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  • 3-D Method Enables Study of Flow in Rocks

Photonics Spectra
Feb 2003
Brent D. Johnson

Subsurface science is concerned with the way that water and contaminants move through rocks. The problem, as Idaho National Engineering and Environmental Laboratory researcher Vance A. Deason explains it, is that the rocks are like a black box. Lack of information about the internal fractures of rocks prevents scientists from understanding the physical processes that are occurring.

Previous geological models were quite crude. In some cases, scientists stacked up piles of bricks and dripped water on top to see how it percolated through the system. The problem was that there was no way to replicate the experiments. Using a synthetic rock that is easily duplicated and optically transparent, however, they can understand the dynamic flow through the internal structure.

The actual dye flow through a model, above left, is very close to that of the computationally predicted flow, above right.

Deason, microbiologist Daphne Stoner and their research group at the laboratory are using complex three-dimensional models that allow them to explore the fracture systems and flow characteristics of rocks and biofilms. They enlisted the help of FineLine Prototyping, a company that specializes in 3-D stereolithography, to make computer-aided designs from which they can produce realistic physical models of simple fracture systems. Scans of actual rock fracture surfaces can be incorporated into the computer-aided models to create even more realistic flow structures.

This technique has yielded good results. When a laser draws on the surface of a liquid photopolymer called WaterShed, the material solidifies. A new cross section of material is added after each pass of the laser, which eventually produces a stack of sections that replicate the computer-aided design model. The resolution can be altered by making the steps smaller (thinner) or by using a finer laser beam. FineLine employs the Viper System from 3D Systems of Valencia, Calif., to make steps that are 2 x 10-3 in., using a beam diameter of 3 x 10-3-in. The system uses an Nd:YVO4 laser operating at 354.7 nm, which allows minimum build layers that are 0.05 mm thick.

In this simple model made from transparent resin, the researchers could track how the dye flows through bumps.

Because the WaterShed resin is transparent, optical measurements enable the researchers to track the flow of material through the system. They can introduce a pulse of colored fluid to see how a section of dye moves through the model. In nature, some of these aquifer systems can take 100 years or more to move water across a valley. However, on a smaller scale in the lab, the movement takes minutes to hours and is recorded with a series of six megapixel pictures using a high-definition digital camera from Eastman Kodak Co. of Rochester, N.Y.

The models are being used to examine the impact of microorganisms on fluid flow within fractures. For these experiments, the researchers inoculated the models with biofilm- forming bacteria and observed the impact on flow. "Understanding the impact of microorganisms on hydrogeological flow is important for maintaining pathogen- and contaminant-free drinking water, cleaning up the environment and enhancing some industrial processes such as oil recovery," Stoner said.

Simple models, such as the transparent flow sample, right, and the intersecting planar fracture system that the model contains, left, were designed to verify the accuracy of the computational effort and to check out the acquisition system.

One of the most promising new techniques for producing synthetic rocks uses material such as powdered wax that is infused with metal or ceramic. A hot laser is applied to a thin dusting of the metal to form a hard metal surface. This is called sintering, or fused deposition modeling. To achieve a realistic chemistry of the rock formations, this may be the best way to go, Deason said.

The scientists are particularly interested in modeling basalt. They could infuse basalt powder into the model, which would give them the desired chemical interaction in addition to the mechanical behavior of the model.

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