Steven L. Bryant
Collaborators: Todd Arbogast (Dept. of Mathematics and Center for Subsurface Modeling) and Jim Jennings (Bureau of Economic Geology)
Vuggy rocks like the one pictured below right present a challenge to traditional models of flow and transport in porous media. In this formation, the vug lengths are 10s of cm, too big for standard core analysis to capture. This research project is seeking to model large scale behavior (effective permeability, contaminant transport) from fine-scale models that account for the geometry of the vugs and the matrix surrounding them.
Sub-mm resolution CT scans of a large chunk of rock (25 cm diameter by 35 cm height) provide the detailed geometry. The vugs are so large that we are forced to consider multiphysics models (Stokes flow in the vugs, Darcy flow in the matrix). An example of the extreme heterogeneity of simulated tracer transport appears in the right panel. The green contour corresponds to a tracer concentration of 80% of the injected value after 0.95 PV of fluid have been injected. The tracer samples only a small part of the sample, and the "swiss cheese" appearance of the contour indicates that much of the pore space will not see the injected fluid until long after the vug-dominated volumes have been swept.
Efforts to validate the mathematical multi-scale and multi-physics models under development present their own challenges. Traditional core floods are not possible because the vugs are so large, so we are conducting "chunk-floods" on samples 25 cm in diameter. Such samples fit conveniently in a five-gallon bucket. The sample came from a creek bed near the town of Pipe Creek, Texas. In the experiments pictured here we discovered that modern sediment has infiltrated the vugs, significantly reducing the permeability and complicating our comparison with model predictions. More experiments are currently underway in CPGE laboratories.
More information is avaiable from the pore-scale modeling web page.