Keith B. Neeves, PhD, Colorado School of Mines
The scientific objective of this proposal is to conduct a fundamental study on the impact of nanoscale features on fluid flow and transport. Specifically, we are measuring how nanoscale features affect fluid and solute transport in dual porosity and random media. Our approach uses nanofabricated porous media models to measure and visualize single phase and multiphase transport. The dispersion of tracers within these models will be interpreted using the formalism of macrotransport theory to gain insight into the mechanisms of solute exchange between microscale and nanoscale pores. The proposed research is significant because until we understand transport mechanism in multiphase flows between microscale fractures and nanoscale matrices, we cannot develop extraction strategies to recover resources in these features.
In Year 1 of the project, we have developed micro- and nanoscale models of porous media and used brightfield and confocal microscopy to quantify multiphase flow in the channels. At the microscale, we have examined the role of network topology under simulated water flood conditions typical of enhanced oil recovery in terms of capillary number and viscosity ratio. Pore scale networks were defined in PDMS using standard soft lithography techniques. Silane chemistry was used to attach alkane groups to the PDMS, rendering the entire pore wall hydrophobic. We considered coordination number of 3-6 in periodic geometries and random geometries defined by Voronoi tesselation in these studies, as well as the effect of heterogeneities (e.g. vugs), in two-phase flow experiments. We found decreased oil recovery with increasing coordination number. However, the effect of heterogeneities had a greater effect than coordination number. A similar set of experiments was conducted using surfactants in the water phase. As expected, the oil recovery improved in homogeneous networks compared to conditions with without surfactants. In a somewhat surprising result, we found that surfactants had a negligible effect in heterogeneous networks that contained large vugs. These results suggest that surfactant flooding is not likely to improve oil recovery in heterogeneous media such as carbonates.
At the nanoscale, we used a combination of reactive ion etching and anodic bonding to make transparent nanochannels in bulk silicon. In our first set of devices, we defined a set 50 of parallel channels with a height of 100 nm. Initially, the network was filled with water or oil. Next, we measure the pressure required to displace the water/oil with air and recorded the shape and velocity of the displacement profile. Interestingly, the hydrodynamic instabilities observed in microscale channels (e.g. viscous fingering) seemed to be suppressed in nanoscale channels. Future work will focus on solute transport in this and more complex nanoscale geometries.
One of the primary objectives of the proposed work was to develop pore scale models that have both micro- and nanoscale pores. Towards that end, we have developed two systems that meet that criterion. The first system is fabricated in bulk silicon using conventional nanofabrication techniques. This system is well-suited for studying gas-liquid flows at high pressures and temperatures. The second system uses agarose, a hydrogel with nanoscale pores, as a matrix for molding microscale pore networks similar to our PDMS devices. This system is well-suited for studying solute transport in the liquid phase between micro- and nanoscale pores. Studies for the upcoming year will focus on measuring tracer transit times within both types of dual porosity models.