Abstract
Geologic carbon storage (GCS) is a process where CO2 emissions from power plants and other point source emitters are injected deep into the subsurface to avoid their release into the atmosphere, where they contribute to climate change. Within the porous rock formations of the deep subsurface, CO2 will displace connate brines creating complex multiphase flow conditions that are impacted by rock characteristics. The connections between the interfacial characteristics of rock surfaces and fluid flow are poorly understood. Using a combination of experimental and modeling approaches, this work explores these connections at the pore (mm) to core (m) scale to provide new insight about fluid trapping and migration at the reservoir (km) scale where the security and efficacy of these injections will be determined.
Small-scale micromodel experiments were carried out to study the effect of nanoscale textures on multiphase flow relative to other factors such as interfacial tension and the wettability of the solid. Glass capillaries were used with their surface wettability or roughness modified. Micromodel experiments were conducted at ambient pressure using to fluid pairs (water/Fluorinert and water/Dodecane) to represent a variety of fluid properties. A modified 2k factorial experimental design was used to test the effect of independent variables on interfacial dynamics and flow. The results suggest that surface roughness and ionic strength have an important impact on multiphase flow and that together they impact flow dynamics more significantly than any other factor. Analysis over interface velocity deviation suggests that surface roughness, wettability and the presence of a water film contribute to over 70% of the variation, which in a more complex porous media, could alter flow directions and capillary pressure.
Intermediate scale column experiments were carried out using a 1-meter-tall high-pressure column packed with layered sand with different properties (e.g., grain size, wettability), to represent a low-contrast stratigraphic horizon. CO2 in supercritical or liquid phase was injected into the bottom of the column at a range of temperatures, pressures, and capillary numbers and the transport of the resulting plume was recorded using electrical resistivity. The effects were measured by the saturation profile at steady state and the relationship between residual and initial saturations. The results show that stratigraphic and residual trapping of CO2 were most strongly impacted by shifting the wettability balance to mixed wet conditions, with lasting impacts on saturation. A 16% increase in the cosine of the contact angle for a mixed wet sand resulted in nearly twice as much residual trapping. Permeability contrast, pressure, and temperature also impacted the residual saturation but to a lesser extent. Flow rate affected the dynamics of saturation profile development, but the effect is transient, suggesting that the other effects observed here could apply to a broad range of leakage conditions.
To understand the impacts of these studies at the largest scales, the results were combined with new data on wettability and pore structure to inform a model of fluid transport. High-pressure experiments were carried out, to investigate a variety of specific wettability and roughness trends that we observed in experiments supporting previous work and considers the aggregate impact they would have on fluid fate and transport in heterogeneous porous media. Capillary pressure and residual saturation curves were synthesized using a combination of previously published results and new experimental data to input into two-phase flow simulations using the TOUGH2 suite of codes to understand how these interfacial phenomena influence fate and transport at the largest scales.
