Computer model for determining fracture porosity and permeability in the Conasauga Group, Oak Ridge National Laboratory, Tennessee

Joint length and spacing, measured in the field, were found to be extremely variable within each exposure and highly dependent upon surficial weathering. The measurements from all locations were combined for detailed analysis and trend prediction. In the siltstone beds, mean joint length varied from 2 cm to 76 cm with a mean range of 6 to 45 joints per meter. The shale beds contained a nearly constant mean joint length of 12 cm with 12 to 28 joints in a one-meter traverse. Results showed that the joint length and spacing increased with increasing bed thickness in the siltstone, while the bed thickness variations in the shale had little effect on the joints.

A computer model was developed by combining the joint orientation, joint spacing, and joint length data collected in the field with subsurface drill core information for the purpose of calculating the fracture porosity and permeability of the rocks. The joint gap width was measured from both outcrop and subsurface samples with ranges from 0.1 mm to 0.7 mm in the siltstones and less than 0.2 mm in the shales. The value for the joint gap width was found to be the major factor in the fracture porosity and permeability calculation. A gap width of 0.19 mm in a pure shale bed had a fracture porosity of 0.09 percent and fracture permeability of 0.09 darcy. In a pure siltstone bed, the porosity and permeability were 0.03 percent and 0.001 darcy. It was determined that increasing the gap width by a factor increases the fracture porosity by the same factor and causes the permeability to increase by the cube of the factor.

The results of the model suggest that the migration of fluids through highly jointed clastic rocks should be considered when defining groundwater flow patterns.