Basic Research Needs for Geosciences - Energetics Meetings and ...

PANEL REPORT: MULTIPHASE FLUID TRANSPORT IN GEOLOGIC MEDIAdeep offshore marine sediments, then the challenges are magnified because geomechanicalresponses of these systems is a largely unexplored topic. Understanding of soft-sedimentdeformation requires interrogation of fundamental questions regarding coupled processesaffecting pore pressure increases and resultant deformation of the sediments, and how these willaffect fluid displacement. Indeed, the rapid and large-scale injection of a fluid like CO 2 into verysoft sediments may be more akin to igneous intrusion than multiphase flow in porous media.A variety of research approaches will be needed to address these challenges, including fieldstudies of geomechanics in a variety of geologic settings, laboratory experiments to constrainrheological behavior of fluid-rock systems, discrete or continuum modeling of deformation, anddevelopment of novel theoretical frameworks where current approaches are inadequate.Coupled pressure, stress, strain, and flow responses in multiphase fluid-rock systemsA multiplicity of deformation mechanisms at different scales may be activated by theanthropogenic perturbations in this geologic regime. In porous clastic and carbonate rocks, grainscaledeformation mechanisms such as microcracking, pore collapse, pressure solution,crystalline plasticity, and aperture dilation and contraction may be operative. The properties offractures must also be understood across a range of scales, and must include both physical andchemical impacts. These include responses to effective stress, as well as feedbacks to the flowfield and associated changes in fluid composition, and therefore geochemical reactions that canresult due to changes in fluid flow. All of these processes can dynamically alter the alreadycomplex hierarchical architecture of fracture networks (e.g., Caine et al. 1996). The interplay ofthese factors controls whether a joint or fault system would provide conduits or barriers for thetransport of a multiphase fluid in the shallow crust and the potential for fault reactivation. Theseare critical considerations for problems involving large-scale pressure perturbations, such as theCO 2 injection system. For injection into deep seabed sediments, a quite different approach mustbe developed, where very large deformations of the sediments may occur. The possibleformation of CO 2 hydrates in these systems, which introduces an additional solid phase into thesystem, would lead to further complexities and represents a largely unexplored area of research.Coupled geochemical and geomechanical responsesMineral dissolution or precipitation can modify rock strength and pore structure. In turn, changesin stress and pore structure can modify the delivery of fresh reactants and removal of reactionproducts, influencing geochemical reaction rates and products. Over long time periods theselocal-scale coupled processes may impact large-scale fluid dynamics and deformation, creating acomplex interplay between basin-wide processes and local-scale coupled geomechanical andgeochemical processes. Chemical and hydrologic gradients in the fluid-rock system cansignificantly impact the kinetics of processes such as pressure solution, subcritical crack growth,and solution-precipitation. The interplay between space and time scales also enters into thesemixed physical-chemical reactions, requiring new approaches to study the comprehensive systemresponse across all of the relevant spatial and temporal scales.Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 17