An interpretation of the difference between aperture estimates derived from hydraulic and tracer tests in a single fracture

Abstract

Recent field efforts involving testing of single fractures indicate that hydraulic and tracer tests provide different estimates of fracture aperture. It is argued that this behavior is a result of the averaging involved in the use of the cubic law for these tests. This argument leads to the following predictions for slow flow through variable aperture fields: (1) For flow parallel to aperture channeling the aperture estimate from a tracer test will be greater than the estimate from a hydraulic test, (2) for flow perpendicular to aperture channeling the tracer test provides the smaller estimate, and (3) for flow in isotropic aperture fields the tracer test again provides the smaller aperture estimate. Lognormally distributed aperture fields are used to demonstrate the magnitude of aperture variability which may be observed under three flow geometries. It is noted that the maximum difference between aperture estimates occurs when the variance of the aperture distribution is large, a result which is consistent with other studies of random aperture fields. Finally, it is suggested that the relative variation of the aperture estimates may provide a means whereby the orientation of anisotropy or channeling may be identified.