When stress is applied to porous media, part of the stress is transmitted through the pore fluid and part of the stress is transmitted through the solid skeleton. Effective stress—the fraction of the total stress that is transmitted through the solid skeleton—controls the mechanical behavior of porous media, from land subsidence due to groundwater pumping to the cohesion of sand in sandcastles. The effective stress concept was introduced a century ago by Karl Terzaghi, the father of soil mechanics. However, it has remained until now inaccessible to direct measurement. We developed the novel manufacturing of millimeter-scale photoelastic spheres that brighten and change color when they are subjected to stress. With these particles, we can visualize and quantify, for the first time, the grain-scale stress state in fluid-filled granular media, and thus measure the macroscale evolution of effective stress. We call this methodology photoporomechanics.

Transport-controlled reactions

Reaction rates are usually measured in the laboratory with a well-mixed condition induced by a stirrer or rotating disk. These rates are often several orders of magnitude higher than those occurring in nature, based on field measurements. The laboratory-field rate discrepancy has been a longstanding topic in the field of geochemistry. One of the factors that were ignored is the limiting effect of transport (advection and diffusion). We improved upon the existing theories to study the evolution of transport-controlled dissolution rate as the conduits (holes and fractures) enlarge. We found that the transport-controlled dissolution rate stays constant in an enlarging hole, while decreases in an enlarging fracture. Here is more on transport-controlled dissolution.

Dissolution kinetics in geosystems

We developed an effluent chemistry monitoring system (ECMS) that is integrated into the top end cap of a triaxial system. This system measures the electric conductivity and temperature of the outflow from the specimen (effluent) during flow tests. This avoids the time-consuming and costly process of effluent sampling. It also eliminates the errors caused by human handling and provides high time-resolution continuous measurement. This system is calibrated to directly monitor the evolution of overall reaction rates in the solid-fluid system under triaxial stress conditions. Here is more on ECMS.

3-D wormhole formation

Underground fluid flow and chemical reactions often result in wormholes, which are channels that look like tree roots. They significantly increase the permeability of the porous media by creating highways for the flow. Wormhole formation is relevant in many natural and industrial processes, including the formation of underground caves, CO2 sequestration and enhanced oil recovery. Our experimental study on wormholes in 3D porous media found a power-law scaling of wormhole lengths with a greater exponent than those in 2D cases. We also improved upon existing models to better predict the relation between wormhole length and permeability. Here is more on wormhole formations.

Discrete fracture network modeling

Enhanced geothermal systems rely on fracture networks to conduct the working fluid and heat to the surface. We developed a heat transfer model for GEOFRAC, a large-scale 3-D geology-based stochastic discrete fracture network model. With these models, we conducted a case study on a geothermal field in Iceland and found a good match between the prediction and field data. This discrete fracture network model also provides a framework for upscaling the subsurface thermo-hydro-chemical models to field scale.