of CO2 invasion dynamics at the pores scale will be done using two-dimensional optically transparent pore networks called “micromodels”, which allow for sub-micron scale resolution imaging of menisci and pore scale capillary dynamics with full-field imaging at milli-second temporal resolution.
To complement and interpret the imaging of CO2 injection, a new set of largely pore scale modeling tools will be developed and applied. The focus will be on lattice Boltzmann techniques for modeling single and multiphase flow at the pore scale, both for the purpose of simulating the highly transient invasion physics as the CO2 moves through the pore structure and for simulating the coupled flow, transport, and chemical processes that can modify the physical and chemical characteristics of the pore structure itself. Lattice Boltzmann modeling will also be used to understand possible pore scale controls on the observed age dependence of subsurface chemical reactions. Direct numerical simulation of pore scale Navier-Stokes flow and reaction will also be employed.
Microscopic imaging and pore scale modeling will be largely in support of a set of carefully controlled experiments in which the injection of CO2 under single and multi-phase conditions will be monitored at multiple scales. One set of experiments will study the physics of CO2 through synthetic and natural porous materials. The differing effects of non-reactive and reactive gas phases will be analyzed. Another set of experiments will compare the coupled chemical and physical (flow and transport) effects of single and multi-phase CO2 injection conditions, with a focus on exactly how the reactions modify the pore structure of the materials. A particular focus will be to understand the role of secondary carbonate phases resulting from the injection of the CO2, building on the fundamental studies of carbonate precipitation and nucleation conducted as part of Thrust Areas 1 and 2. Such studies provide the basis for building macroscale models from microscopic scale observations.
The integration of microscopic imaging, pore-scale modeling, and carefully controlled experimentation will provide an unprecedented new understanding of how fundamental molecular-scale processes of flow, transport, and reaction play out at the pore scale. New insights into the pore scale dynamics that merge under the far from equilibrium conditions associated with the injection of CO2 will greatly improve our ability to develop scientifically defensible predictive models.
The Nanopore Processes Research Team performs laboratory experiments and molecular simulations on carbon dioxide-aqueous brine mixtures at high temperature and pressure in porous media relevant to geological sequestration. The principal research tasks to be undertaken by the team during the first year or two of the Center's operation are identified in a set of downloadable slides.