Usage of “Equivalent apertures” for rock fractures as derived from hydraulic and tracer tests

Abstract

In the literature during the past several years there appear numerous references to the “equivalent aperture” of a rough‐walled rock fracture as derived from various hydraulic and tracer tests. However, the similar or even identical terms used by different researchers for “equivalent aperture” often do not have the same meaning. This has led to some confusion in the comparison of their results. In particular, there is a serious apparent contradiction in the claims of some authors that “equivalent apertures” derived from tracer tests are much larger than those derived from hydraulic tests, and the findings of others that apertures estimated from tracer tests are consistently smaller than those estimated from hydraulic tests. This apparent contradiction of the field results in fact arises from the different definitions of the so‐called “tracer aperture” as employed by different researchers. In this short technical note I have attempted to sort out the different definitions, denotations and usage of the various “equivalent apertures” and show that there are mainly three alternative definitions used in the literature. The meaning of each as related to experimental measurements is explained and their interrelationship discussed. It is shown that once the specific definition of “equivalent aperture” referred to by each researcher is identified, then the relative magnitudes of these “equivalent apertures” as reported by different groups of researchers are perfectly consistent with each other.

Phase II Transport Model of Corrective Action Unit 98: Frenchman Flat, Nevada Test Site, Nye County, Nevada, Revision 1

“…This difference is a function of the portion of the aperture distribution within the fracture that participates during hydraulic and tracer tests. For hydraulic tests, the effective aperture will be most impacted by the smaller aperture regions because those are the areas that offer the most resistance to flow (Tsang, 1992). The influence of fracture roughness and contact area, resulting in tortuous flow and reduction in flow rate, is largest when a larger fraction of small-aperture regions exists (Tsang, 1984).…”

“…Therefore, the migration during tracer tests is controlled by the arithmetic mean of the fracture apertures (Tsang, 1992). Hydraulic conductivities from hydraulic tests will be controlled by the small aperture regions in the fracture system, and they will yield smaller fracture porosities calculated using the cubic law than will fracture porosities calculated from tracer tests that are controlled by the mean fracture aperture.…”

Description of Fracture Systems for External Criticality Reports

“…At "q" standard deviations, the aperture is: that is the geometric average hydraulic fracture aperture has a probability of 0.15% of being in excess of 1.35 millimeters and similarly of being below 7 pm. It can be proven theoretically (Tsang 1992) that mass balance apertures are always larger than hydraulic apertures. A relationship between hydraulic and transport apertures is given by Gelhar (1993, Eq.…”

Far-Field Accumulation of Fissile Material From Waste Packages Containing Plutonium Disposition Waste Form

“…The later is always larger than the former (Ref. 43). Flow tends to be impeded by constrictions (that are measured as hydraulic aperture) while transport is more function of the largest openings where the solute has to diffuse (or disperse).…”