ON ESTIMATION OF THE REPRESENTATIVE ELEMENTARY VOLUME FOR BEDROCK SYSTEMS

The size of the Representative Elementary Volume (REV) for a porous media is larger than individual pores or soil grains, and common references establish it as, “greater than 1 cm³” or “30-50 grain diameters on a side”. Fractured bedrock with low primary porosity exhibits its dominant water flow in the fractures. However commonly, these systems are modeled as porous media systems and with porous media models. Philosophically, there is no reason why the “30 to 50 grain diameters” approach cannot be extended to fractured bedrock systems. In this case, the "30 to 50" metric is more realistically associated with fracture characteristics and interconnectedness.

Wells in competent, fractured bedrock were constructed and then complete borehole geophysical measurements taken, sufficient to describe borehole fracture characteristics. The wells then had multiple hydraulics packer tests performed. These tests included over 200 slug and 10 pumping tests. One pumping test was a site-wide test that involved wells in overburden, weathered bedrock, and competent bedrock. This information provided a database with which to make statistical inferences about bedrock hydraulic parameters and the size of the REV. Analysis of the slug test data utilized a heterogeneous conceptual model rather than the more common homogeneous models. The REV was identifiable from slug tests in single wells, for single intervals, without the use of nearby monitoring wells. At this site, the REV was on the order of 1 to 30 times the median fracture spacing. This REV from hydraulic tests is compatible with what can be estimated from fracture density observed in cores, outcrops, and geophysical logs. The interrelation between hydraulic testing of well bore intervals and testing the entire open well bore may in fact overlap in the description of the REV. If one considers that an open borehole is a linear, vertical investigation zone in which to study hydraulic parameters, and that this zone is on the order of the REV, then open well bore hydraulic tests may yield REV-scale information. Arithmetic and geometric sums of well bore interval transmissivity compared favorably to the open borehole transmissivity, which supports this conclusion.